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Effects of dynamical couplings in hydrogen bond systems in the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals
Chemical Physics 368 (2010) 133–145
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Effects of dynamical couplings in hydrogen bond systems in the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals Henryk T. Flakus *, Barbara Hachuła Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl-40-006 Katowice, Poland
a r t i c l e
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Article history: Received 9 November 2009 In final form 27 January 2010 Available online 1 February 2010 Keywords: Hydrogen bond Molecular crystals Polarized IR spectra H/D isotopic effects Linear dichroic effects Temperature effects Isotopic dilution Dynamical co-operative interactions
a b s t r a c t This paper presents the investigation results of the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals measured at 293 and 77 K. Analysis of the results concerned the linear dichroic, H/D isotopic and temperature effects observed in the spectra of the hydrogen and deuterium bond at the frequency ranges of the mO–H and the mO–D bands, respectively. The main spectral properties of the crystals were interpreted in terms of the ‘‘strong-coupling” theory on the basis of the hydrogen bond dimer model. The spectra revealed that the strongest vibrational exciton coupling involved the closelyspaced hydrogen bonds, each belonging to a different chain of associated molecules. The reason for two different crystalline systems, are characterized by almost identical mO–H and mO–D band shapes, is explained. It was proved that a random distribution of the protons and deuterons took place in the lattices of the isotopically diluted crystals. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Over the last six decades IR spectroscopy has been considered to be a powerful tool in investigating hydrogen bonds. The spectral studies mainly concern the middle frequency range of IR where the mX–H bands, ascribed to the proton stretching vibrations in the X–H Y hydrogen bonds, appear [1–5]. Complex fine structure patterns of the mX–H bands are highly susceptible to influences exerted by diverse interaction mechanisms such as the intra- and inter-hydrogen bond ones [1–5]. According to the contemporary quantitative theories of IR spectra of hydrogen bonded systems these bands are treated as an abundant source of information about the complex dynamics of hydrogen bonds. In terms of these theories, strong anharmonic couplings involving motions of different forms and of different energies are considered to be responsible for the generation of the main spectral properties of hydrogen bond systems. To understand the spectral properties of the hydrogen bond systems the socalled ‘‘strong-coupling” [6–8] and the ‘‘relaxation” theory [9,10] played an important role. These theoretical models allowed us to succeed in the quantitative interpretation of the IR spectra of the hydrogen bond in simple molecular aggregates like dimers [7,11– 13] as well as in hydrogen-bonded molecular crystals [14–16].
* Corresponding author. Fax: +48 32 2599 978. E-mail address: fl[email protected] (H.T. Flakus). 0301-0104/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2010.01.024
The H/D isotopic effects obtained due to IR spectroscopy have been used to verify the validity of theoretical models subsequently introduced for the quantitative interpretation of vibrational spectra of hydrogen bond systems [6–10]. From earlier spectral studies of hydrogen bond dimers it resulted that mechanisms non-revealed yet might also co-decide about the generation of the system spectra [17,18]. It seemed that IR spectroscopy in polarized light, applied for investigation of crystalline spectra should provide essential data in this matter. Polarized IR spectra of molecular crystals measured for spatially oriented lattices of hydrogen bonds seem to be the source of the a complete data system concerning the inter- as well as the intra-hydrogen bond interactions in the systems. However, the solid-state introduces its own new effects considerably complicating the crystal spectra interpretation, related to the inter-hydrogen bond couplings in the excited vibrational state. Overcoming these interpretation problems may allow us to extend our knowledge about the coupling mechanisms, involving motions of diverse forms in hydrogen bond systems. Recent studies of polarized IR spectra of the hydrogen bond in diverse crystalline systems have exhibited a rich diversity of their spectral properties and have allowed us to reveal a number of new, non-conventional effects in the spectra. These new effects were found when model calculations, mainly performed in terms of the so-called ‘‘strong-coupling” theory [6–8], were applied to the quantitative interpretation of the crystalline spectra. Among these effects are the so-called H/D isotopic ‘‘ self-organization” effects, identified in the polarized IR spectra of isotopically diluted
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hydrogen-bonded molecular crystals [19,20]. They result from a non-random distribution of protons and deuterons in the hydrogen bridge systems in lattices of isotopically diluted molecular crystals. It looks as if some non-identified ‘‘attraction” forces appear, which involve identical hydrogen isotope atoms in coupled hydrogen bond systems. Therefore, the invariability of the mX–H proton stretching vibration bands, registered independently of the increasing concentrations of deuterons in a crystalline sample could be observed in the spectra of isotopically diluted crystalline samples. The latter effect is the result of the newly revealed mechanisms governing the inter-hydrogen bond co-operative interaction in molecular crystals, i.e., the so-called dynamical cooperation interaction mechanisms [19,20]. Vibronic coupling in hydrogen bonded molecular systems is considered to be the most probable source of these interactions [19,20]. In the spectral studies of H/D isotopic effects for hydrogen bonded systems carried out so far the problem of the influence of the isotopic dilution on to the IR spectra was treated as rather marginal. This was the result of the studies performed in the past, which significantly influenced later investigations in this area [21–23]. Detailed studies of the dynamical co-operative interaction mechanisms performed for diverse solid-state systems have proved that the H/D isotopic ‘‘self-organization” processes did not occur in one precisely defined way. This remark mainly concerns a particular group of hydrogen-bonded molecular crystals in whose lattices hydrogen bonds form infinitely long chains. It appeared that in relation to the electronic structure of the associating molecules, the H/D isotopic ‘‘self-organization” processes, provided they take place, might proceed in two different ways. In the first one they may involve the adjacent hydrogen bonds in each individual chain. In the other case, pairs of closely-spaced hydrogen bonds participate, where each moiety in an individual pair belongs to another chain. The first group of crystals comprises crystals of pyrazole [24], imidazole [25] and 4-thiopyridone [26]. These molecules contain large delocalized ‘‘p”-electronic systems. Surprisingly, also formic acid crystals belong to this group [27]. This way of the occurrence of the discussed processes is evidenced by the fact that linear dichroic effects are retained. This in turn depends on the differentiation of the dichroic properties of the two opposite branches of the mX–H ‘‘residual” bands in the polarized spectra of isotopically diluted crystals. Also the intensity distribution pattern of these bands, characteristic of linear hydrogen bond dimers, supports this conclusion. The second group comprises crystals of N-methylthioacetamide [28], N-methylacetamide [29] and crystals of acetic acid [30]. For these latter systems the distribution of protons and deuterons in the hydrogen bridge chains appeared to be fully random. This was deduced from the disappearance of the Davydow-splitting effects [31,32] in the mX–H ‘‘residual” bands in the spectra of the isotopically diluted crystals, attributed to the in-chain exciton couplings. Also the fine structure intensity pattern of these bands, similar to the one observed for cyclic hydrogen bond dimers appears in these spectra. In this case no essential differences in the dichroic properties between the opposite mX–H ‘‘residual” band branches can be observed [28,29]. This fact supports the hypothesis about the inter-chain H/D isotopic ‘‘self-organization” in this group of crystals. In this article, we present the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals. Aiming at their quantitative interpretation in the initial theoretical approach it seems possible that the process proceeds in two different ways. The lattices of both these crystals are composed with infinite hydrogen bond chains, differing however, from one another in their space-symmetry groups and in the mutual hydrogen bond arrangement. Therefore, these systems seemed to pose interesting
objects for the investigation of the H/D isotopic ‘‘self-organization” process mechanisms in relation to the electronic structure of the associated molecules and the space-symmetry of the crystals. One may expect that the substituent atomic groups, O–H and CHO, linked to the benzene rings in two different positions, 1,3and 1,4-, can significantly influence the electronic properties of O–H O hydrogen bonds in these crystalline systems. In this way they may affect the magnitude of the dynamical co-operative interaction energies since these mechanisms are basically of a vibronic nature [19,20]. Therefore, one can expect that the H/D isotopic ‘‘ self-organization” processes occur in two different ways for these two different isomeric system crystals. Similar electronic properties of the two akin molecular systems may facilitate a fully quantitative interpretation of the crystal spectra and therefore, might allow for a deeper insight into the very nature of the H/D isotopic ‘‘ self-organization” processes in hydrogen bond systems [19,20]. 2. X-ray structures of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde Crystals of 3-hydroxybenzaldehyde (3-HBA) belong to the orthorhombic system. Space-symmetry group is Pna21 ¼ C 92v . The Factor-group is C2v. Melting point is 100–103 °C. A unit cell contains four molecules (Z = 4). The crystal lattice is composed of infinitely long chains of associated 3-HBA molecules of a ‘‘zig-zag” form, linked by O–H O hydrogen bonds. These molecular chains elongate along the ‘‘c” axis direction. The lattice constants are: a = 18.858(7) Å, b = 3.864(1) Å, c = 8.190(7) Å and a = b = c = 90° [33]. A view of the 3-HBA crystal lattice is shown in Fig. 1. Crystals of 4-hydroxybenzaldehyde (4-HBA) are monoclinic. The space-symmetry group is P21 =c ¼ C 52h and Z = 4. The crystal Factor-group is C2h. The unit cell parameters determined at 295 K are: a = 6.453(5) Å, b = 13.810(8) Å, c = 7.044(6) Å, a = c = 90°, b = 107.94(9)°. 4-HBA molecules are linked by O–H O hydrogen bonds forming infinitely long ‘‘zig -zag”-type chains elongating along the ‘‘b” axis [34]. Fig. 2 presents the arrangement of the hydrogen-bonded 4-HBA molecules in the lattice of 4-HBA crystal. 3. Experimental 3-Hydroxybenzaldehyde (3-HBA) and 4-hydroxybenzaldehyde (4-HBA) (the general chemical formula HO–C6H4–CHO), used for our studies were the commercial substances (Sigma–Aldrich). The samples were used without their further purification.
Fig. 1. View of a unit cell in the crystal lattice of 3-hydroxybenzaldehyde. A pair of the closely-spaced O–H O hydrogen bonds governed by the C2v site-symmetry group is marked by an ellipse.
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Fig. 2. Unit cell in the lattice of 4-hydroxybenzaldehyde crystal. A centrosymmetric pair of the O–H O hydrogen bonds (Ci site-symmetry) is marked by an ellipse.
The single crystals of 3-HBA and 4-HBA and crystals of their deuterium isotopomers were obtained by crystallization from melt, occurring between two closely spaced CaF2 windows. In this way we prepared thin enough crystals, characterized by their maximum absorbance close to 0.5 at the mO–H band frequency
range. Next, suitable monocrystalline fragments from the crystalline mosaic were selected and then spatially oriented, using a polarization microscope. These selected single crystals were exposed for the experiment by placing them on a metal plate diaphragm with a 1.5 mm diameter hole. The two different forms of
Fig. 3. The mO–H bands in the IR spectra of the polycrystalline samples of 3-hydroxybenzaldehyde (3-HBA) and 4-hydroxybenzaldehyde (4-HBA) dispersed in KBr pellets, measured at 293 and 77 K. (a) 3-HBA. (b) 4-HBA. The Raman spectrum of the polycrystalline samples is also plotted in order to indicate the influence of the mC–H bands on the spectra.
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3-HBA crystals developed either the ‘‘bc” or ‘‘ac” crystalline faces, whereas two crystalline forms of 4-HBA developed the ‘‘bc” or ‘‘ab” planes of the lattice. In a similar way measurements of spectra were performed for 3-HBA and 4-HBA deuterium derivative crystals, which were obtained by evaporation of the compound solution in D2O solution at room temperature and under reduced pressure. The deuterium substitution rate for different crystals investigated varied in a relatively wide range (from 20% to 80%). The IR spectra were recorded with the FT-IR Nicolet Magna 560 spectrometer by transmission method with 2 cm1 resolution. Measurements of the spectra were performed in the temperature range from 293 K, to the temperature of liquid nitrogen for two different orientations of the electric field vector ‘E’. For 3-HBA crystals with the ‘‘bc” face developed the spectra were recorded for the ‘‘E” vector parallel to the ‘‘b” axis of the lattice, and in the other case, for the one perpendicular to it, i.e., parallel to the ‘‘c” identity period. For the other crystal form of 3-HBA developing the ‘‘ac” plane, the spectra were measured for the ‘‘E” vector parallel to the ‘‘c” axis and then for the one perpendicular to it, i.e., parallel to the ‘‘a” identity period. In the case when 4-HBA crystals developed the ‘‘bc” face, the spectra were measured for the ‘‘E” vector parallel to the ‘‘b” axis of the lattice and then for the one perpendicular to it, i.e., parallel
to the ‘‘c” identity period. For the other crystal form, with the ‘‘ab” face developed, the polarized spectra were recorded for the ‘‘E” vector parallel to the ‘‘b” axis of the lattice, and then for the one perpendicular to it, i.e., parallel to the ‘‘a” identity period. For each 3-HBA and 4-HBA case, the measurements were repeated for ca. 10 different single crystals. The Raman spectra of polycrystalline samples of 3-HBA and 4-HBA were measured at room temperature with the use of the Raman Accessory for the Nicolet Magna 560 spectrometer. 4. The results of the spectral studies The IR spectra of polycrystalline samples of 3-HBA and 4-HBA, measured in the frequency range of the proton stretching vibration band mO–H, with the use of the KBr pellet technique at room temperature and the temperature of liquid nitrogen, are shown in Fig. 3. In the same picture the Raman spectrum of polycrystalline samples of these compounds was plotted to facilitate the identification of the C–H bond stretching vibration lines, overlapping the mO–H, band contours. In Fig. 4 spectra of isotopically diluted 3-HBA and 4-HBA polycrystalline samples are given, measured with the use of the KBR pellet technique in the frequency ranges of the proton and the deuteron stretching vibration bands, mO–H and mO–D, respectively.
Fig. 4. The mO–D bands in the IR spectra of partially deuterated polycrystalline samples of KBr pellets, measured at 293 and 77 K. (a) 3-HBA. (b) 4-HBA.
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In the next stage of the study polarized spectra of 3-HBA and 4HBA single crystals were measured for isotopically neat as well as for isotopically diluted samples. The polarized spectra of 3-HBA crystals recorded at 77 K in the frequency range of the mO–H band, for two mutually perpendicular electric vector ‘‘E” orientations, are presented in Fig. 5. Spectra of two crystal forms, with two different ‘‘bc” and ‘‘ac” crystalline faces, were measured. The polarized IR spectra of 4-HBA crystals polarized spectra measured at 77 K in the frequency range of the mO–H band, for two different crystal forms, characterized by two developed planes ‘‘bc” and ‘‘ab”, were measured and are shown in Fig. 6. The low-temperature spectra of the two different forms of isotopically diluted 3-HBA single crystals recorded in the ‘‘residual” mO–H and mO–D band frequency ranges are shown in Fig. 7. In Fig. 8, the low-temperature polarized spectra of the two different forms of isotopically diluted 4-HBA single crystals measured in the ‘‘residual” mO–H and mO–D band frequency ranges are presented.
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5. Discussion 5.1. Linear dichroic effects in the mO–H bands The corresponding spectra of the two substances, of different Xray structures, isotopically neat as well as isotopically diluted, are characterized by fairly similar fine structure patterns of their polarized mO–H and mO–D band contours. Polarized spectra of monocrystalline samples of 3-HBA exhibit the characteristic two-branch intensity distribution patterns of their mO–H bands. The polarized spectra measured for the two different crystalline faces are of qualitatively similar mO–H band shapes but markedly differ in the relative intensities of their spectral branches, i.e., of the shorter- to the longer-wave one. In the case of the ‘‘ac” plane spectra the higher-frequency branch is narrow and of a relatively high intensity, whereas in the spectra of the ‘‘bc” face this spectral branch is significantly wider and also of a relatively higher intensity in relation to the lower-frequency branch properties. These effects may result from vibrational
Fig. 5. Polarized IR spectra of 3-hydroxybenzaldehyde (3-HBA) single crystals, measured by a transmission method at 77 K, in the mO–H and the mC–H band frequency ranges. The IR beam of normal incidence with respect to different crystalline faces ‘‘bc” and ‘‘ac” was applied. In each case the component spectra were obtained for two different orientations of the electric field vector E. (a) ‘‘ac” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘a”). (b) ‘‘bc” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘b”). Spectra (I) and (II) were drawn on a common scale.
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Fig. 6. Polarized IR spectra of 4-hydroxybenzaldehyde (4-HBA) single crystals, measured by a transmission method at 77 K, in the mO–H and the mC–H band frequency ranges. The IR beam of normal incidence with respect to different crystalline faces ‘‘bc” or ‘‘ab” was applied. In each case the component spectra were obtained for two different orientations of the electric field vector E. (a) ‘‘bc” plane: (I) Ek‘‘b”; (II) E\‘‘b”, i.e. (Ek‘‘c”). (b) ‘‘ab” plane: (I) Ek‘‘a”; (II) E\‘‘a”, i.e. (Ek‘‘a”). Spectra (I) and (II) were drawn on a common scale.
exciton interactions involving translationally non-equivalent hydrogen bonds in each unit cell of the crystal lattice (Davydovsplitting effect). In the polarized IR spectra the main linear dichroic effects can be seen connected to the orientation of the electric field vector ‘‘E” of the incident beam of IR radiation with respect to the spatially oriented hydrogen bond chains in the experimental conditions. This is the polarization effect of the ‘‘first kind”. In each case the most intense polarized component of the crystal spectra was recorded on the ‘‘E” vector polarization parallel to the ‘‘c” axis, i.e., parallel to the direction of hydrogen bond chains in the lattices. Considerably weaker polarized components of the crystalline spectra were measured for the ‘‘E” vector polarization perpendicular to the chain directions. For the ‘‘bc” crystal face of 3-HBA it is the ‘‘b” axis direction and for the ‘‘ac” form it is the ‘‘a” axis direction. From comparison of the dichroic properties of the two spectral branches of the mO–H band it results that in the whole band frequency range the direction of the transition moment vector is almost constant. It means that the crystal spectral properties are
not governed by the C2v Factor-group. The difference in the higher-frequency band branch width in the spectra of the two crystal forms proves that vibrational exciton coupling mechanism involves four hydrogen bonds from a unit cell. Therefore, the approach assuming a weak exciton coupling limit [31,32] for the interpretation of the 3-HBA crystal vibrational spectra seems to be the most appropriate. In the case of the 4-HBA crystal qualitatively similar spectral effects can be observed. Also in these spectra the higher-frequency branch of the mO–H band is more intense in comparison with the lower-frequency band branch intensity. The spectra of the two crystal forms markedly differ in the width of their higher-frequency mO–H band branch. Therefore, the exciton interaction mechanism, similar in nature to the one found in 3-HBA crystal spectra, seems to govern the spectral properties of 4-HBA crystal. On comparing the polarized IR spectra of the hydrogen bond in 3-HBA and 4-HBA crystals, measured in the mO–H band frequency range, a fair similarity between them can be noticed. The common property of the compared spectra is also an almost proportional
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Fig. 7. Polarized IR spectra of isotopically diluted 3-hydroxybenzaldehyde (3-HBA) crystals measured at 77 K in the mO–D and the ‘‘residual” mO–H band frequency range. Common scale. The spectra were measured for the normal incidence of the IR beam with respect to different crystallographic planes, respectively, for two polarizations of the electric field vector E. (a) ‘‘ac” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘a”) (ca. 30%H, 70%D). (b) ‘‘bc” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘b”) (ca. 15%H, 85%D).
change in their intensity distribution when the polarization of the electric field vector ‘‘E” of the incident beam is changed in the experimental conditions. At this point it seems noteworthy to add that the decrease in temperature from 293 to 77 K does not noticeably change the linear dichroic properties of the analyzed spectra. The linear dichroic effects observed in the 3-HBA and 4-HBA crystal spectra seem to be puzzling enough, since the two crystalline systems, each of a chain hydrogen bond arrangement in its lattice, differ in their space-symmetry groups. In the case of spatially-oriented hydrogen bond chains, the basic dichroic effect in their polarized IR spectra should be strictly connected to the orientation of the ‘‘E” vector with the respect to the chains. In result of the sufficiently strong vibrational exciton coupling, involving hydrogen bonds arranged in the chain of the ‘‘zig-zag” type motif, the polarized spectra obtained for the two orientation of ‘‘E”, i.e., parallel and perpendicular to the chain direction, should strongly differ in their dichroic properties. In this case the higher- and the lower-frequency mO–H band branch in the polarized spectra should be characterized by the mutually perpendicular transition moment
directions. This would be the linear dichroic effect of the ‘‘second kind”. Lack of such effects in the polarized spectra of 3-HBA and 4-HBA crystals requires a convincing explanation. 5.2. Linear dichroic effects in the mO–D bands The mO–D bands in the crystalline spectra of 3-HBA and 4-HBA are characterized by relatively pffiffiffisimple fine structure patterns of frequencies related by the 1= 2 factor with the corresponding frequency values of the spectral branches in the ‘‘residual” mO–H bands. This is the familiar H/D isotopic effect in the area of the IR spectroscopy of the hydrogen bond [1–5]. It can be noticed that the mO–D band in the polarized spectra of 3-HBA crystals measured in the higher-frequency branch frequency range for the ‘‘bc” crystal face is wider than the band of crystals with the ‘‘ac” plane developed. This is most probably the effect of the vibrational exciton interactions on to the spectra, i.e., of the Davydov-splitting effect. In these circumstances the ‘‘residual” mO–H band contours became practically undistinguishable. This means that the ‘‘residual” mO–H bands are devoid of the
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Fig. 8. Polarized IR spectra of isotopically diluted 4-hydroxybenzaldehyde (4-HBA) crystals measured at 77 K in the mO–D and the ‘‘residual” mO–H band frequency range. Common scale. The spectra were measured for the normal incidence of the IR beam with respect to different crystallographic planes, respectively, for two polarizations of the electric field vector E. (a) ‘‘bc” plane: (I) Ek‘‘b”; (II) E\‘‘b”, i.e. (Ek‘‘c”) (ca. 25%H, 75%D). (b) ‘‘ab” plane: (I) Ek‘‘a”; (II) E\‘‘a”, i.e. (Ek‘‘a”) (ca. 30%H, 70%D).
effect of some exciton interactions involving hydrogen bonds, whereas the mO–D bands are visibly influenced by the inter-deuterium bond exciton couplings. Qualitatively very similar spectral effects can be found in the mO–D and the ‘‘residual” mO–H bands in the polarized spectra of 4HBA crystals. Similarly to the mO–H ones, the mO–D bands exhibit almost homogeneous linear dichroic properties in the whole frequency range of the band in the spectra of the two molecular systems. This is expressed by the fair proportionality relation between the intensity distribution of the polarized component bands in the crystalline spectra. 5.3. Isotopic dilution effects in the crystalline spectra On comparing the spectral properties of two forms of isotopically neat crystals of 3-HBA and 4-HBA with the related properties of isotopically diluted crystals, it can be found that the effect of the isotopic dilution in the spectra is relatively strong. It can be noticed that along with the increase in concentration ratio of deuterons, the ‘‘residual” mO–H band contours in the spectra of the two crystal-
line forms of each individual compound, 3-HBA or 4-HBA, become indistinguishable. This is due to the narrowing of the contours of the higher-frequency band branches accompanied by the disappearance of the longer-wave branches of the bands. Consequently, for each compound the spectra of the two crystal forms exhibit identical widths. This effect may be due to the fact that interhydrogen bond vibrational exciton interactions in the lattice accompanying the isotopic dilution vanish. However, even for the highest concentrations of deuterons in the samples, the basic linear dichroic effects characterizing the ‘‘residual” mO–H bands practically remain unchanged in comparison with the corresponding properties of spectra measured for isotopically neat crystals. The observed changes in the mO–H band contour shapes in the polarized spectra due to the isotopic dilution are evidence of the effects of relatively weak dynamical co-operative interactions involving the closely-spaced hydrogen bonds on to the spectra [19,20]. In 3-HBA and 4-HBA crystals non-conventional ‘‘attraction” forces, responsible for holding together identical hydrogen isotope atoms in neighboring hydrogen bonds, if exist, are too weak to generate the H/D isotopic ‘‘self-organization” effects in the spectra. In these two cases the non-conventional dynamical co-operative
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interactions involving hydrogen bonds are probably much weaker than 1.5 kcal/mol of hydrogen bond dimers [18]. Therefore, the distribution of protons and deuterons in the hydrogen bond lattices is practically random [19,20]. 6. Model calculation of the mO–H and mO–D band shapes 6.1. The 4-hydroxybenzaldehyde crystal spectra Our quantitative analysis of the crystalline spectra started with calculations of the mO–H and mO–D band shapes for 4-HBA. Model calculations, aiming at the quantitative reconstitution of the main properties of the bands from the polarized IR spectra of 4-HBA single crystals were performed in terms of the ‘‘strong-coupling” theory [6,7,17]. The simplest hydrogen bond aggregate, exhibiting the crystal spectral properties discussed above, is the dimeric system of hydrogen bonds (see Fig. 1). It allows to retain the complex structure of the analyzed bands in the isotopic dilution. Therefore, suitable model calculations of the band contour shapes ought to be performed by assuming a hydrogen-bond dimer model. Within the ‘‘strong-coupling” theory the mO–H band shape for a dimer, composed of two O–H O hydrogen bonds, depends on the following system of coupling parameters: (i) on the distortion parameter ‘‘bH”, and on (ii) the resonance interaction parameters ‘‘C0” and ‘‘C1” [6,7,17]. Each parameter has a precisely defined physical meaning. The ‘‘bH” parameter describes the change in the equilibrium geometry for the low-energy hydrogen bond stretching vibrations, accompanied by the excitation of the highfrequency proton stretching vibrations mO–H. The ‘‘C0” and ‘‘C1” parameters are responsible for the mutual interactions between the hydrogen bonds in a dimer in its vibrationally excited state. They denote subsequent expansion coefficients in the series, on developing the resonance interaction integral ‘‘C”, with respect to the normal coordinates of the low-frequency, hydrogen bond stretching vibrations mO O:
C ¼ C0 þ C1Q 1; where Q1 represents the totally symmetric normal coordinate for the low-frequency stretching vibration in a dimer. These parameter values remain in close relation to the intensity distribution in the dimeric mO–H band: that is ‘‘bH” and ‘‘C1” parameters are directly connected with the dimeric mO–H bandwidth. The ‘‘C0” parameter determines the splitting of the component bands of the dimeric spectrum, corresponding to the excitation of the proton vibrational motions of different-symmetry [6,7,17]. In its original version, the ‘‘strong-coupling” model predicts diminution of the distortion parameter value for deuterium bond systems, according to the relation:
bH ¼
pffiffiffi 2bD :
For the ‘‘C0” and ‘‘C1” resonance interaction parameters, the simplest version of the theory predicts pffiffiffi the H/D isotopic effect expressed by diminution by 1.0 to 2 times of the parameter values. The model calculations performed with the use of this relatively simple model will allow us to understand better the generation mechanism of the crystalline spectra. In particular owing to them the individual hydrogen bond pairs in each unit cell, involved in the strongest vibrational exciton interactions, can be identified. These exciton-coupling energies affect most strongly the IR spectra of the hydrogen or the deuterium bond in 4-HBA single crystals and determine their basic properties. 6.1.1. Linear dimer approximation The first-step analysis of the geometry relations in the lattice of the 4-HBA crystal, basing on its X-ray structure geometry relations
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might suggest that the basic structural unit of the lattice, responsible for the generation of the spectra is the linear hydrogen-bond dimer. In this dimer the two moieties are linked together in the ‘‘ tail-to-head” order. In this case the mO–H and the mO–D bands should be considered to be a superposition of two component bands, each of different origin. On the basis of the ‘‘strong-coupling” model [6,7,17] for such a linear dimer the higher-frequency branch of each band corresponds with the non-totally symmetric (out-ofphase) proton (or deuteron) stretching vibrations, whereas the lower-frequency branch is connected with the totally symmetric (in-phase) vibrations. The intense, lower-frequency band branch is generated by the dipole-allowed transition and the lower-intensity, higher-frequency branch corresponds to the forbidden transition for the ideally axial symmetry of the dimer. Its intensity increases along the deviation of the dimer geometry from the ideal axial symmetry. For this model dimeric system the two vibrational transitions participating in the generation of the mO–H and mO–D bands should strongly differ in their polarization properties. Moreover, their individual transition moment vectors should be mutually perpendicular. Therefore as can be seen the 4-HBA crystal polarized spectra do not satisfy these requirements and therefore the linear dimer model cannot be treated as the potential source of the main spectral properties of the crystal. This model seems to be inadequate for the quantitative reconstitution of the band fine structure patterns. It also does not explain whether the linear dichroic properties of 4-HBA crystalline spectra as the ‘‘residual” mO–H and mO–D bands exhibit constant polarization properties in their whole band frequency ranges. However, from our studies it results that the polarization properties and the mO–H and mO–D band shapes in the 4-HBA crystal spectra resemble the corresponding properties of typical centrosymmetric hydrogen bond cyclic dimers found in diverse molecular crystals, e.g., in carboxylic acid crystals [35–38]. This fact suggests that another model hydrogen bond aggregate should be accepted as a reliable bearer of the basic spectral properties of the crystal. 6.1.2. Dimer of a ‘‘side-to-side”-type, anti-parallel arrangement of hydrogen bonds In the next step the centrosymmetric dimer model composed of two hydrogen bonds of the ‘‘side-to-side”-type, with anti-parallel arrangement of the moieties, is responsible for the generation of the main spectral properties of 4-HBA crystals. In this dimer each hydrogen bond belongs to a different chain of associated molecules passing the crystal unit cell. Also for this model system each analyzed band, mO–H and mO–D one, is treated as a superposition of two component bands, each of different origin. The higher-frequency branch of each band, corresponding to the Au excited state, is generated by the excitation of the non-totally symmetric proton (or deuteron) stretching vibrations in the centrosymmetric dimers. In turn, the lower-frequency band branch corresponds to the totally symmetric vibrations. This latter transition should be forbidden by the symmetry rules, since it occurs at the Ag excited state of the dimer. It is activated by a vibronic coupling mechanism involving proton stretching vibrations and the electronic motions in the dimer hydrogen bonds [39]. The forbidden transition promotion mechanism is a kind of reverse of the familiar Herzberg–Teller mechanism [40] originating from the theory of the UV spectra of aromatic hydrocarbons, ‘‘borrowing” its intensity from the allowed transition. Therefore, the two component transition bands are characterized by identical dichroic properties [39]. The problem of the forbidden transition activation in IR spectra of centrosymmetric hydrogen bond systems has been intensively studied for diverse crystalline system spectra. It was experimentally evidenced that the forbidden transition activation rate
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depends in a predictable way on the electronic properties of the associating molecules [37–39]. This fact remains in agreement with the predictions derived from the vibronic model [39]. In our calculations the two dimeric component subbands, denoted by the ‘‘minus” and ‘‘plus” symbols, contributed to the mN–H band generation mechanism with their appropriate statistical
weights given by the F and F+ parameter values, respectively. The so-called ‘‘plus” band mainly reproduces the higher-frequency band branch attributed the allowed transition, whereas the ‘‘minus” band reproduces the lower-frequency branch of the band connected to the forbidden transition. In Figs. 9 and 10, the results of model calculations aiming at the theoretical reconstitution of the mO–H and mO–D band contour shapes from the crystalline spectra of 4-HBA crystals, performed on using the ‘‘strong-coupling” model, are shown. The mO–H band contour was reproduced for the following coupling parameter values: bH = 1.6, C0 = 1.5, C1 = 0.4, F+ = 1.0, F = 0.5, XO O = 100 cm1, where the XO O parameter denotes the O O hydrogen bond stretching frequency. For the quantitative simulation of the mN-D band contour for the spectra of solid-state samples of 4-HBA the following coupling parameter values were used: bD = 0.3, C0 = 0.6, C1 = 0.2, F+ = 1.0, F = 0.1, XO O = 100 cm1. Our calculations managed to reproduce the observation that the higher-frequency branch is of higher intensity and diffused, whereas the lower-frequency branch is considerably weaker. It also exhibits a well-developed fine structure pattern, qualitatively resembling a typical Franck–Condon-type progression of low-frequency. The two component bands in the polarized spectra, measured for the ‘‘residual” bands should be characterized by identical transition moment directions since they ‘‘borrow” its intensity from the allowed vibrational transition in centrosymmetric hydrogen-bond dimers [39]. 6.2. The 3-hydroxybenzaldehyde crystal spectra
Fig. 9. Theoretical reconstitution of the most intense mO–H band component from the low-temperature spectra of 4-hydroxybenzaldehyde (4-HBA) crystals. (I) The ‘‘minus” dimeric band reconstituting the lower-frequency branch of the mO–H band, (II) the ‘‘plus” dimeric band reproducing the higher-frequency branch, and (III) the superposition of the ‘‘minus” and ‘‘plus” bands contributing proportionally to their respective statistical weight parameters F and F+. The coupling parameter values: bH = 1.6, C0 = 1.5, C1 = 0.4, F+ = 1.0, F = 0.5, XO O = 100 cm1. The corresponding experimental spectrum is given in inset.
The spectra of the 3-HBA crystals, with properties similar to the spectra of 4-HBA crystals, can also be quantitatively reproduced in terms of the ‘‘strong-coupling” theory [6,7,17] in the approximation of the hydrogen-bond dimer model. This fact remains in contrast to the quasi-parallel arrangement of the moieties in the model hydrogen bond pair. In this case the crystal lattice is also characterized by the ‘‘side-to-side”-type arrangement of the moieties of the C2v symmetry (see Fig. 2). This is the property which differs 3-HBA crystals from the 4-HBA ones. The coupling parameter value systems used for the quantitative interpretation of the mO–H and mO–D band contour shapes appeared to be almost identical with those reproducing the spectra of 4-HBA crystals. However, for the C2vsymmetry of the model dimer the two component transitions forming the band contours should differ in their polarization properties and in the decline of the hydrogen bond arrangement from the ideally parallel one. 7. Why are the spectra of both crystalline systems fairly similar?
Fig. 10. Theoretical prediction of the most intense mO–H band component from the low-temperature spectra of the 4-hydroxybenzaldehyde (4-HBA) crystals. (I) The ‘‘minus” dimeric band, (II) the plus band, and (III) the superposition of the ‘‘minus” and ‘‘plus” bands taken with the integral intensities proportional to their statistical weight parameters, F and F+. The coupling parameter values: bD = 0.3, C0 = 0.6, C1 = 0.2, F+ = 1.0, F = 0.1, XO O = 100 cm1. The corresponding experimental spectrum is shown in inset.
We will prove that the fair similarity of the mO–H band shapes from the spectra of 3-HBA and 4-HBA crystals is not incidental. These spectral properties result from general regularities derived from symmetry rules governing the IR spectra generation mechanisms in the two hydrogen-bonded crystals. For this purpose the problem of the geometry influence on to the hydrogen bond IR spectra should be analyzed for two different model dimeric systems of hydrogen bonds. These two model dimers are characterized by the ‘‘side-to-side”-type of the hydrogen bond arrangement but they differ in their point symmetry groups. The first dimer is of the Ci symmetry and the other is described by the C2v symmetry as shown in Fig. 11. For each model dimeric system the energy relations corresponding to the electric transition moment directions ‘‘d” for the proton stretching vibrations in the dimer should be analyzed for the normal vibrations belonging to the different irreducible representations of the dimer point groups.
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Fig. 11. The two model hydrogen-bond dimers of different-symmetry: (a) the Ci symmetry; (b) the C2v symmetry.
of the A1 symmetry is related with the vibrational excited state of a higher energy than the energy value corresponding to the excited state of the B1 vibration. The dipole–dipole interaction model, as shown in Table 2, predicted such energy relation governing the vibrational transition moment dipoles for the dimer. In this case the vibrational transition to the A1 excited state generates the most intense higher-frequency branch of the mO–H band in the IR spectra of the dimeric system. In the limits of the ‘‘strongcoupling” theory this spectral branch can be reproduced by the socalled ‘‘plus” band [6,7,19]. Vibrational transition to the B1 state, formally forbidden by the symmetry rules for the ideally parallel arrangement of the hydrogen bonds in the system, can be activated when the dimer geometry deviates from the parallel arrangement of hydrogen bonds. The forbidden transition in IR can also be promoted by the vibronic mechanism [39]. In terms of the ‘‘strong-coupling” theory this band contour can be quantitatively reconstituted by the so-called ‘‘minus” theoretical band [6,7,17].
7.1. The centrosymmetric dimer case The dipole–dipole-type-coupling model for the description of the interaction between the transition moment vector can explain the sequence of the energy levels of the excited state of the proton vibrations in the dimer. In the Coulomb force approximation, for the dimer of the Ci symmetry the anti-parallel arrangement of the transition moment vectors ‘‘d” in the dimer, occurring to the Agstate, is of lower energy than with the energy corresponding to the parallel arrangement of the vectors to the Au-state. This is shown in Table 1. The transition in IR occurring to the Ag-state in centrosymmetric hydrogen-bond dimers, which is formally forbidden by the symmetry rules, can be activated by the vibronic mechanism [39]. In this way the weaker, lower-frequency mO–H band branch is generated. In terms of the ‘‘strong-coupling” model this transition spectrum can be reproduced by the so-called ‘‘minus” theoretical band [6,7,17]. The symmetry-allowed transition occurring to the Au-state generates the higher-frequency mO–H band branch in the dimer IR spectra. In terms of the ‘‘strong-coupling” theory this latter transition spectrum can be reproduced by the so-called ‘‘plus” band [6,7,17].
7.3. The reason of the spectra similarity For the two compared dimeric system spectra the two lowerfrequency mO–H band branches exhibit the characteristic intensity distribution patterns expressed by the existence in the band contours of a well-developed regular vibrational progression of lowfrequency. In each case this spectral branch can be quantitatively reproduced by the so-called ‘‘minus” band from the ‘‘strong-coupling” theory. In turn, in each dimeric system case the higher-frequency mO–H band branch, with a diffused band contour, can be quantitatively reproduced by the so-called ‘‘plus” band from the ‘‘strong-coupling” theory [6,7,17]. The ‘‘plus” band reproduces the higher-frequency branch of the spectrum of the Ci -symmetry dimer for the Au-type proton vibrations and also reproduces the higher-frequency branch of the of the A1 vibration spectrum for the C2v symmetry dimer. In terms of the ‘‘strong-coupling” theory, this is the result of the identical sign of the exciton interaction energy parameter C0 governing the generation mechanism in the two compared system spectra. In each case this branch of the mO–H band is quantitatively reproduced by the theoretically derived ‘‘plus” band which corresponds with the symmetry-allowed transition in each model dimer. It also
7.2. The C2v-symmetry dimer case For the model dimer of the C2v symmetry, characterized by a quasi-parallel arrangement of hydrogen bonds, proton vibration Table 1 The arrangement of the vibrational transition moment vectors for the O–H bond stretching vibrations in the model centrosymmetric hydrogen-bond dimeric system. Normal vibration form
Normal vibration symmetry
Arrangement of the transition moment vectors
Table 2 The arrangement of the vibrational transition moment vectors for the O–H bond stretching vibrations in the model hydrogen-bond dimeric system of the C2v symmetry. Normal vibration form
Normal vibration symmetry
Arrangement of the transition moment vectors
A1
Ag
B1 Au
The symbols dA and dB denote the transition moment vectors. The ‘‘+” and ‘‘” symbols denote the electronic charges on the ends of the transition moment dipoles.
The symbols dA and dB denote the transition moment vectors. The ‘‘+” and ‘‘” symbols denote the electronic charges on the ends of the transition moment dipoles.
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corresponds with the positive value of the ‘‘C0” parameter from the ‘‘strong-coupling” model [6,7,17]. Therefore, the two compared band contours are similar one to the other. The discussed properties of the lower-frequency branches of the two compared spectra can be explained in the same way. The ‘‘minus” band can reproduce the lower-frequency branch of each dimer spectrum of a unique intensity distribution pattern, in each case corresponding to the quasi-forbidden transition. It can be done on assuming the negative ‘‘C0” parameter value in the model calculations [6,7,17]. Identical results can be obtained on the basis of the ‘‘relaxation” theory [9,10]. The similarity of the compared spectra of the two different crystalline systems also finds its another explanation. In the interaction between the electric field vector ‘‘E” and the oscillating transition moment dipoles during the act of the optical transition, the ‘‘E” vector can only ‘‘see” the mutual arrangement of the transition moment vectors, ‘‘dA” and ‘‘dB”, in the two model hydrogen-bond dimers. In these two cases this arrangement is approximately similar, i.e., quasi-parallel or quasi-anti-parallel, in consequence determining the two-branch mO–H band structures of a similar intensity distribution pattern. The mutual relative arrangement of the O–H groups in the two hydrogen-bond model dimeric systems plays a secondary role in the spectra generation mechanism.
8. The spectra generation mechanism for crystals In the polarized spectra of 3-HBA and 4-HBA crystals no typical effects of the exciton interactions attributed to crystals characterized by Z = 4 and by the chain arrangement of hydrogen bonds can be identified. Lack of well-pronounced linear dichroic effects in the spectra, characteristic for chain hydrogen bond systems proves that only some selected hydrogen bond pairs from the unit cells are involved in relatively strong vibrational exciton interactions in the crystals and, hence, determining their main spectral properties. Also the H/B isotopic ‘‘self-organization” effects absent in the spectra of isotopically diluted crystals prove that the distribution of protons and deuterons in the hydrogen bond systems is random. The latter statement results from the observed changes of the analyzed band contour shapes in the condition of the increasing H/D replacement ratios. The nature of the polarized spectra of 3-HBA and 4-HBA crystals can be explained by assuming that in both crystals the strongest exciton coupling mechanism involves the hydrogen bonds belonging to two different closely-spaced hydrogen bond chains, penetrating each unit cell, in ‘‘side-to-side”-type interactions. These interactions determine the basic dichroic properties of the crystalline systems. As the consequence of these ‘‘side-to-side”-type exciton interactions in the crystals only slight differences in the dichroic properties of the two opposite branches of each band, mO–H and mO–D, appear in the crystalline spectra which exhibit a fair resemblance to the centrosymmetric dimer-type spectra. The quantitative analysis of the crystalline spectra has proved that hydrogen bond pairs of the ‘‘side-to-side”-type arrangement of the moieties, the anti-parallel and the parallel ones, are the bearers of the crystal spectral properties. These non-conventional interactions appeared to be too weak for grouping together the identical hydrogen isotope atoms, proton or deuterons, in the mutually coupled hydrogen bond pairs in the isotopically diluted crystals. The spectral properties of 3-HBA and 4-HBA crystals fairly resemble the spectral properties of decyl alcohol crystals found in the past, which affected for many years our knowledge about the inter-hydrogen bond interactions in hydrogen-bonded crystals
of mixed H/D isotope contents [21,22]. These properties seem to result from electronic properties of O–H groups in alcohols and phenols. For the interpretation of the main crystal spectral properties we did not consider the effect of the hypothetical Fermi resonance mechanism [41–43] on the spectra. In our opinion this influence, provided it exists, is in this case of secondary importance.
9. Conclusion Our studies have shown that O–H O hydrogen bond pairs (formally treated as dimers), composed of two closely-spaced hydrogen bonds of the ‘‘side-to-side”-type arrangement of the moieties, where each belongs to another chain of the associated molecules, are responsible for generation of the basic spectral properties of 3-HBA and 4-HBA crystals. Such a structural unit is responsible for the intensity distribution pattern in the ‘‘residual” mO–H and mO–D bands and for the linear dichroic effect. This effect is characterized by almost identical polarization properties of the two opposite band branches (the higher- and the lower-frequency one) of each mentioned band. The two-branch structure of the ‘‘residual” mO–H and mO–D bands disappears due to the annihilation of the vibrational exciton interactions involving hydrogen bonds in the isotopically diluted crystals with a random distribution of protons an deuterons in their lattices. These band spectral properties provide evidence for ineffective H/D isotopic ‘‘self-organization” processes resulting from weak dynamical co-operative interactions in the hydrogen bond lattices of 3-HBA and 4-HBA crystals.
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Effects of dynamical couplings in hydrogen bond systems in the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals Henryk T. Flakus *, Barbara Hachuła Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl-40-006 Katowice, Poland
a r t i c l e
i n f o
Article history: Received 9 November 2009 In final form 27 January 2010 Available online 1 February 2010 Keywords: Hydrogen bond Molecular crystals Polarized IR spectra H/D isotopic effects Linear dichroic effects Temperature effects Isotopic dilution Dynamical co-operative interactions
a b s t r a c t This paper presents the investigation results of the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals measured at 293 and 77 K. Analysis of the results concerned the linear dichroic, H/D isotopic and temperature effects observed in the spectra of the hydrogen and deuterium bond at the frequency ranges of the mO–H and the mO–D bands, respectively. The main spectral properties of the crystals were interpreted in terms of the ‘‘strong-coupling” theory on the basis of the hydrogen bond dimer model. The spectra revealed that the strongest vibrational exciton coupling involved the closelyspaced hydrogen bonds, each belonging to a different chain of associated molecules. The reason for two different crystalline systems, are characterized by almost identical mO–H and mO–D band shapes, is explained. It was proved that a random distribution of the protons and deuterons took place in the lattices of the isotopically diluted crystals. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Over the last six decades IR spectroscopy has been considered to be a powerful tool in investigating hydrogen bonds. The spectral studies mainly concern the middle frequency range of IR where the mX–H bands, ascribed to the proton stretching vibrations in the X–H Y hydrogen bonds, appear [1–5]. Complex fine structure patterns of the mX–H bands are highly susceptible to influences exerted by diverse interaction mechanisms such as the intra- and inter-hydrogen bond ones [1–5]. According to the contemporary quantitative theories of IR spectra of hydrogen bonded systems these bands are treated as an abundant source of information about the complex dynamics of hydrogen bonds. In terms of these theories, strong anharmonic couplings involving motions of different forms and of different energies are considered to be responsible for the generation of the main spectral properties of hydrogen bond systems. To understand the spectral properties of the hydrogen bond systems the socalled ‘‘strong-coupling” [6–8] and the ‘‘relaxation” theory [9,10] played an important role. These theoretical models allowed us to succeed in the quantitative interpretation of the IR spectra of the hydrogen bond in simple molecular aggregates like dimers [7,11– 13] as well as in hydrogen-bonded molecular crystals [14–16].
* Corresponding author. Fax: +48 32 2599 978. E-mail address: fl[email protected] (H.T. Flakus). 0301-0104/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2010.01.024
The H/D isotopic effects obtained due to IR spectroscopy have been used to verify the validity of theoretical models subsequently introduced for the quantitative interpretation of vibrational spectra of hydrogen bond systems [6–10]. From earlier spectral studies of hydrogen bond dimers it resulted that mechanisms non-revealed yet might also co-decide about the generation of the system spectra [17,18]. It seemed that IR spectroscopy in polarized light, applied for investigation of crystalline spectra should provide essential data in this matter. Polarized IR spectra of molecular crystals measured for spatially oriented lattices of hydrogen bonds seem to be the source of the a complete data system concerning the inter- as well as the intra-hydrogen bond interactions in the systems. However, the solid-state introduces its own new effects considerably complicating the crystal spectra interpretation, related to the inter-hydrogen bond couplings in the excited vibrational state. Overcoming these interpretation problems may allow us to extend our knowledge about the coupling mechanisms, involving motions of diverse forms in hydrogen bond systems. Recent studies of polarized IR spectra of the hydrogen bond in diverse crystalline systems have exhibited a rich diversity of their spectral properties and have allowed us to reveal a number of new, non-conventional effects in the spectra. These new effects were found when model calculations, mainly performed in terms of the so-called ‘‘strong-coupling” theory [6–8], were applied to the quantitative interpretation of the crystalline spectra. Among these effects are the so-called H/D isotopic ‘‘ self-organization” effects, identified in the polarized IR spectra of isotopically diluted
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hydrogen-bonded molecular crystals [19,20]. They result from a non-random distribution of protons and deuterons in the hydrogen bridge systems in lattices of isotopically diluted molecular crystals. It looks as if some non-identified ‘‘attraction” forces appear, which involve identical hydrogen isotope atoms in coupled hydrogen bond systems. Therefore, the invariability of the mX–H proton stretching vibration bands, registered independently of the increasing concentrations of deuterons in a crystalline sample could be observed in the spectra of isotopically diluted crystalline samples. The latter effect is the result of the newly revealed mechanisms governing the inter-hydrogen bond co-operative interaction in molecular crystals, i.e., the so-called dynamical cooperation interaction mechanisms [19,20]. Vibronic coupling in hydrogen bonded molecular systems is considered to be the most probable source of these interactions [19,20]. In the spectral studies of H/D isotopic effects for hydrogen bonded systems carried out so far the problem of the influence of the isotopic dilution on to the IR spectra was treated as rather marginal. This was the result of the studies performed in the past, which significantly influenced later investigations in this area [21–23]. Detailed studies of the dynamical co-operative interaction mechanisms performed for diverse solid-state systems have proved that the H/D isotopic ‘‘self-organization” processes did not occur in one precisely defined way. This remark mainly concerns a particular group of hydrogen-bonded molecular crystals in whose lattices hydrogen bonds form infinitely long chains. It appeared that in relation to the electronic structure of the associating molecules, the H/D isotopic ‘‘self-organization” processes, provided they take place, might proceed in two different ways. In the first one they may involve the adjacent hydrogen bonds in each individual chain. In the other case, pairs of closely-spaced hydrogen bonds participate, where each moiety in an individual pair belongs to another chain. The first group of crystals comprises crystals of pyrazole [24], imidazole [25] and 4-thiopyridone [26]. These molecules contain large delocalized ‘‘p”-electronic systems. Surprisingly, also formic acid crystals belong to this group [27]. This way of the occurrence of the discussed processes is evidenced by the fact that linear dichroic effects are retained. This in turn depends on the differentiation of the dichroic properties of the two opposite branches of the mX–H ‘‘residual” bands in the polarized spectra of isotopically diluted crystals. Also the intensity distribution pattern of these bands, characteristic of linear hydrogen bond dimers, supports this conclusion. The second group comprises crystals of N-methylthioacetamide [28], N-methylacetamide [29] and crystals of acetic acid [30]. For these latter systems the distribution of protons and deuterons in the hydrogen bridge chains appeared to be fully random. This was deduced from the disappearance of the Davydow-splitting effects [31,32] in the mX–H ‘‘residual” bands in the spectra of the isotopically diluted crystals, attributed to the in-chain exciton couplings. Also the fine structure intensity pattern of these bands, similar to the one observed for cyclic hydrogen bond dimers appears in these spectra. In this case no essential differences in the dichroic properties between the opposite mX–H ‘‘residual” band branches can be observed [28,29]. This fact supports the hypothesis about the inter-chain H/D isotopic ‘‘self-organization” in this group of crystals. In this article, we present the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals. Aiming at their quantitative interpretation in the initial theoretical approach it seems possible that the process proceeds in two different ways. The lattices of both these crystals are composed with infinite hydrogen bond chains, differing however, from one another in their space-symmetry groups and in the mutual hydrogen bond arrangement. Therefore, these systems seemed to pose interesting
objects for the investigation of the H/D isotopic ‘‘self-organization” process mechanisms in relation to the electronic structure of the associated molecules and the space-symmetry of the crystals. One may expect that the substituent atomic groups, O–H and CHO, linked to the benzene rings in two different positions, 1,3and 1,4-, can significantly influence the electronic properties of O–H O hydrogen bonds in these crystalline systems. In this way they may affect the magnitude of the dynamical co-operative interaction energies since these mechanisms are basically of a vibronic nature [19,20]. Therefore, one can expect that the H/D isotopic ‘‘ self-organization” processes occur in two different ways for these two different isomeric system crystals. Similar electronic properties of the two akin molecular systems may facilitate a fully quantitative interpretation of the crystal spectra and therefore, might allow for a deeper insight into the very nature of the H/D isotopic ‘‘ self-organization” processes in hydrogen bond systems [19,20]. 2. X-ray structures of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde Crystals of 3-hydroxybenzaldehyde (3-HBA) belong to the orthorhombic system. Space-symmetry group is Pna21 ¼ C 92v . The Factor-group is C2v. Melting point is 100–103 °C. A unit cell contains four molecules (Z = 4). The crystal lattice is composed of infinitely long chains of associated 3-HBA molecules of a ‘‘zig-zag” form, linked by O–H O hydrogen bonds. These molecular chains elongate along the ‘‘c” axis direction. The lattice constants are: a = 18.858(7) Å, b = 3.864(1) Å, c = 8.190(7) Å and a = b = c = 90° [33]. A view of the 3-HBA crystal lattice is shown in Fig. 1. Crystals of 4-hydroxybenzaldehyde (4-HBA) are monoclinic. The space-symmetry group is P21 =c ¼ C 52h and Z = 4. The crystal Factor-group is C2h. The unit cell parameters determined at 295 K are: a = 6.453(5) Å, b = 13.810(8) Å, c = 7.044(6) Å, a = c = 90°, b = 107.94(9)°. 4-HBA molecules are linked by O–H O hydrogen bonds forming infinitely long ‘‘zig -zag”-type chains elongating along the ‘‘b” axis [34]. Fig. 2 presents the arrangement of the hydrogen-bonded 4-HBA molecules in the lattice of 4-HBA crystal. 3. Experimental 3-Hydroxybenzaldehyde (3-HBA) and 4-hydroxybenzaldehyde (4-HBA) (the general chemical formula HO–C6H4–CHO), used for our studies were the commercial substances (Sigma–Aldrich). The samples were used without their further purification.
Fig. 1. View of a unit cell in the crystal lattice of 3-hydroxybenzaldehyde. A pair of the closely-spaced O–H O hydrogen bonds governed by the C2v site-symmetry group is marked by an ellipse.
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Fig. 2. Unit cell in the lattice of 4-hydroxybenzaldehyde crystal. A centrosymmetric pair of the O–H O hydrogen bonds (Ci site-symmetry) is marked by an ellipse.
The single crystals of 3-HBA and 4-HBA and crystals of their deuterium isotopomers were obtained by crystallization from melt, occurring between two closely spaced CaF2 windows. In this way we prepared thin enough crystals, characterized by their maximum absorbance close to 0.5 at the mO–H band frequency
range. Next, suitable monocrystalline fragments from the crystalline mosaic were selected and then spatially oriented, using a polarization microscope. These selected single crystals were exposed for the experiment by placing them on a metal plate diaphragm with a 1.5 mm diameter hole. The two different forms of
Fig. 3. The mO–H bands in the IR spectra of the polycrystalline samples of 3-hydroxybenzaldehyde (3-HBA) and 4-hydroxybenzaldehyde (4-HBA) dispersed in KBr pellets, measured at 293 and 77 K. (a) 3-HBA. (b) 4-HBA. The Raman spectrum of the polycrystalline samples is also plotted in order to indicate the influence of the mC–H bands on the spectra.
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3-HBA crystals developed either the ‘‘bc” or ‘‘ac” crystalline faces, whereas two crystalline forms of 4-HBA developed the ‘‘bc” or ‘‘ab” planes of the lattice. In a similar way measurements of spectra were performed for 3-HBA and 4-HBA deuterium derivative crystals, which were obtained by evaporation of the compound solution in D2O solution at room temperature and under reduced pressure. The deuterium substitution rate for different crystals investigated varied in a relatively wide range (from 20% to 80%). The IR spectra were recorded with the FT-IR Nicolet Magna 560 spectrometer by transmission method with 2 cm1 resolution. Measurements of the spectra were performed in the temperature range from 293 K, to the temperature of liquid nitrogen for two different orientations of the electric field vector ‘E’. For 3-HBA crystals with the ‘‘bc” face developed the spectra were recorded for the ‘‘E” vector parallel to the ‘‘b” axis of the lattice, and in the other case, for the one perpendicular to it, i.e., parallel to the ‘‘c” identity period. For the other crystal form of 3-HBA developing the ‘‘ac” plane, the spectra were measured for the ‘‘E” vector parallel to the ‘‘c” axis and then for the one perpendicular to it, i.e., parallel to the ‘‘a” identity period. In the case when 4-HBA crystals developed the ‘‘bc” face, the spectra were measured for the ‘‘E” vector parallel to the ‘‘b” axis of the lattice and then for the one perpendicular to it, i.e., parallel
to the ‘‘c” identity period. For the other crystal form, with the ‘‘ab” face developed, the polarized spectra were recorded for the ‘‘E” vector parallel to the ‘‘b” axis of the lattice, and then for the one perpendicular to it, i.e., parallel to the ‘‘a” identity period. For each 3-HBA and 4-HBA case, the measurements were repeated for ca. 10 different single crystals. The Raman spectra of polycrystalline samples of 3-HBA and 4-HBA were measured at room temperature with the use of the Raman Accessory for the Nicolet Magna 560 spectrometer. 4. The results of the spectral studies The IR spectra of polycrystalline samples of 3-HBA and 4-HBA, measured in the frequency range of the proton stretching vibration band mO–H, with the use of the KBr pellet technique at room temperature and the temperature of liquid nitrogen, are shown in Fig. 3. In the same picture the Raman spectrum of polycrystalline samples of these compounds was plotted to facilitate the identification of the C–H bond stretching vibration lines, overlapping the mO–H, band contours. In Fig. 4 spectra of isotopically diluted 3-HBA and 4-HBA polycrystalline samples are given, measured with the use of the KBR pellet technique in the frequency ranges of the proton and the deuteron stretching vibration bands, mO–H and mO–D, respectively.
Fig. 4. The mO–D bands in the IR spectra of partially deuterated polycrystalline samples of KBr pellets, measured at 293 and 77 K. (a) 3-HBA. (b) 4-HBA.
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In the next stage of the study polarized spectra of 3-HBA and 4HBA single crystals were measured for isotopically neat as well as for isotopically diluted samples. The polarized spectra of 3-HBA crystals recorded at 77 K in the frequency range of the mO–H band, for two mutually perpendicular electric vector ‘‘E” orientations, are presented in Fig. 5. Spectra of two crystal forms, with two different ‘‘bc” and ‘‘ac” crystalline faces, were measured. The polarized IR spectra of 4-HBA crystals polarized spectra measured at 77 K in the frequency range of the mO–H band, for two different crystal forms, characterized by two developed planes ‘‘bc” and ‘‘ab”, were measured and are shown in Fig. 6. The low-temperature spectra of the two different forms of isotopically diluted 3-HBA single crystals recorded in the ‘‘residual” mO–H and mO–D band frequency ranges are shown in Fig. 7. In Fig. 8, the low-temperature polarized spectra of the two different forms of isotopically diluted 4-HBA single crystals measured in the ‘‘residual” mO–H and mO–D band frequency ranges are presented.
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5. Discussion 5.1. Linear dichroic effects in the mO–H bands The corresponding spectra of the two substances, of different Xray structures, isotopically neat as well as isotopically diluted, are characterized by fairly similar fine structure patterns of their polarized mO–H and mO–D band contours. Polarized spectra of monocrystalline samples of 3-HBA exhibit the characteristic two-branch intensity distribution patterns of their mO–H bands. The polarized spectra measured for the two different crystalline faces are of qualitatively similar mO–H band shapes but markedly differ in the relative intensities of their spectral branches, i.e., of the shorter- to the longer-wave one. In the case of the ‘‘ac” plane spectra the higher-frequency branch is narrow and of a relatively high intensity, whereas in the spectra of the ‘‘bc” face this spectral branch is significantly wider and also of a relatively higher intensity in relation to the lower-frequency branch properties. These effects may result from vibrational
Fig. 5. Polarized IR spectra of 3-hydroxybenzaldehyde (3-HBA) single crystals, measured by a transmission method at 77 K, in the mO–H and the mC–H band frequency ranges. The IR beam of normal incidence with respect to different crystalline faces ‘‘bc” and ‘‘ac” was applied. In each case the component spectra were obtained for two different orientations of the electric field vector E. (a) ‘‘ac” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘a”). (b) ‘‘bc” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘b”). Spectra (I) and (II) were drawn on a common scale.
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Fig. 6. Polarized IR spectra of 4-hydroxybenzaldehyde (4-HBA) single crystals, measured by a transmission method at 77 K, in the mO–H and the mC–H band frequency ranges. The IR beam of normal incidence with respect to different crystalline faces ‘‘bc” or ‘‘ab” was applied. In each case the component spectra were obtained for two different orientations of the electric field vector E. (a) ‘‘bc” plane: (I) Ek‘‘b”; (II) E\‘‘b”, i.e. (Ek‘‘c”). (b) ‘‘ab” plane: (I) Ek‘‘a”; (II) E\‘‘a”, i.e. (Ek‘‘a”). Spectra (I) and (II) were drawn on a common scale.
exciton interactions involving translationally non-equivalent hydrogen bonds in each unit cell of the crystal lattice (Davydovsplitting effect). In the polarized IR spectra the main linear dichroic effects can be seen connected to the orientation of the electric field vector ‘‘E” of the incident beam of IR radiation with respect to the spatially oriented hydrogen bond chains in the experimental conditions. This is the polarization effect of the ‘‘first kind”. In each case the most intense polarized component of the crystal spectra was recorded on the ‘‘E” vector polarization parallel to the ‘‘c” axis, i.e., parallel to the direction of hydrogen bond chains in the lattices. Considerably weaker polarized components of the crystalline spectra were measured for the ‘‘E” vector polarization perpendicular to the chain directions. For the ‘‘bc” crystal face of 3-HBA it is the ‘‘b” axis direction and for the ‘‘ac” form it is the ‘‘a” axis direction. From comparison of the dichroic properties of the two spectral branches of the mO–H band it results that in the whole band frequency range the direction of the transition moment vector is almost constant. It means that the crystal spectral properties are
not governed by the C2v Factor-group. The difference in the higher-frequency band branch width in the spectra of the two crystal forms proves that vibrational exciton coupling mechanism involves four hydrogen bonds from a unit cell. Therefore, the approach assuming a weak exciton coupling limit [31,32] for the interpretation of the 3-HBA crystal vibrational spectra seems to be the most appropriate. In the case of the 4-HBA crystal qualitatively similar spectral effects can be observed. Also in these spectra the higher-frequency branch of the mO–H band is more intense in comparison with the lower-frequency band branch intensity. The spectra of the two crystal forms markedly differ in the width of their higher-frequency mO–H band branch. Therefore, the exciton interaction mechanism, similar in nature to the one found in 3-HBA crystal spectra, seems to govern the spectral properties of 4-HBA crystal. On comparing the polarized IR spectra of the hydrogen bond in 3-HBA and 4-HBA crystals, measured in the mO–H band frequency range, a fair similarity between them can be noticed. The common property of the compared spectra is also an almost proportional
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Fig. 7. Polarized IR spectra of isotopically diluted 3-hydroxybenzaldehyde (3-HBA) crystals measured at 77 K in the mO–D and the ‘‘residual” mO–H band frequency range. Common scale. The spectra were measured for the normal incidence of the IR beam with respect to different crystallographic planes, respectively, for two polarizations of the electric field vector E. (a) ‘‘ac” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘a”) (ca. 30%H, 70%D). (b) ‘‘bc” plane: (I) Ek‘‘c”; (II) E\‘‘c”, i.e. (Ek‘‘b”) (ca. 15%H, 85%D).
change in their intensity distribution when the polarization of the electric field vector ‘‘E” of the incident beam is changed in the experimental conditions. At this point it seems noteworthy to add that the decrease in temperature from 293 to 77 K does not noticeably change the linear dichroic properties of the analyzed spectra. The linear dichroic effects observed in the 3-HBA and 4-HBA crystal spectra seem to be puzzling enough, since the two crystalline systems, each of a chain hydrogen bond arrangement in its lattice, differ in their space-symmetry groups. In the case of spatially-oriented hydrogen bond chains, the basic dichroic effect in their polarized IR spectra should be strictly connected to the orientation of the ‘‘E” vector with the respect to the chains. In result of the sufficiently strong vibrational exciton coupling, involving hydrogen bonds arranged in the chain of the ‘‘zig-zag” type motif, the polarized spectra obtained for the two orientation of ‘‘E”, i.e., parallel and perpendicular to the chain direction, should strongly differ in their dichroic properties. In this case the higher- and the lower-frequency mO–H band branch in the polarized spectra should be characterized by the mutually perpendicular transition moment
directions. This would be the linear dichroic effect of the ‘‘second kind”. Lack of such effects in the polarized spectra of 3-HBA and 4-HBA crystals requires a convincing explanation. 5.2. Linear dichroic effects in the mO–D bands The mO–D bands in the crystalline spectra of 3-HBA and 4-HBA are characterized by relatively pffiffiffisimple fine structure patterns of frequencies related by the 1= 2 factor with the corresponding frequency values of the spectral branches in the ‘‘residual” mO–H bands. This is the familiar H/D isotopic effect in the area of the IR spectroscopy of the hydrogen bond [1–5]. It can be noticed that the mO–D band in the polarized spectra of 3-HBA crystals measured in the higher-frequency branch frequency range for the ‘‘bc” crystal face is wider than the band of crystals with the ‘‘ac” plane developed. This is most probably the effect of the vibrational exciton interactions on to the spectra, i.e., of the Davydov-splitting effect. In these circumstances the ‘‘residual” mO–H band contours became practically undistinguishable. This means that the ‘‘residual” mO–H bands are devoid of the
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Fig. 8. Polarized IR spectra of isotopically diluted 4-hydroxybenzaldehyde (4-HBA) crystals measured at 77 K in the mO–D and the ‘‘residual” mO–H band frequency range. Common scale. The spectra were measured for the normal incidence of the IR beam with respect to different crystallographic planes, respectively, for two polarizations of the electric field vector E. (a) ‘‘bc” plane: (I) Ek‘‘b”; (II) E\‘‘b”, i.e. (Ek‘‘c”) (ca. 25%H, 75%D). (b) ‘‘ab” plane: (I) Ek‘‘a”; (II) E\‘‘a”, i.e. (Ek‘‘a”) (ca. 30%H, 70%D).
effect of some exciton interactions involving hydrogen bonds, whereas the mO–D bands are visibly influenced by the inter-deuterium bond exciton couplings. Qualitatively very similar spectral effects can be found in the mO–D and the ‘‘residual” mO–H bands in the polarized spectra of 4HBA crystals. Similarly to the mO–H ones, the mO–D bands exhibit almost homogeneous linear dichroic properties in the whole frequency range of the band in the spectra of the two molecular systems. This is expressed by the fair proportionality relation between the intensity distribution of the polarized component bands in the crystalline spectra. 5.3. Isotopic dilution effects in the crystalline spectra On comparing the spectral properties of two forms of isotopically neat crystals of 3-HBA and 4-HBA with the related properties of isotopically diluted crystals, it can be found that the effect of the isotopic dilution in the spectra is relatively strong. It can be noticed that along with the increase in concentration ratio of deuterons, the ‘‘residual” mO–H band contours in the spectra of the two crystal-
line forms of each individual compound, 3-HBA or 4-HBA, become indistinguishable. This is due to the narrowing of the contours of the higher-frequency band branches accompanied by the disappearance of the longer-wave branches of the bands. Consequently, for each compound the spectra of the two crystal forms exhibit identical widths. This effect may be due to the fact that interhydrogen bond vibrational exciton interactions in the lattice accompanying the isotopic dilution vanish. However, even for the highest concentrations of deuterons in the samples, the basic linear dichroic effects characterizing the ‘‘residual” mO–H bands practically remain unchanged in comparison with the corresponding properties of spectra measured for isotopically neat crystals. The observed changes in the mO–H band contour shapes in the polarized spectra due to the isotopic dilution are evidence of the effects of relatively weak dynamical co-operative interactions involving the closely-spaced hydrogen bonds on to the spectra [19,20]. In 3-HBA and 4-HBA crystals non-conventional ‘‘attraction” forces, responsible for holding together identical hydrogen isotope atoms in neighboring hydrogen bonds, if exist, are too weak to generate the H/D isotopic ‘‘self-organization” effects in the spectra. In these two cases the non-conventional dynamical co-operative
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interactions involving hydrogen bonds are probably much weaker than 1.5 kcal/mol of hydrogen bond dimers [18]. Therefore, the distribution of protons and deuterons in the hydrogen bond lattices is practically random [19,20]. 6. Model calculation of the mO–H and mO–D band shapes 6.1. The 4-hydroxybenzaldehyde crystal spectra Our quantitative analysis of the crystalline spectra started with calculations of the mO–H and mO–D band shapes for 4-HBA. Model calculations, aiming at the quantitative reconstitution of the main properties of the bands from the polarized IR spectra of 4-HBA single crystals were performed in terms of the ‘‘strong-coupling” theory [6,7,17]. The simplest hydrogen bond aggregate, exhibiting the crystal spectral properties discussed above, is the dimeric system of hydrogen bonds (see Fig. 1). It allows to retain the complex structure of the analyzed bands in the isotopic dilution. Therefore, suitable model calculations of the band contour shapes ought to be performed by assuming a hydrogen-bond dimer model. Within the ‘‘strong-coupling” theory the mO–H band shape for a dimer, composed of two O–H O hydrogen bonds, depends on the following system of coupling parameters: (i) on the distortion parameter ‘‘bH”, and on (ii) the resonance interaction parameters ‘‘C0” and ‘‘C1” [6,7,17]. Each parameter has a precisely defined physical meaning. The ‘‘bH” parameter describes the change in the equilibrium geometry for the low-energy hydrogen bond stretching vibrations, accompanied by the excitation of the highfrequency proton stretching vibrations mO–H. The ‘‘C0” and ‘‘C1” parameters are responsible for the mutual interactions between the hydrogen bonds in a dimer in its vibrationally excited state. They denote subsequent expansion coefficients in the series, on developing the resonance interaction integral ‘‘C”, with respect to the normal coordinates of the low-frequency, hydrogen bond stretching vibrations mO O:
C ¼ C0 þ C1Q 1; where Q1 represents the totally symmetric normal coordinate for the low-frequency stretching vibration in a dimer. These parameter values remain in close relation to the intensity distribution in the dimeric mO–H band: that is ‘‘bH” and ‘‘C1” parameters are directly connected with the dimeric mO–H bandwidth. The ‘‘C0” parameter determines the splitting of the component bands of the dimeric spectrum, corresponding to the excitation of the proton vibrational motions of different-symmetry [6,7,17]. In its original version, the ‘‘strong-coupling” model predicts diminution of the distortion parameter value for deuterium bond systems, according to the relation:
bH ¼
pffiffiffi 2bD :
For the ‘‘C0” and ‘‘C1” resonance interaction parameters, the simplest version of the theory predicts pffiffiffi the H/D isotopic effect expressed by diminution by 1.0 to 2 times of the parameter values. The model calculations performed with the use of this relatively simple model will allow us to understand better the generation mechanism of the crystalline spectra. In particular owing to them the individual hydrogen bond pairs in each unit cell, involved in the strongest vibrational exciton interactions, can be identified. These exciton-coupling energies affect most strongly the IR spectra of the hydrogen or the deuterium bond in 4-HBA single crystals and determine their basic properties. 6.1.1. Linear dimer approximation The first-step analysis of the geometry relations in the lattice of the 4-HBA crystal, basing on its X-ray structure geometry relations
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might suggest that the basic structural unit of the lattice, responsible for the generation of the spectra is the linear hydrogen-bond dimer. In this dimer the two moieties are linked together in the ‘‘ tail-to-head” order. In this case the mO–H and the mO–D bands should be considered to be a superposition of two component bands, each of different origin. On the basis of the ‘‘strong-coupling” model [6,7,17] for such a linear dimer the higher-frequency branch of each band corresponds with the non-totally symmetric (out-ofphase) proton (or deuteron) stretching vibrations, whereas the lower-frequency branch is connected with the totally symmetric (in-phase) vibrations. The intense, lower-frequency band branch is generated by the dipole-allowed transition and the lower-intensity, higher-frequency branch corresponds to the forbidden transition for the ideally axial symmetry of the dimer. Its intensity increases along the deviation of the dimer geometry from the ideal axial symmetry. For this model dimeric system the two vibrational transitions participating in the generation of the mO–H and mO–D bands should strongly differ in their polarization properties. Moreover, their individual transition moment vectors should be mutually perpendicular. Therefore as can be seen the 4-HBA crystal polarized spectra do not satisfy these requirements and therefore the linear dimer model cannot be treated as the potential source of the main spectral properties of the crystal. This model seems to be inadequate for the quantitative reconstitution of the band fine structure patterns. It also does not explain whether the linear dichroic properties of 4-HBA crystalline spectra as the ‘‘residual” mO–H and mO–D bands exhibit constant polarization properties in their whole band frequency ranges. However, from our studies it results that the polarization properties and the mO–H and mO–D band shapes in the 4-HBA crystal spectra resemble the corresponding properties of typical centrosymmetric hydrogen bond cyclic dimers found in diverse molecular crystals, e.g., in carboxylic acid crystals [35–38]. This fact suggests that another model hydrogen bond aggregate should be accepted as a reliable bearer of the basic spectral properties of the crystal. 6.1.2. Dimer of a ‘‘side-to-side”-type, anti-parallel arrangement of hydrogen bonds In the next step the centrosymmetric dimer model composed of two hydrogen bonds of the ‘‘side-to-side”-type, with anti-parallel arrangement of the moieties, is responsible for the generation of the main spectral properties of 4-HBA crystals. In this dimer each hydrogen bond belongs to a different chain of associated molecules passing the crystal unit cell. Also for this model system each analyzed band, mO–H and mO–D one, is treated as a superposition of two component bands, each of different origin. The higher-frequency branch of each band, corresponding to the Au excited state, is generated by the excitation of the non-totally symmetric proton (or deuteron) stretching vibrations in the centrosymmetric dimers. In turn, the lower-frequency band branch corresponds to the totally symmetric vibrations. This latter transition should be forbidden by the symmetry rules, since it occurs at the Ag excited state of the dimer. It is activated by a vibronic coupling mechanism involving proton stretching vibrations and the electronic motions in the dimer hydrogen bonds [39]. The forbidden transition promotion mechanism is a kind of reverse of the familiar Herzberg–Teller mechanism [40] originating from the theory of the UV spectra of aromatic hydrocarbons, ‘‘borrowing” its intensity from the allowed transition. Therefore, the two component transition bands are characterized by identical dichroic properties [39]. The problem of the forbidden transition activation in IR spectra of centrosymmetric hydrogen bond systems has been intensively studied for diverse crystalline system spectra. It was experimentally evidenced that the forbidden transition activation rate
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depends in a predictable way on the electronic properties of the associating molecules [37–39]. This fact remains in agreement with the predictions derived from the vibronic model [39]. In our calculations the two dimeric component subbands, denoted by the ‘‘minus” and ‘‘plus” symbols, contributed to the mN–H band generation mechanism with their appropriate statistical
weights given by the F and F+ parameter values, respectively. The so-called ‘‘plus” band mainly reproduces the higher-frequency band branch attributed the allowed transition, whereas the ‘‘minus” band reproduces the lower-frequency branch of the band connected to the forbidden transition. In Figs. 9 and 10, the results of model calculations aiming at the theoretical reconstitution of the mO–H and mO–D band contour shapes from the crystalline spectra of 4-HBA crystals, performed on using the ‘‘strong-coupling” model, are shown. The mO–H band contour was reproduced for the following coupling parameter values: bH = 1.6, C0 = 1.5, C1 = 0.4, F+ = 1.0, F = 0.5, XO O = 100 cm1, where the XO O parameter denotes the O O hydrogen bond stretching frequency. For the quantitative simulation of the mN-D band contour for the spectra of solid-state samples of 4-HBA the following coupling parameter values were used: bD = 0.3, C0 = 0.6, C1 = 0.2, F+ = 1.0, F = 0.1, XO O = 100 cm1. Our calculations managed to reproduce the observation that the higher-frequency branch is of higher intensity and diffused, whereas the lower-frequency branch is considerably weaker. It also exhibits a well-developed fine structure pattern, qualitatively resembling a typical Franck–Condon-type progression of low-frequency. The two component bands in the polarized spectra, measured for the ‘‘residual” bands should be characterized by identical transition moment directions since they ‘‘borrow” its intensity from the allowed vibrational transition in centrosymmetric hydrogen-bond dimers [39]. 6.2. The 3-hydroxybenzaldehyde crystal spectra
Fig. 9. Theoretical reconstitution of the most intense mO–H band component from the low-temperature spectra of 4-hydroxybenzaldehyde (4-HBA) crystals. (I) The ‘‘minus” dimeric band reconstituting the lower-frequency branch of the mO–H band, (II) the ‘‘plus” dimeric band reproducing the higher-frequency branch, and (III) the superposition of the ‘‘minus” and ‘‘plus” bands contributing proportionally to their respective statistical weight parameters F and F+. The coupling parameter values: bH = 1.6, C0 = 1.5, C1 = 0.4, F+ = 1.0, F = 0.5, XO O = 100 cm1. The corresponding experimental spectrum is given in inset.
The spectra of the 3-HBA crystals, with properties similar to the spectra of 4-HBA crystals, can also be quantitatively reproduced in terms of the ‘‘strong-coupling” theory [6,7,17] in the approximation of the hydrogen-bond dimer model. This fact remains in contrast to the quasi-parallel arrangement of the moieties in the model hydrogen bond pair. In this case the crystal lattice is also characterized by the ‘‘side-to-side”-type arrangement of the moieties of the C2v symmetry (see Fig. 2). This is the property which differs 3-HBA crystals from the 4-HBA ones. The coupling parameter value systems used for the quantitative interpretation of the mO–H and mO–D band contour shapes appeared to be almost identical with those reproducing the spectra of 4-HBA crystals. However, for the C2vsymmetry of the model dimer the two component transitions forming the band contours should differ in their polarization properties and in the decline of the hydrogen bond arrangement from the ideally parallel one. 7. Why are the spectra of both crystalline systems fairly similar?
Fig. 10. Theoretical prediction of the most intense mO–H band component from the low-temperature spectra of the 4-hydroxybenzaldehyde (4-HBA) crystals. (I) The ‘‘minus” dimeric band, (II) the plus band, and (III) the superposition of the ‘‘minus” and ‘‘plus” bands taken with the integral intensities proportional to their statistical weight parameters, F and F+. The coupling parameter values: bD = 0.3, C0 = 0.6, C1 = 0.2, F+ = 1.0, F = 0.1, XO O = 100 cm1. The corresponding experimental spectrum is shown in inset.
We will prove that the fair similarity of the mO–H band shapes from the spectra of 3-HBA and 4-HBA crystals is not incidental. These spectral properties result from general regularities derived from symmetry rules governing the IR spectra generation mechanisms in the two hydrogen-bonded crystals. For this purpose the problem of the geometry influence on to the hydrogen bond IR spectra should be analyzed for two different model dimeric systems of hydrogen bonds. These two model dimers are characterized by the ‘‘side-to-side”-type of the hydrogen bond arrangement but they differ in their point symmetry groups. The first dimer is of the Ci symmetry and the other is described by the C2v symmetry as shown in Fig. 11. For each model dimeric system the energy relations corresponding to the electric transition moment directions ‘‘d” for the proton stretching vibrations in the dimer should be analyzed for the normal vibrations belonging to the different irreducible representations of the dimer point groups.
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Fig. 11. The two model hydrogen-bond dimers of different-symmetry: (a) the Ci symmetry; (b) the C2v symmetry.
of the A1 symmetry is related with the vibrational excited state of a higher energy than the energy value corresponding to the excited state of the B1 vibration. The dipole–dipole interaction model, as shown in Table 2, predicted such energy relation governing the vibrational transition moment dipoles for the dimer. In this case the vibrational transition to the A1 excited state generates the most intense higher-frequency branch of the mO–H band in the IR spectra of the dimeric system. In the limits of the ‘‘strongcoupling” theory this spectral branch can be reproduced by the socalled ‘‘plus” band [6,7,19]. Vibrational transition to the B1 state, formally forbidden by the symmetry rules for the ideally parallel arrangement of the hydrogen bonds in the system, can be activated when the dimer geometry deviates from the parallel arrangement of hydrogen bonds. The forbidden transition in IR can also be promoted by the vibronic mechanism [39]. In terms of the ‘‘strong-coupling” theory this band contour can be quantitatively reconstituted by the so-called ‘‘minus” theoretical band [6,7,17].
7.1. The centrosymmetric dimer case The dipole–dipole-type-coupling model for the description of the interaction between the transition moment vector can explain the sequence of the energy levels of the excited state of the proton vibrations in the dimer. In the Coulomb force approximation, for the dimer of the Ci symmetry the anti-parallel arrangement of the transition moment vectors ‘‘d” in the dimer, occurring to the Agstate, is of lower energy than with the energy corresponding to the parallel arrangement of the vectors to the Au-state. This is shown in Table 1. The transition in IR occurring to the Ag-state in centrosymmetric hydrogen-bond dimers, which is formally forbidden by the symmetry rules, can be activated by the vibronic mechanism [39]. In this way the weaker, lower-frequency mO–H band branch is generated. In terms of the ‘‘strong-coupling” model this transition spectrum can be reproduced by the so-called ‘‘minus” theoretical band [6,7,17]. The symmetry-allowed transition occurring to the Au-state generates the higher-frequency mO–H band branch in the dimer IR spectra. In terms of the ‘‘strong-coupling” theory this latter transition spectrum can be reproduced by the so-called ‘‘plus” band [6,7,17].
7.3. The reason of the spectra similarity For the two compared dimeric system spectra the two lowerfrequency mO–H band branches exhibit the characteristic intensity distribution patterns expressed by the existence in the band contours of a well-developed regular vibrational progression of lowfrequency. In each case this spectral branch can be quantitatively reproduced by the so-called ‘‘minus” band from the ‘‘strong-coupling” theory. In turn, in each dimeric system case the higher-frequency mO–H band branch, with a diffused band contour, can be quantitatively reproduced by the so-called ‘‘plus” band from the ‘‘strong-coupling” theory [6,7,17]. The ‘‘plus” band reproduces the higher-frequency branch of the spectrum of the Ci -symmetry dimer for the Au-type proton vibrations and also reproduces the higher-frequency branch of the of the A1 vibration spectrum for the C2v symmetry dimer. In terms of the ‘‘strong-coupling” theory, this is the result of the identical sign of the exciton interaction energy parameter C0 governing the generation mechanism in the two compared system spectra. In each case this branch of the mO–H band is quantitatively reproduced by the theoretically derived ‘‘plus” band which corresponds with the symmetry-allowed transition in each model dimer. It also
7.2. The C2v-symmetry dimer case For the model dimer of the C2v symmetry, characterized by a quasi-parallel arrangement of hydrogen bonds, proton vibration Table 1 The arrangement of the vibrational transition moment vectors for the O–H bond stretching vibrations in the model centrosymmetric hydrogen-bond dimeric system. Normal vibration form
Normal vibration symmetry
Arrangement of the transition moment vectors
Table 2 The arrangement of the vibrational transition moment vectors for the O–H bond stretching vibrations in the model hydrogen-bond dimeric system of the C2v symmetry. Normal vibration form
Normal vibration symmetry
Arrangement of the transition moment vectors
A1
Ag
B1 Au
The symbols dA and dB denote the transition moment vectors. The ‘‘+” and ‘‘” symbols denote the electronic charges on the ends of the transition moment dipoles.
The symbols dA and dB denote the transition moment vectors. The ‘‘+” and ‘‘” symbols denote the electronic charges on the ends of the transition moment dipoles.
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corresponds with the positive value of the ‘‘C0” parameter from the ‘‘strong-coupling” model [6,7,17]. Therefore, the two compared band contours are similar one to the other. The discussed properties of the lower-frequency branches of the two compared spectra can be explained in the same way. The ‘‘minus” band can reproduce the lower-frequency branch of each dimer spectrum of a unique intensity distribution pattern, in each case corresponding to the quasi-forbidden transition. It can be done on assuming the negative ‘‘C0” parameter value in the model calculations [6,7,17]. Identical results can be obtained on the basis of the ‘‘relaxation” theory [9,10]. The similarity of the compared spectra of the two different crystalline systems also finds its another explanation. In the interaction between the electric field vector ‘‘E” and the oscillating transition moment dipoles during the act of the optical transition, the ‘‘E” vector can only ‘‘see” the mutual arrangement of the transition moment vectors, ‘‘dA” and ‘‘dB”, in the two model hydrogen-bond dimers. In these two cases this arrangement is approximately similar, i.e., quasi-parallel or quasi-anti-parallel, in consequence determining the two-branch mO–H band structures of a similar intensity distribution pattern. The mutual relative arrangement of the O–H groups in the two hydrogen-bond model dimeric systems plays a secondary role in the spectra generation mechanism.
8. The spectra generation mechanism for crystals In the polarized spectra of 3-HBA and 4-HBA crystals no typical effects of the exciton interactions attributed to crystals characterized by Z = 4 and by the chain arrangement of hydrogen bonds can be identified. Lack of well-pronounced linear dichroic effects in the spectra, characteristic for chain hydrogen bond systems proves that only some selected hydrogen bond pairs from the unit cells are involved in relatively strong vibrational exciton interactions in the crystals and, hence, determining their main spectral properties. Also the H/B isotopic ‘‘self-organization” effects absent in the spectra of isotopically diluted crystals prove that the distribution of protons and deuterons in the hydrogen bond systems is random. The latter statement results from the observed changes of the analyzed band contour shapes in the condition of the increasing H/D replacement ratios. The nature of the polarized spectra of 3-HBA and 4-HBA crystals can be explained by assuming that in both crystals the strongest exciton coupling mechanism involves the hydrogen bonds belonging to two different closely-spaced hydrogen bond chains, penetrating each unit cell, in ‘‘side-to-side”-type interactions. These interactions determine the basic dichroic properties of the crystalline systems. As the consequence of these ‘‘side-to-side”-type exciton interactions in the crystals only slight differences in the dichroic properties of the two opposite branches of each band, mO–H and mO–D, appear in the crystalline spectra which exhibit a fair resemblance to the centrosymmetric dimer-type spectra. The quantitative analysis of the crystalline spectra has proved that hydrogen bond pairs of the ‘‘side-to-side”-type arrangement of the moieties, the anti-parallel and the parallel ones, are the bearers of the crystal spectral properties. These non-conventional interactions appeared to be too weak for grouping together the identical hydrogen isotope atoms, proton or deuterons, in the mutually coupled hydrogen bond pairs in the isotopically diluted crystals. The spectral properties of 3-HBA and 4-HBA crystals fairly resemble the spectral properties of decyl alcohol crystals found in the past, which affected for many years our knowledge about the inter-hydrogen bond interactions in hydrogen-bonded crystals
of mixed H/D isotope contents [21,22]. These properties seem to result from electronic properties of O–H groups in alcohols and phenols. For the interpretation of the main crystal spectral properties we did not consider the effect of the hypothetical Fermi resonance mechanism [41–43] on the spectra. In our opinion this influence, provided it exists, is in this case of secondary importance.
9. Conclusion Our studies have shown that O–H O hydrogen bond pairs (formally treated as dimers), composed of two closely-spaced hydrogen bonds of the ‘‘side-to-side”-type arrangement of the moieties, where each belongs to another chain of the associated molecules, are responsible for generation of the basic spectral properties of 3-HBA and 4-HBA crystals. Such a structural unit is responsible for the intensity distribution pattern in the ‘‘residual” mO–H and mO–D bands and for the linear dichroic effect. This effect is characterized by almost identical polarization properties of the two opposite band branches (the higher- and the lower-frequency one) of each mentioned band. The two-branch structure of the ‘‘residual” mO–H and mO–D bands disappears due to the annihilation of the vibrational exciton interactions involving hydrogen bonds in the isotopically diluted crystals with a random distribution of protons an deuterons in their lattices. These band spectral properties provide evidence for ineffective H/D isotopic ‘‘self-organization” processes resulting from weak dynamical co-operative interactions in the hydrogen bond lattices of 3-HBA and 4-HBA crystals.
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