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Distribution of hydrogen isotopes in proton and deuteron mixtures in open-chain hydrogen bond systems: polarization IR spectra of 3-hydroxypyridine crystals
Vibrational Spectroscopy 33 (2003) 163–175
Distribution of hydrogen isotopes in proton and deuteron mixtures in open-chain hydrogen bond systems: polarization IR spectra of 3-hydroxypyridine crystals Henryk T. Flakusa,*, Aleksandra Tyla, Peter G. Jonesb a
b
Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl-40 006 Katowice, Poland Institut fu¨r Anorganische und Analytische Chemie, Technische Universita¨t Braunschweig, Postfach 3329, Braunschweig, Germany Received 24 March 2003; received in revised form 1 August 2003; accepted 7 August 2003
Abstract This paper is devoted to IR spectroscopic studies in polarized light of hydrogen bond systems in 3-hydroxypyridine crystals. These investigations were preceded by re-determination of the 3-hydroxypyridine X-ray structure; in contrast to the previous report, no significant differences were observed between the two independent molecules. Polarization spectra of 3-hydroxypyridine crystals were measured in the frequency ranges of nO–H and nO–D bands at the room temperature, and also at the temperature of liquid nitrogen, for the ‘‘ac’’ crystalline face. Investigations of the ‘‘residual’’ nO–H band shapes for crystals that were diluted by deuterium revealed characteristic changes in the ‘‘residual’’ band shape, indicating an essentially random distribution of protons and deuterons between the hydrogen bonds in the lattice and ruling out the so-called ‘‘self-organization’’ isotopic effects, which are an attribute of some crystalline systems involving chains of hydrogen bonds in their lattices. The random distribution of the protons and deuterons in the crystal is ascribed to disappearance of effective coupling of the hydroxyl group proton and deuteron movements with the pyridine ring p- electrons, which seems to be the reason for the particular spectral behavior of the 3-hydroxypyridine crystal. # 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen bond; Molecular crystals; Polarization IR spectra; H/D isotopic effects; Strong-coupling model; Linear dichroic effects; Temperature effects; Self-organization effects
1. Introduction Infrared spectroscopy still remains a basic tool in the research of the unique dynamics of the atoms forming hydrogen bonds in associated molecular systems. The most spectacular spectral changes that accompany hydrogen bond formation concern the nX–H band frequency range [1–3]. Very often well-developed * Corresponding author. Fax: þ48-32-2599-978. E-mail address: [email protected] (H.T. Flakus).
and complex fine structures of the nX–H bands appear in IR spectra. This latter effect is considered as a manifestation of a strong anharmonic coupling mechanism involving the hydrogen bond normal vibrations of different frequencies [2–5]. Although the problem of spectral properties of hydrogen bonded systems presently appears to interest only a limited circle of researchers (the apogee of the hydrogen bond spectral properties studies was in the late sixties and in the early seventies), we consider that these particular studies warrant continued study.
0924-2031/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2003.08.003
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The most severe tests of theories proposed for quantitative interpretation of spectral properties of hydrogen bond systems include prediction of the H/D isotopic effects in spectra, associated with the replacement of protons by deuterons. This simplest kind of isotopic effect has been widely investigated and discussed [2–5]. Nevertheless, most recent studies have proved that some other, more complex H/D isotopic effects in hydrogen bond spectra can be recognized [6,7]. Recent studies have shown that contemporary theoretical models are unable to provide a complete explanation of spectral properties of even simple hydrogen bond aggregates, cyclic dimers included. This probably does not result from imperfections of the theories, but may rather result from the history of hydrogen bond research, as during the pioneer period certain spectral effects were misinterpreted, or even unnoticed. Unfortunately, this remark mainly concerns more complex systems of hydrogen bonds, namely hydrogen bonded molecular crystals. Only recently, a problem of an abnormal hydrogen isotope distribution between hydrogen bonds in cyclic dimers has appeared, deduced from a quantitative analysis of the IR spectra of the partially deuterated samples [7]. This novel effect seems to be of particular importance. It was found for partially deuterated, cyclic, dimeric hydrogen bond systems that the shapes of the ‘‘residual’’ nX–H bands do not undergo any substantial changes associated with increasing deuteration rate for the samples. The ‘‘residual’’ nX–H bands are attributed just to the residual protons in the samples. This spectral effect was totally unexpected, as the latest theoretical models would predict some noticeable changes in the band contour shapes [3,5,8–15]. According to the theory, the exciton interactions are co-responsible for the mechanism of nX–H band shape formation [3,5,8–15]. When hydrogen and deuterium bonds coexist in a dimer, exciton interactions between them should vanish because of the different frequencies of the nX–H and nX–D normal vibrations. As no essential changes in the nX–H band contour shapes were observed, it was concluded that the hydrogen isotope distribution is not random. It was shown that this effect is probably due to the different dimer formation energies for the three types of isotopic derivatives: ‘‘HH’’, ‘‘HD’’ and ‘‘DD’’ [7,16] (These symbols represent the hydrogen isotopes present in the cyclic dimer hydrogen bridges).
The non-symmetric ‘‘HD’’-type dimers, being of a higher energy, exist in much lower concentration than deduced from the statistical model, and were nondetectable in the IR spectra of the cyclic, dimeric systems of hydrogen bonds [7,17,18]. The above spectral effect was ascribed to a new kind of the H/D isotope effect for the hydrogen bond systems— the ‘‘self-organization’’ effect in proton and deuteron mixture systems. They represent a new type of cooperative effect in the hydrogen bond systems [16].
2. Systematic studies of the ‘‘self-organization’’ processes Considering the potential importance of the H/D isotopic ‘‘self-organization’’ effects for hydrogen bond studies, and also for related problems in biophysics and biochemistry, the concept of isotope ‘‘selforganization’’ effects inspires further research in this area, with the aim of formulating a proper theoretical model for the phenomenon. However, there is no reason to consider the effect to be common for diverse open-chain systems of hydrogen bonds. The following question arise: What kind of spectral isotope effects are attributes of open-chain hydrogen bond systems in molecular crystals? Are there any spectral effects similar to those that were found for the cyclic dimeric systems of hydrogen bonds? To solve this problem, we propose to perform suitable spectral studies for some carefully selected solid-state systems. Investigations should include measurements of crystalline spectra in polarized light, also in a relatively wide temperature range. This kind of study should allow the identification of ‘‘self-organization’’ isotope effects, if they genuinely exist, in crystalline systems involving chains of hydrogen bonds. In the literature devoted to hydrogen bond problems there are widely cited papers by Jakobsen et al. [19] and Mikhailov et al. [20], who studied effects of the isotopic dilution onto the shapes of the ‘‘residual’’ nO–H bands, in the spectra of ethanol crystals. In the solid-state ethanol molecules form infinite chains; in each unit cell there are two molecules. Vibrational exciton interactions between adjacent hydrogen bonds in the chains are responsible for the appearance of a doublet nO–H band structure, with a splitting
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magnitude of ca. 60 cm1. Jakobsen et al. [19] and Mikhailov et al. [20] proved that, with increasing D/H isotope exchange rate, the nO–H band shape gradually altered, and for a composition of ca. 70% deuterons, the ‘‘residual’’ band took the form of a singlet. Such a dramatic band shape evolution was ascribed to the disappearance of the exciton interactions between the OH O hydrogen bonds in the chains of associated alcohol molecules. Linear dichroic effects in the spectra vanished for a deuterium substitution rate of close to ca. 60% [20], because of a strong diminution of ‘‘Davydow-splitting’’ effects in the nO–H ‘‘residual’’ band, in turn attributable to separation of OH O hydrogen bonds by those involving OD O. A random distribution of hydrogen and deuterium bonds in each chain was shown to prevent an effective vibrational exciton coupling between hydrogen bonds [19,20]. Measurements of the IR spectra in the polarized light, for the nX–H ‘‘residual’’ band frequency ranges, might confirm or contradict the random distribution of hydrogen isotope atoms in chains of associated molecules. This might be helpful in explaining mechanisms governing ‘‘self-organization’’ effects in proton and deuteron systems in hydrogen bonded crystals. In this paper, we intend to answer the following questions: (i) Do ‘‘residual’’ nX–H bands and nX–H bands, measured for oriented hydrogen bond chain systems, differ in shape or not? (ii) Are there any linear dichroic effects in the ‘‘residual’’ nX–H band frequency ranges? If yes, what kind of the polarization effects are there? From a number of suitable systems considered we chose 3-hydroxypyridine; its spectrum satisfies well with the demands of the project.
3. Crystal structure determination of 3-hydroxypyridine 3.1. Crystal data Monoclinic, space group P21/c, a ¼ 7:6672ð11Þ, ˚ , b ¼ 98:628ð3Þ8, b ¼ 7:0148ð8Þ, c ¼ 17:803ð2ÞA ˚ 3, Z ¼ 8,m ¼ 0:095 mm1, T ¼ 140 8C. U ¼ 946:7 A 3.2. Data collection A crystal ca. 0:37 mm 0:20 mm 0:12 mm was used to record 12377 intensities on a Bruker SMART
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1000 CCD diffractometer (Mo Ka radiation: 2ymax, 608); 2767 reflections were independent (Rint: 0.071). 3.3. Structure refinement The structure was refined anisotropically on F2 (program SHELXL-97, G.M. Sheldrick, University of Go˜ ttingen) to wR2 0.124, R1 0.0415 for 135 para˚ 3. The hydrogen meters: S ¼ 1:05; max. Dr, 0.41 eA bonded to oxygen was refined freely, others using a riding model [21].1 The two independent molecules of 3-hydroxypyridine are shown in Fig. 1. The molecules form twisting open chains (Fig. 2) associated with hydrogen bonds of the form OH N (Table 1). In the chains, the two independent molecules alternate with each other and are perpendicular to each other (interplanar angle: 88.03(3)8). Neighboring pairs of molecules are linked by the c-glide operation, and the overall chain direction is [1 0 1]. The chains themselves are interlinked by five CH O interactions, of which three are shown in Fig. 3 (the third and fifth in Table 1 are omitted from this figure because they involve chains farther translated parallel to the ˚) y-axis). The uncorrected H O distances (ca. 2.6 A indicate that these interactions are quite weak. An earlier investigation of the same structure (Ohms et al. henceforth OGT, [22]) at room temperature led, as would be expected, to qualitatively similar results; the planned low-temperature investigation referred to by OGT at the time has to the best of our knowledge never been published. The advantages of low temperature data can be seen both in the number of reflections available for refinement (here 2767, OGT 958) and in the refutation of some of OGT’s conclusions: 1. That the molecules show significant differences in geometry. The C–O bond lengths were reported as ˚ ; this major difference disap1.306, 1.350(3) A pears in the current study, with values of 1.3462, ˚ . Similarly, N–C2 bond lengths were 1.3466(12) A ˚ [he re 1. 337 3(14 ), 1.2 96(3 ), 1 .334 (4) A ˚ ˚ [here 1.3403(13) A] and C4–C5 1.331, 1.380(4) A ˚ 1.3859(14), 1.3809(15) A]. We conclude that low 1 Complete crystallographic data (excluding structure factors) have been deposited at the Cambridge Crystallographic Data Center under the number CCDC 165614, and can be obtained free of charge.
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Fig. 1. The two independent molecules of 3-hydroxypyridine in the crystals. Ellipsoids represent 50% probability levels. Hydrogen atom radii ˚ ) and angles (8), with unprimed values first: C3–O1, 1.3462, 1.3466(12); N1–C6, 1.3392, 1.3361(14); are arbitrary. Selected bond lengths (A N1–C2 1.3373(14), 1,3403(13); C2–N1–C6 119.05(8), 119.13(9); C2–C3–C4, 117.75, 117.87(9). A rigid-body libration correction led to bond ˚ (a full list is given in the deposited cif file). length corrections from þ0.004 to þ0.006 A
Fig. 2. Association of molecules of 3-hydroxypyridine to form a chain parallel to [1 0 1]. Hydrogen bonds are indicated by dashed lines.
temperature data provide no evidence for significant differences between the two molecules. 2. That the average C–C bond length is shorter than in pyridine [23]. OGT quote values of 1.363 and ˚ compared to 1.395 A ˚ in pyridine. The cur1.376 A ˚, rent study leads to values of 1.3908 and 1.3914 A but rigid-body libration corrections [24]) lead to an ˚ . The correoverall corrected average of 1.396 A sponding value of Mootz and Wussow [23] (also
˚ . We conclude that libration-corrected) is 1.391 A any apparent shortening observed by OGT may probably be attributed to libration effects.
4. Measurement of the spectra The 3-hydroxypyridine crystals were obtained by melting the commercial sample (Sigma–Aldrich)
Table 1 ˚ and 8) Hydrogen bond geometry parameters for 3-hydroxypyridine crystal (A DH A 0
O(1)–Hð01Þ N(1 ) O(10 )–Hð010 Þ N(1) #1 C(20 )–Hð20 Þ O(1) #2 C(6)–Hð6Þ O(10 ) #3 C(5)–Hð5Þ O(1) #4 C(60 )–Hð60 Þ O(10 ) #5 C(2)–Hð2Þ O(10 ) #2
d(D–H)
D(H A)
d(D A)
1.02 (2) 1.000 (17) 0.95 0.95 0.95 0.95 0.95
1.64 (2) 1.672 (18) 2.60 2.59 2.62 2.69 2.71
2.6601 2.6688 3.5229 3.4121 3.4715 3.6132 3.4028
178.2 (18) 174.0 (16) 165.2 144.9 149.8 164.0 130.4
(11) (11) (14) (13) (13) (14) (13)
Symmetric transformations used to generate equivalent atoms: #1: x 1, y þ 32, z 12; #2: x þ 1, y þ 1, z þ 1; #3: x þ 1, y þ 2, z þ 1; #4: x, y þ 1, z; #5: x þ 1, y, z.
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Fig. 3. Association of hydrogen bonded chains by CH O interactions (thin dashed lines).
between two CaF2 windows, followed by a very slow cooling of the melt, when the plates were pressed tightly together. From the crystalline mosaic, monocrystalline fragments were selected and spatially oriented with the help of a polarization microscope. From an oriented single crystal a restricted area was exposed to the spectral studies using a tin diaphragm, with hole diameter 1.5 mm. The identity periods for the developed crystalline faces were confirmed by Xray diffraction. It was found that in the experimental conditions crystals most frequently develop the ‘‘ac’’ face. Measurements of the spectra were performed by a transmission method, in the temperature range from room temperature down to the temperature of liquid nitrogen, using polarized light. Polarization spectra were measured for two orientations of the electric field vector E of the incident beam, namely for E parallel to the c-axis and for E perpendicular to the ‘‘c’’-axis (parallel to the a -axis, where a is a reciprocal lattice vector). We were able to record two polarized component only, corresponding to the ‘‘ac’’ plane. The deuterated derivative, obtained by evaporation under a reduced pressure of a solution of 3-hydroxypyridine in D2O, was investigated in the same way. The polarized spectra of the crystals were measured by a transmission method, using the Nicolet Magna 560
FT–IR spectrometer. The studies were repeated for a number of crystals to check the reproducibility of the results. The Raman spectra were measured with the help of the Raman Accessory for the FT–IR spectrometer. The exciting line frequency was 9600 cm1.
5. Results The results of our initial studies are shown in Fig. 4, which depict the spectra of 3-hydroxypyridine measured as KBr pellets. The room temperature polarization spectra of 3hydroxypyridine crystals in the nO–H band frequency ranges are presented in Fig. 5, and the low-temperature spectra are shown in Fig. 6. The polarized spectra of the crystals of the deuterated derivative in the nO–D band frequency ranges, recorded at low temperature, are shown in Fig. 7.
6. Discussion 6.1. Choice of a model At the beginning one has to decide which general mechanism is responsible for generation of basic
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1,8 Raman
1,6
77K Absorbance
1,4 1,2 1,0
298K
0,8 0,6 0,4 0,2 0,0 3000
2500 Wavenumbers (cm-1)
2000
Fig. 4. The nO–H bands in the IR spectra of the polycrystalline samples of 3-hydroxypyridine dispersed in KBr pellets. Also, the Raman spectra of the polycrystalline samples are drawn in order to indicate the nC–H band influences on the spectra.
properties of the nO–H band structure in the crystalline spectra. A doublet structure of this band, according to elder theoretical models, might have been connected with a Fermi resonance mechanism, concerning proton stretching vibrations and proton ‘‘bending-inplane’’ vibrations dO–H in their first overtone state (their frequencies are being close to 2700 cm1). Nevertheless, it appears that such band shape forming mechanism seems to have no deeper justification and
must be excluded. The arguments against the Fermi resonance mechanism are the following: the nO–H band splitting is too large, one order of magnitude larger than typical band splitting magnitudes, observed in Fermi resonance cases for different molecular systems. The Fermi resonance model is also unable to explain a strong temperature dependence of the band intensity distribution, or the band shape evolution, due to the partial isotopic dilution of the samples.
1,2 1,0
Absorbance
I 0,8 0,6
II
0,4 0,2
3000
2500 Wavenumbers (cm-1)
2000
Fig. 5. The polarized spectra of 3-hydroxypyridine crystal measured at room temperature in the nO–H band frequency range. The IR beam of normal incidence with respect to the ‘‘ac’’ plane was used. The component spectra were obtained for the two orientations of the electric field vector E: (I) E parallel to the ‘‘c’’-axis; (II) E perpendicular to the ‘‘c’’-axis (parallel to the ‘‘a ’’-axis).
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169
1,2 I
Absorbance
1,0 0,8 II
0,6 0,4 0,2 0,0 3000
2500 Wavenumbers (cm-1)
2000
Fig. 6. The nO–H bands in the polarized spectra of 3-hydroxypyridine crystals measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 5.
On the other hand, vibrational exciton interaction energies, in their proton stretching vibration excited states, should dominate over anharmonic coupling energies. As the energies of exciton interactions decrease very fast, when the inter-hydrogen bond distances increase, the observed nO–H band doublet structure most probably must result from the strongest vibrational exciton interactions, involving pairs of the most closely spaced hydrogen bonds, each belonging to a different chain of the associated molecules.
6.2. Model calculations for the nN–H bands For the space group P21/c and with Z ¼ 8, eight exciton states of the proton stretching vibrations should exist, belonging to four different irreducible representations of C2h,which is isomorphic with the crystal factor-group. The Au and Bu exciton states of should be directly available by absorption of the IR polarized radiation, generating respective component bands contributing to the nO–H band contour.
1,0 0,9
I
0,8 Absorbance
0,7 0,6 0,5
II
0,4 0,3 0,2 0,1 0,0 2800
2600
2400 2200 2000 Wavenumbers (cm-1)
1800
Fig. 7. The nO–D bands in the polarized crystalline spectra of the deuterium derivative of 3-hydroxypyridine crystals (ca.80% of D-atoms) measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 5.
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When some exciton coupling energies for the hydrogen bonds from one unit cell are negligible, the number of different subbands in the crystal spectra strongly reduces. As generation mechanisms of the nX–H bands in vibrational spectra of centrosymmetric hydrogen bond dimers (e.g. (COOH)2 systems of carboxylic acid dimers) may involve mechanisms of dynamic coupling, enabling promotion of symmetry-forbidden transitions in the IR [25], even forbidden transitions may effectively contribute to the nO–H band contour. Calculations of the nO–H and nO–D band contours from the spectra of the crystal were performed in the limits of the ‘‘strong-coupling’’ model in the hydrogen bond dimer approximation. The method utilized was identical to those in [8,9,11]. According to the formalism of the theory, the band contour shapes depend on the set of three coupling parameters: bH (bD), C0 and C1. These are not variation parameters in the theory, as they have precisely defined physical meanings: The distortion parameter of a hydrogen (or deuterium) bond bH (or bD) denotes change in the O O bond equilibrium geometry, accompanying excitation of the proton (deuteron) stretching vibration nO–H (nO–D) to its first excited level. The bH and bD parameter values satisfy, within the simplest version of the ‘‘strongcoupling’’ theory, the following relation: bH pffiffiffi ¼ 2 bD The resonance interaction parameters C0 and C1 for the vibrationally excited hydrogen (or deuterium) bonds of a dimer are consecutive terms of the resonance interaction integral C, for a pair of dimeric hydrogen bonds. The constant term C0 can be estimated from the splitting of the component subbands of the dimer spectrum, corresponding to the transitions to proton (deuteron) vibrational states of different symmetry. The C1 parameter is the developing coefficient in the linear term with respect to the totally symmetric coordinate Q1 of the low-energy O O bond stretching vibrations of the dimer. This parameter is responsible for differentiation of the component subband widths in the dimer spectra [8,9,11]. For the deuterium bonded dimer spectra, the C0 and C1 parameter values p should decrease by a factor within ffiffiffi the range of 1.0– 2 [8,9,11]. The results of our model calculations for the nO–H band contour shapes of 3-hydroxypyridine crystal are
shown in Fig. 8. The nO–D band contour shape deduced from the nO–H band calculation results is shown in Fig. 9. Our calculation suggests that each band is a superposition of two component bands, which on the ‘‘strong-coupling’’ theory were assigned as ‘‘plus’’ and ‘‘minus’’ [8,9,11]. The longer-wave branch of each spectrum is reproduced by the dimeric ‘‘minus’’ band, which according to the theory corresponds to the symmetry-forbidden excitation of the totally symmetric proton vibrations in the dimer [8,9,11]. The shorter-wave branch of each nO–H band is reproduced by the dimeric ‘‘plus’’ band, corresponding to the dipole-allowed excitation of the non-totally symmetric proton vibrations in the dimer [8,9,11]. Each nO–D band is satisfactorily reproduced solely by the ‘‘plus’’ band, corresponding to the symmetry allowed transition [8,9,11]. The ‘‘minus’’ band makes essentially no contribution to the analyzed band contours. A complex structure of the nO–H band also finds its confirmation in the temperature-induced changes in the band (see Figs. 5 and 6). The temperature affects the two spectral branches, however, in a different way. For the longer-wavelength branch of the nO–H band a ca. two-fold increase of intensity can be observed, compared to the shorter-wavelength branch integral intensity. For the shorter-wavelength branch, some relatively weak temperature effects can be noticed, indicating a more complex nature of this spectral branch. 6.3. Linear dichroic effects The linear dichroic effects, as observed in the nO–H band frequency range of the 3-hydroxypyridine crystal spectra, are very characteristic: this band exhibits a doublet structure, composed of two well-separated spectral branches (their centers of gravity located at 2450 and 1900 cm1, respectively). The splitting magnitude of the nO–H band (see Figs. 5 and 6) suggests the existence of a strong vibrational exciton coupling in the crystal, although the two branches display almost identical polarization properties. The dichroic properties of these bands simply result from the hydrogen bond orientations in the crystalline lattice: the integral intensity ratio of the polarized component bands in the solid-state spectra
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12.00
3000
8.00
171
Wavenumbers (cm-1) 2000
Intensity
III
4.00
II
I
0.00 8.00
4.00
0.00
-4.00
-8.00
_
hω O---N Fig. 8. Theoretical reconstitution of the most intense nO–H band component from the low-temperature spectra of 3-hydroxypyridine crystals. (I) The ‘‘plus’’ dimeric band reconstituting the symmetry-allowed transition; (II) the ‘‘minus’’ dimeric band reproducing the forbidden transition; and (III) the superposition of the ‘‘plus’’ and ‘‘minus’’ bands taken with their statistical weight parameters Fþ and F. The coupling parameter values: bH ¼ 1:8, C0 ¼ 3:0, C1 ¼ þ0:3, F þ ¼ 1:0, F ¼ 0:9, OOO ¼ 90 cm1. The experimental spectrum is presented at the right upper edge of the picture.
(see Figs. 5 and 6) is approximately proportional to the ratio of squares of the direction cosines of the O N vector, calculated in relation to the electric field E vector direction in the experimental conditions. A lack of differences of the polarization properties between the two branches of the nO–H band makes difficult a detailed analysis of the crystal spectra. The peculiar dichroic properties of the spectra directly result from the chain arrangement of the hydrogen bonds in the lattice and from the chain geometry. To a good approximation, neighboring hydrogen bonds from one chain are parallel (the angle between the O N vectors is 38). This fact may be helpful in
explanation of identical vibrational transition moment directions, characterizing the longer- and shorterwavelength branches of the nO–H band in the 3-hydroxypyridine crystal spectra. In our discussion of the dichroic effects, a solution of the two-branch spectrum generation mechanism for the nO–H band seems to be crucial. In this task, the parallel arrangement of all hydrogen bonds is a substantial problem. A considerable difference of the branch frequencies, approximately equal to 550 cm1, is most probably connected with the strongest hydrogen bond exciton interactions, concerning the most closely spaced hydrogen bonds from one unit cell.
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10.00
8.00
Intensity
2000
6.00
I
1800
Wavenumbers (cm-1)
II
4.00
2.00
0.00 8.00
4.00
0.00
-4.00
-8.00
_
hωO---N Fig. 9. Theoretical prediction of the most intense nO–D band component from the low-temperature spectra of 3-hydroxypyridine crystals. (I) The ‘‘plus’’ dimeric band; (II) the ‘‘minus’’ dimeric band reproducing the forbidden transition. The coupling parameter values: bD ¼ 1:3, C0 ¼ 3:0, C1 ¼ þ0:3, F þ ¼ 1:0, F ¼ 0:0, ONO ¼ 90 cm1. The experimental spectrum is presented at the right upper edge of the picture.
In this approach, the influence on the spectra of the weak and non-conventional hydrogen bonds CH O (see Table 1) seems to be of second order and, therefore, negligible in our considerations. At this point, it must be decided whether the strongest exciton interaction concerns the neighboring hydrogen bonds from one chain, or connects hydrogen bonds belonging to two different chains (see Fig. 3). The temperature effects in the spectra strongly support the latter option. The intensity of the longer-wavelength branch of the nO–H band increased when the temperature decreased, whereas the intensity of the shorter-wavelength branch did not undergo such strong changes with the decreasing temperature. Such behavior is typical of centrosymmetric hydrogen bond dimers with a side-to-side placement of the hydrogen bonds in a dimer [8,9,11,17,18]. In the case of strongest interactions between adjacent hydrogen bonds in one chain, the temperature effect on the spectra would be of opposite character; the shorter-wavelength branch, ascribed to the symmetry forbidden transition
for non-totally symmetric proton vibrations in a chain (the hydrogen bonds in one chain are almost parallel), would be most temperature-affected. Therefore, the longer-wavelength branch of the 3-hydroxypyridine crystal spectrum is most probably associated with the symmetry-forbidden transition in the IR. It is connected with the excitation of the totally symmetric proton vibrations in centrosymmetric hydrogen bond dimers, formed by hydrogen bonds belonging to adjacent chains. Such a transition would become allowed thanks to a vibronic promotion mechanism, considered as a reversal of the familiar Herzberg-Teller effect from the electronic molecular spectroscopy [26]. The above-presented interpretation is supported by the isotope effect in the solid-state spectra (see Fig. 7). The nO–D band in the spectra of the deuterium derivative of 3-hydroxypyridine is located at ca. 1900 cm1, overlapping with the longer-wavelength branch of the ‘‘residual’’ nO–H band. This nO–D band pffiffiffi frequency is about 2 times lower than the nO–H band
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higher-energy branch frequency (see Figs. 5 and 6), which represents a regular H/D isotopic effect, usually observed in the vibrational spectroscopy. However, the nO–D band is a singlet, with no second spectral branch corresponding to the lower-frequency branch of the nO–H band; in the nO–D band the longer-wavelength branch is extinguished. For centrosymmetric hydrogen bond dimers, the lower-energy transition in the spectra is forbidden by symmetry rules. The probability of this transition and also the longer-wavelength branch intensity for each band, namely the nO–H and nO–H bands, depends on the electronic properties of hydrogen bonds in the dimers. The vibronic mechanism responsible for promotion of the forbidden transition will also strongly depend on the hydrogen atom masses in the hydrogen bonds [25]. This latter factor can explain the strong diminution of the forbidden transition intensity in the dimer system spectra and in some cases it explains its total extinction [11,18], as observed in the nO–D band frequency range for 3-hydroxypyridine (see Fig. 7). 6.4. Isotopic dilution effects in the crystalline spectra The H/D isotope effects in the solid-state spectra of 3-hydroxypyridine are relatively complex. Besides, the above-mentioned relation between the nO–D and nO–H band structures, a very substantial isotope effect concerns the nO–H ‘‘residual’’ band properties, measured for samples characterized by high deuterium content (see Fig. 7). Clearly, the loss of the protons in a sample by deuteration is responsible for proportional diminution of the nO–H ‘‘residual’’ band intensity. This relation is valid for the shorter-wavelength branch of the band. However, the nO–H and nO–D bands meet at a frequency near 1900 cm1; in this frequency range the spectrum intensity is not simply proportional to the proton concentration, as protons and deuterons introduce opposite trends. An increasing concentration of deuterons in a crystalline sample is accompanied by the appearance of a relatively narrow band at ca. 2130 cm1. This new band is characterized by identical polarization properties to those of the nO–H band. Its intensity follows the proton concentrations in a sample. Furthermore, the pffiffiffinew band frequency is not related by the factor 2 to any of the nO–H band spectral branch frequencies. The narrow band
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frequency corresponds almost to the average frequency of the gravity centers of the nO–H band spectral branches. Thus, exciton interactions between the OH N hydrogen bonds play no essential role in generating this band, as these bonds are effectively separated by the deuterium bonds. 6.5. Distribution of protons and deuterons in the lattice The spectra of isotopically diluted samples of 3hydroxypyridine indicate that distribution of protons and deuterons within the chain system of the hydrogen bonds is totally random. A similar conclusion is valid for the hydrogen isotope distribution over the hydrogen bonds of the adjacent chains in the lattice. This means that in this case no so-called isotopic ‘‘selforganization’’ effects exist, in contrast to some other dimeric hydrogen bond systems [7,17,18], or for some crystals with chains of hydrogen bonds in their lattices, e.g. for 4-mercaptopyridine [27], pyrazole [28], imidazole [29], or for the high-temperature form of 4-hydroxypyridine [29]. It was suggested recently that ‘‘self-organization’’ effects in the IR spectra of the hydrogen bond are most probably a result of a mechanism that generates additional attracting forces between identical hydrogen isotope atoms, in systems of mutually coupled hydrogen bonds. The new kind of co-operative interaction mechanism in the hydrogen bond area seems to be generally of a dynamic, vibronic nature [16]. This statement is supported by the observed strong hydrogen atom mass-dependence of this effect. The ‘‘self-organization’’ energy effects are not accessible even using the most sophisticated calculation methods of contemporary quantum chemistry. They find no counterpart even in the latest monographs concerning hydrogen bonds [1–5,30,31]. In the spectra of 3-hydroxypyridine crystals, the isotopic ‘‘self-organization’’ effects are not observed. With an increasing deuterium content, the doublet structure of the nO–H band transforms towards a singlet band, located exactly in the center of the doublet. This line correlates with isolated OH N hydrogen bonds, not coupled with other, similar hydrogen bonds in the lattice. This isotope effect is very similar to those observed in the case of the ethanol crystal spectra, where those effects were ascribed to a fully random distribution of protons and deuterons in chains
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of OH O hydrogen bonds [19,20]. Also, the spectra of the low-temperature crystalline phase of 4-hydroxypyridine indicate no isotopic ‘‘self-organization’’ effects. These results will be published elsewhere. 6.6. Possible nature of the ‘‘self-organization’’ mechanisms for chain systems Detailed analysis of the all cases where the ‘‘selforganization’’ H/D isotopic effects were detected in the crystalline spectra [26–28], in contrast to the 3hydroxypyridine and the ethanol crystal [19,20] cases, might facilitate proposing a hypothesis concerning the mechanisms of these processes. It seems that structural factor is crucial in these mechanisms, which are most probably of a vibronic nature. In the ‘‘self-organization’’ mechanisms, a prominent role is played by couplings between the proton stretching vibrations and electronic motions in the hydrogen bonds, p-electrons of the associated molecules included [25]. When the proton-donor atoms are directly connected to a p-electronic system, such coupling may be effective, in consequence becoming a source of the ‘‘self-organization’’ effects in the spectra. In such a case, a totally symmetric and coordinated movement of protons, coupled with electron systems of the hydrogen bonds in a chain, might be responsible for appearance of some relatively weak attraction forces between identical hydrogen isotope atoms from the neighboring hydrogen bonds [16]. In the case of solid-state 3-hydroxypyridine, the protons of the hydrogen bonds are separated from the pyridine aromatic rings by oxygen atoms. Thus, the vibronic mechanisms become much less effective. For ethanol crystals, with no electrons in p-orbitals, such mechanism is also expected to be ineffective. All these analyzed spectral effects may be helpful in elaboration in the future of a general theory of ‘‘selforganization’’ effects in the IR spectroscopy of hydrogen bonded crystalline systems.
7. Conclusion Our investigations of the IR spectra of the hydrogen bond in 3-hydroxypyridine crystals suggest a lack of ‘‘self-organization’’ effects in the spectra of samples
characterized by relatively high deuterium content in the hydrogen bonds. Such effects were previously found for some solid-state systems with a chain arrangement of hydrogen bonds in their lattices. The spectra of deuterium-substituted 3-hydroxypyridine crystals demonstrate with high probability a random distribution of protons and deuterons among the hydrogen bonds. The search for a correlation between spectral properties of a number of crystalline systems (namely, the ‘‘self-organization’’ effects in the spectra), with molecular structures of the hydrogen-bonded molecules suggests that such a correlation is valid in terms of a simple vibronic model. These effects seem to be an attribute of this particular kind of hydrogen bonds, in which the proton-donor atoms are directly attached to p-electronic systems and the proton-acceptor atoms also belong to the same system.
Acknowledgements P.G. Jones thanks the Fonds der Chemischen Industrie for financial support. References [1] G.C. Pimentel, A.L. McClellan, The Hydrogen Bond, Freeman, San Francisco, 1960. [2] P. Schuster, G. Zundel, C. Sandorfy (Eds.), The Hydrogen Bond, Recent Developments in the Theory and Experiment. Parts I–III, North-Holland, Amsterdam, 1976. [3] G.L. Hofacker, Y. Marechal, M.A. Ratner, The dynamical aspects of hydrogen bonds, in: P. Schuster, G. Zundel, C. Sandorfy (Eds.), The Hydrogen Bond, Recent Developments in Theory and Experiment. Part I, North-Holland, Amsterdam, 1976, p. 295. [4] P. Schuster, W. Mikenda (Eds.), Hydrogen Bond Research, Monatshefte fuˆ r Chemie, Chemical Monthly, 130 (No. 8), Springer, Vien, New York, 1999. [5] D. Hadz˙ i (Ed.), Theoretical Treatments of Hydrogen Bonding, Wiley, New York, 1997. [6] H.T. Flakus, A. Machelska, J. Mol. Struct. 447 (1998) 97. [7] H.T. Flakus, A. Ban´ czyk, J. Mol. Struct. 476 (1999) 57. [8] A. Witkowski, J. Chem. Phys. 47 (1967) 3645. [9] Y. Marechal, A. Witkowski, J. Chem. Phys. 48 (1968) 3697. [10] S.F. Fisher, G.L. Hofacker, M.A. Ratner, J. Chem. Phys. 52 (1970) 1934. [11] H.T. Flakus, Chem. Phys. 62 (1981) 103. [12] O. Henri-Rousseau, P. Blaise, The infrared density of weak hydrogen bonds within the Linear response theory, in: I. Prigogine, S.A. Rice (Eds.), Advances in Chemical Physics, vol. 103, Wiley, New York, 1998.
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[22]
D. Chamma, O. Henri-Rousseau, Chem. Phys. 248 (1999) 53. D. Chamma, O. Henri-Rousseau, Chem. Phys. 248 (1999) 71. D. Chamma, O. Henri-Rousseau, Chem. Phys. 248 (1999) 91. H.T. Flakus, J. Mol. Struct. 646 (2003) 15. H.T. Flakus, A. Miros, J. Mol. Struct. 484 (1999) 103. H.T. Flakus, A. Miros, P.G. Jones, J. Mol. Struct. 604 (2002) 29–44. R.J. Jakobsen, J.W. Brasch, Y. Mikawa, J. Mol. Struct. 1 (1967) 309. I.D. Mikhailov, V.A. Savelev, N.D. Sokolov, N.G. Bokh, Phys. Status Solidi 57 (1973) 719. No. CCDC 165614, The Director, The Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, England, UK (e-mail: [email protected]). U. Ohms, H. Guth, W. Treutmann, Z. Kristallogr. 162 (1983) 299.
[23] [24] [25] [26] [27] [28] [29] [30] [31]
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D. Mootz, H.-G. Wussow, J. Chem. Phys. 75 (1981) 1517. V. Schomaker, K.N. Trueblood, Acta Cryst. B 24 (1968) 63. H.T. Flakus, J. Mol. Struct. 187 (1989) 35. G. Fisher, Vibronic Coupling, Academic Press, London, 1984. H.T. Flakus, A. Tyl, P.G. Jones, Spectrochim. Acta A 58 (2002) 299–310. H.T. Flakus, A. Machelska, Spectrochim. Acta A 58 (2002) 555–568. H.T. Flakus, A. Tyl, unpublished results. H. Ratajczak, W.J. Orville-Thomas (Eds.), Molecular Interactions, vol. 1, Wiley, New York, 1980. S.S. Xantheas (Ed.), Recent Theoretical and Experimental Advances in Hydrogen Bonded Clusters, NATO ASI Series, Series C: Mathematical and Physical Sciences, vol. 561, Kluwer Academic Publishers, Dordrecht, 2000.
Distribution of hydrogen isotopes in proton and deuteron mixtures in open-chain hydrogen bond systems: polarization IR spectra of 3-hydroxypyridine crystals Henryk T. Flakusa,*, Aleksandra Tyla, Peter G. Jonesb a
b
Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl-40 006 Katowice, Poland Institut fu¨r Anorganische und Analytische Chemie, Technische Universita¨t Braunschweig, Postfach 3329, Braunschweig, Germany Received 24 March 2003; received in revised form 1 August 2003; accepted 7 August 2003
Abstract This paper is devoted to IR spectroscopic studies in polarized light of hydrogen bond systems in 3-hydroxypyridine crystals. These investigations were preceded by re-determination of the 3-hydroxypyridine X-ray structure; in contrast to the previous report, no significant differences were observed between the two independent molecules. Polarization spectra of 3-hydroxypyridine crystals were measured in the frequency ranges of nO–H and nO–D bands at the room temperature, and also at the temperature of liquid nitrogen, for the ‘‘ac’’ crystalline face. Investigations of the ‘‘residual’’ nO–H band shapes for crystals that were diluted by deuterium revealed characteristic changes in the ‘‘residual’’ band shape, indicating an essentially random distribution of protons and deuterons between the hydrogen bonds in the lattice and ruling out the so-called ‘‘self-organization’’ isotopic effects, which are an attribute of some crystalline systems involving chains of hydrogen bonds in their lattices. The random distribution of the protons and deuterons in the crystal is ascribed to disappearance of effective coupling of the hydroxyl group proton and deuteron movements with the pyridine ring p- electrons, which seems to be the reason for the particular spectral behavior of the 3-hydroxypyridine crystal. # 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen bond; Molecular crystals; Polarization IR spectra; H/D isotopic effects; Strong-coupling model; Linear dichroic effects; Temperature effects; Self-organization effects
1. Introduction Infrared spectroscopy still remains a basic tool in the research of the unique dynamics of the atoms forming hydrogen bonds in associated molecular systems. The most spectacular spectral changes that accompany hydrogen bond formation concern the nX–H band frequency range [1–3]. Very often well-developed * Corresponding author. Fax: þ48-32-2599-978. E-mail address: [email protected] (H.T. Flakus).
and complex fine structures of the nX–H bands appear in IR spectra. This latter effect is considered as a manifestation of a strong anharmonic coupling mechanism involving the hydrogen bond normal vibrations of different frequencies [2–5]. Although the problem of spectral properties of hydrogen bonded systems presently appears to interest only a limited circle of researchers (the apogee of the hydrogen bond spectral properties studies was in the late sixties and in the early seventies), we consider that these particular studies warrant continued study.
0924-2031/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2003.08.003
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The most severe tests of theories proposed for quantitative interpretation of spectral properties of hydrogen bond systems include prediction of the H/D isotopic effects in spectra, associated with the replacement of protons by deuterons. This simplest kind of isotopic effect has been widely investigated and discussed [2–5]. Nevertheless, most recent studies have proved that some other, more complex H/D isotopic effects in hydrogen bond spectra can be recognized [6,7]. Recent studies have shown that contemporary theoretical models are unable to provide a complete explanation of spectral properties of even simple hydrogen bond aggregates, cyclic dimers included. This probably does not result from imperfections of the theories, but may rather result from the history of hydrogen bond research, as during the pioneer period certain spectral effects were misinterpreted, or even unnoticed. Unfortunately, this remark mainly concerns more complex systems of hydrogen bonds, namely hydrogen bonded molecular crystals. Only recently, a problem of an abnormal hydrogen isotope distribution between hydrogen bonds in cyclic dimers has appeared, deduced from a quantitative analysis of the IR spectra of the partially deuterated samples [7]. This novel effect seems to be of particular importance. It was found for partially deuterated, cyclic, dimeric hydrogen bond systems that the shapes of the ‘‘residual’’ nX–H bands do not undergo any substantial changes associated with increasing deuteration rate for the samples. The ‘‘residual’’ nX–H bands are attributed just to the residual protons in the samples. This spectral effect was totally unexpected, as the latest theoretical models would predict some noticeable changes in the band contour shapes [3,5,8–15]. According to the theory, the exciton interactions are co-responsible for the mechanism of nX–H band shape formation [3,5,8–15]. When hydrogen and deuterium bonds coexist in a dimer, exciton interactions between them should vanish because of the different frequencies of the nX–H and nX–D normal vibrations. As no essential changes in the nX–H band contour shapes were observed, it was concluded that the hydrogen isotope distribution is not random. It was shown that this effect is probably due to the different dimer formation energies for the three types of isotopic derivatives: ‘‘HH’’, ‘‘HD’’ and ‘‘DD’’ [7,16] (These symbols represent the hydrogen isotopes present in the cyclic dimer hydrogen bridges).
The non-symmetric ‘‘HD’’-type dimers, being of a higher energy, exist in much lower concentration than deduced from the statistical model, and were nondetectable in the IR spectra of the cyclic, dimeric systems of hydrogen bonds [7,17,18]. The above spectral effect was ascribed to a new kind of the H/D isotope effect for the hydrogen bond systems— the ‘‘self-organization’’ effect in proton and deuteron mixture systems. They represent a new type of cooperative effect in the hydrogen bond systems [16].
2. Systematic studies of the ‘‘self-organization’’ processes Considering the potential importance of the H/D isotopic ‘‘self-organization’’ effects for hydrogen bond studies, and also for related problems in biophysics and biochemistry, the concept of isotope ‘‘selforganization’’ effects inspires further research in this area, with the aim of formulating a proper theoretical model for the phenomenon. However, there is no reason to consider the effect to be common for diverse open-chain systems of hydrogen bonds. The following question arise: What kind of spectral isotope effects are attributes of open-chain hydrogen bond systems in molecular crystals? Are there any spectral effects similar to those that were found for the cyclic dimeric systems of hydrogen bonds? To solve this problem, we propose to perform suitable spectral studies for some carefully selected solid-state systems. Investigations should include measurements of crystalline spectra in polarized light, also in a relatively wide temperature range. This kind of study should allow the identification of ‘‘self-organization’’ isotope effects, if they genuinely exist, in crystalline systems involving chains of hydrogen bonds. In the literature devoted to hydrogen bond problems there are widely cited papers by Jakobsen et al. [19] and Mikhailov et al. [20], who studied effects of the isotopic dilution onto the shapes of the ‘‘residual’’ nO–H bands, in the spectra of ethanol crystals. In the solid-state ethanol molecules form infinite chains; in each unit cell there are two molecules. Vibrational exciton interactions between adjacent hydrogen bonds in the chains are responsible for the appearance of a doublet nO–H band structure, with a splitting
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magnitude of ca. 60 cm1. Jakobsen et al. [19] and Mikhailov et al. [20] proved that, with increasing D/H isotope exchange rate, the nO–H band shape gradually altered, and for a composition of ca. 70% deuterons, the ‘‘residual’’ band took the form of a singlet. Such a dramatic band shape evolution was ascribed to the disappearance of the exciton interactions between the OH O hydrogen bonds in the chains of associated alcohol molecules. Linear dichroic effects in the spectra vanished for a deuterium substitution rate of close to ca. 60% [20], because of a strong diminution of ‘‘Davydow-splitting’’ effects in the nO–H ‘‘residual’’ band, in turn attributable to separation of OH O hydrogen bonds by those involving OD O. A random distribution of hydrogen and deuterium bonds in each chain was shown to prevent an effective vibrational exciton coupling between hydrogen bonds [19,20]. Measurements of the IR spectra in the polarized light, for the nX–H ‘‘residual’’ band frequency ranges, might confirm or contradict the random distribution of hydrogen isotope atoms in chains of associated molecules. This might be helpful in explaining mechanisms governing ‘‘self-organization’’ effects in proton and deuteron systems in hydrogen bonded crystals. In this paper, we intend to answer the following questions: (i) Do ‘‘residual’’ nX–H bands and nX–H bands, measured for oriented hydrogen bond chain systems, differ in shape or not? (ii) Are there any linear dichroic effects in the ‘‘residual’’ nX–H band frequency ranges? If yes, what kind of the polarization effects are there? From a number of suitable systems considered we chose 3-hydroxypyridine; its spectrum satisfies well with the demands of the project.
3. Crystal structure determination of 3-hydroxypyridine 3.1. Crystal data Monoclinic, space group P21/c, a ¼ 7:6672ð11Þ, ˚ , b ¼ 98:628ð3Þ8, b ¼ 7:0148ð8Þ, c ¼ 17:803ð2ÞA ˚ 3, Z ¼ 8,m ¼ 0:095 mm1, T ¼ 140 8C. U ¼ 946:7 A 3.2. Data collection A crystal ca. 0:37 mm 0:20 mm 0:12 mm was used to record 12377 intensities on a Bruker SMART
165
1000 CCD diffractometer (Mo Ka radiation: 2ymax, 608); 2767 reflections were independent (Rint: 0.071). 3.3. Structure refinement The structure was refined anisotropically on F2 (program SHELXL-97, G.M. Sheldrick, University of Go˜ ttingen) to wR2 0.124, R1 0.0415 for 135 para˚ 3. The hydrogen meters: S ¼ 1:05; max. Dr, 0.41 eA bonded to oxygen was refined freely, others using a riding model [21].1 The two independent molecules of 3-hydroxypyridine are shown in Fig. 1. The molecules form twisting open chains (Fig. 2) associated with hydrogen bonds of the form OH N (Table 1). In the chains, the two independent molecules alternate with each other and are perpendicular to each other (interplanar angle: 88.03(3)8). Neighboring pairs of molecules are linked by the c-glide operation, and the overall chain direction is [1 0 1]. The chains themselves are interlinked by five CH O interactions, of which three are shown in Fig. 3 (the third and fifth in Table 1 are omitted from this figure because they involve chains farther translated parallel to the ˚) y-axis). The uncorrected H O distances (ca. 2.6 A indicate that these interactions are quite weak. An earlier investigation of the same structure (Ohms et al. henceforth OGT, [22]) at room temperature led, as would be expected, to qualitatively similar results; the planned low-temperature investigation referred to by OGT at the time has to the best of our knowledge never been published. The advantages of low temperature data can be seen both in the number of reflections available for refinement (here 2767, OGT 958) and in the refutation of some of OGT’s conclusions: 1. That the molecules show significant differences in geometry. The C–O bond lengths were reported as ˚ ; this major difference disap1.306, 1.350(3) A pears in the current study, with values of 1.3462, ˚ . Similarly, N–C2 bond lengths were 1.3466(12) A ˚ [he re 1. 337 3(14 ), 1.2 96(3 ), 1 .334 (4) A ˚ ˚ [here 1.3403(13) A] and C4–C5 1.331, 1.380(4) A ˚ 1.3859(14), 1.3809(15) A]. We conclude that low 1 Complete crystallographic data (excluding structure factors) have been deposited at the Cambridge Crystallographic Data Center under the number CCDC 165614, and can be obtained free of charge.
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Fig. 1. The two independent molecules of 3-hydroxypyridine in the crystals. Ellipsoids represent 50% probability levels. Hydrogen atom radii ˚ ) and angles (8), with unprimed values first: C3–O1, 1.3462, 1.3466(12); N1–C6, 1.3392, 1.3361(14); are arbitrary. Selected bond lengths (A N1–C2 1.3373(14), 1,3403(13); C2–N1–C6 119.05(8), 119.13(9); C2–C3–C4, 117.75, 117.87(9). A rigid-body libration correction led to bond ˚ (a full list is given in the deposited cif file). length corrections from þ0.004 to þ0.006 A
Fig. 2. Association of molecules of 3-hydroxypyridine to form a chain parallel to [1 0 1]. Hydrogen bonds are indicated by dashed lines.
temperature data provide no evidence for significant differences between the two molecules. 2. That the average C–C bond length is shorter than in pyridine [23]. OGT quote values of 1.363 and ˚ compared to 1.395 A ˚ in pyridine. The cur1.376 A ˚, rent study leads to values of 1.3908 and 1.3914 A but rigid-body libration corrections [24]) lead to an ˚ . The correoverall corrected average of 1.396 A sponding value of Mootz and Wussow [23] (also
˚ . We conclude that libration-corrected) is 1.391 A any apparent shortening observed by OGT may probably be attributed to libration effects.
4. Measurement of the spectra The 3-hydroxypyridine crystals were obtained by melting the commercial sample (Sigma–Aldrich)
Table 1 ˚ and 8) Hydrogen bond geometry parameters for 3-hydroxypyridine crystal (A DH A 0
O(1)–Hð01Þ N(1 ) O(10 )–Hð010 Þ N(1) #1 C(20 )–Hð20 Þ O(1) #2 C(6)–Hð6Þ O(10 ) #3 C(5)–Hð5Þ O(1) #4 C(60 )–Hð60 Þ O(10 ) #5 C(2)–Hð2Þ O(10 ) #2
d(D–H)
D(H A)
d(D A)
1.02 (2) 1.000 (17) 0.95 0.95 0.95 0.95 0.95
1.64 (2) 1.672 (18) 2.60 2.59 2.62 2.69 2.71
2.6601 2.6688 3.5229 3.4121 3.4715 3.6132 3.4028
178.2 (18) 174.0 (16) 165.2 144.9 149.8 164.0 130.4
(11) (11) (14) (13) (13) (14) (13)
Symmetric transformations used to generate equivalent atoms: #1: x 1, y þ 32, z 12; #2: x þ 1, y þ 1, z þ 1; #3: x þ 1, y þ 2, z þ 1; #4: x, y þ 1, z; #5: x þ 1, y, z.
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Fig. 3. Association of hydrogen bonded chains by CH O interactions (thin dashed lines).
between two CaF2 windows, followed by a very slow cooling of the melt, when the plates were pressed tightly together. From the crystalline mosaic, monocrystalline fragments were selected and spatially oriented with the help of a polarization microscope. From an oriented single crystal a restricted area was exposed to the spectral studies using a tin diaphragm, with hole diameter 1.5 mm. The identity periods for the developed crystalline faces were confirmed by Xray diffraction. It was found that in the experimental conditions crystals most frequently develop the ‘‘ac’’ face. Measurements of the spectra were performed by a transmission method, in the temperature range from room temperature down to the temperature of liquid nitrogen, using polarized light. Polarization spectra were measured for two orientations of the electric field vector E of the incident beam, namely for E parallel to the c-axis and for E perpendicular to the ‘‘c’’-axis (parallel to the a -axis, where a is a reciprocal lattice vector). We were able to record two polarized component only, corresponding to the ‘‘ac’’ plane. The deuterated derivative, obtained by evaporation under a reduced pressure of a solution of 3-hydroxypyridine in D2O, was investigated in the same way. The polarized spectra of the crystals were measured by a transmission method, using the Nicolet Magna 560
FT–IR spectrometer. The studies were repeated for a number of crystals to check the reproducibility of the results. The Raman spectra were measured with the help of the Raman Accessory for the FT–IR spectrometer. The exciting line frequency was 9600 cm1.
5. Results The results of our initial studies are shown in Fig. 4, which depict the spectra of 3-hydroxypyridine measured as KBr pellets. The room temperature polarization spectra of 3hydroxypyridine crystals in the nO–H band frequency ranges are presented in Fig. 5, and the low-temperature spectra are shown in Fig. 6. The polarized spectra of the crystals of the deuterated derivative in the nO–D band frequency ranges, recorded at low temperature, are shown in Fig. 7.
6. Discussion 6.1. Choice of a model At the beginning one has to decide which general mechanism is responsible for generation of basic
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1,8 Raman
1,6
77K Absorbance
1,4 1,2 1,0
298K
0,8 0,6 0,4 0,2 0,0 3000
2500 Wavenumbers (cm-1)
2000
Fig. 4. The nO–H bands in the IR spectra of the polycrystalline samples of 3-hydroxypyridine dispersed in KBr pellets. Also, the Raman spectra of the polycrystalline samples are drawn in order to indicate the nC–H band influences on the spectra.
properties of the nO–H band structure in the crystalline spectra. A doublet structure of this band, according to elder theoretical models, might have been connected with a Fermi resonance mechanism, concerning proton stretching vibrations and proton ‘‘bending-inplane’’ vibrations dO–H in their first overtone state (their frequencies are being close to 2700 cm1). Nevertheless, it appears that such band shape forming mechanism seems to have no deeper justification and
must be excluded. The arguments against the Fermi resonance mechanism are the following: the nO–H band splitting is too large, one order of magnitude larger than typical band splitting magnitudes, observed in Fermi resonance cases for different molecular systems. The Fermi resonance model is also unable to explain a strong temperature dependence of the band intensity distribution, or the band shape evolution, due to the partial isotopic dilution of the samples.
1,2 1,0
Absorbance
I 0,8 0,6
II
0,4 0,2
3000
2500 Wavenumbers (cm-1)
2000
Fig. 5. The polarized spectra of 3-hydroxypyridine crystal measured at room temperature in the nO–H band frequency range. The IR beam of normal incidence with respect to the ‘‘ac’’ plane was used. The component spectra were obtained for the two orientations of the electric field vector E: (I) E parallel to the ‘‘c’’-axis; (II) E perpendicular to the ‘‘c’’-axis (parallel to the ‘‘a ’’-axis).
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169
1,2 I
Absorbance
1,0 0,8 II
0,6 0,4 0,2 0,0 3000
2500 Wavenumbers (cm-1)
2000
Fig. 6. The nO–H bands in the polarized spectra of 3-hydroxypyridine crystals measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 5.
On the other hand, vibrational exciton interaction energies, in their proton stretching vibration excited states, should dominate over anharmonic coupling energies. As the energies of exciton interactions decrease very fast, when the inter-hydrogen bond distances increase, the observed nO–H band doublet structure most probably must result from the strongest vibrational exciton interactions, involving pairs of the most closely spaced hydrogen bonds, each belonging to a different chain of the associated molecules.
6.2. Model calculations for the nN–H bands For the space group P21/c and with Z ¼ 8, eight exciton states of the proton stretching vibrations should exist, belonging to four different irreducible representations of C2h,which is isomorphic with the crystal factor-group. The Au and Bu exciton states of should be directly available by absorption of the IR polarized radiation, generating respective component bands contributing to the nO–H band contour.
1,0 0,9
I
0,8 Absorbance
0,7 0,6 0,5
II
0,4 0,3 0,2 0,1 0,0 2800
2600
2400 2200 2000 Wavenumbers (cm-1)
1800
Fig. 7. The nO–D bands in the polarized crystalline spectra of the deuterium derivative of 3-hydroxypyridine crystals (ca.80% of D-atoms) measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 5.
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When some exciton coupling energies for the hydrogen bonds from one unit cell are negligible, the number of different subbands in the crystal spectra strongly reduces. As generation mechanisms of the nX–H bands in vibrational spectra of centrosymmetric hydrogen bond dimers (e.g. (COOH)2 systems of carboxylic acid dimers) may involve mechanisms of dynamic coupling, enabling promotion of symmetry-forbidden transitions in the IR [25], even forbidden transitions may effectively contribute to the nO–H band contour. Calculations of the nO–H and nO–D band contours from the spectra of the crystal were performed in the limits of the ‘‘strong-coupling’’ model in the hydrogen bond dimer approximation. The method utilized was identical to those in [8,9,11]. According to the formalism of the theory, the band contour shapes depend on the set of three coupling parameters: bH (bD), C0 and C1. These are not variation parameters in the theory, as they have precisely defined physical meanings: The distortion parameter of a hydrogen (or deuterium) bond bH (or bD) denotes change in the O O bond equilibrium geometry, accompanying excitation of the proton (deuteron) stretching vibration nO–H (nO–D) to its first excited level. The bH and bD parameter values satisfy, within the simplest version of the ‘‘strongcoupling’’ theory, the following relation: bH pffiffiffi ¼ 2 bD The resonance interaction parameters C0 and C1 for the vibrationally excited hydrogen (or deuterium) bonds of a dimer are consecutive terms of the resonance interaction integral C, for a pair of dimeric hydrogen bonds. The constant term C0 can be estimated from the splitting of the component subbands of the dimer spectrum, corresponding to the transitions to proton (deuteron) vibrational states of different symmetry. The C1 parameter is the developing coefficient in the linear term with respect to the totally symmetric coordinate Q1 of the low-energy O O bond stretching vibrations of the dimer. This parameter is responsible for differentiation of the component subband widths in the dimer spectra [8,9,11]. For the deuterium bonded dimer spectra, the C0 and C1 parameter values p should decrease by a factor within ffiffiffi the range of 1.0– 2 [8,9,11]. The results of our model calculations for the nO–H band contour shapes of 3-hydroxypyridine crystal are
shown in Fig. 8. The nO–D band contour shape deduced from the nO–H band calculation results is shown in Fig. 9. Our calculation suggests that each band is a superposition of two component bands, which on the ‘‘strong-coupling’’ theory were assigned as ‘‘plus’’ and ‘‘minus’’ [8,9,11]. The longer-wave branch of each spectrum is reproduced by the dimeric ‘‘minus’’ band, which according to the theory corresponds to the symmetry-forbidden excitation of the totally symmetric proton vibrations in the dimer [8,9,11]. The shorter-wave branch of each nO–H band is reproduced by the dimeric ‘‘plus’’ band, corresponding to the dipole-allowed excitation of the non-totally symmetric proton vibrations in the dimer [8,9,11]. Each nO–D band is satisfactorily reproduced solely by the ‘‘plus’’ band, corresponding to the symmetry allowed transition [8,9,11]. The ‘‘minus’’ band makes essentially no contribution to the analyzed band contours. A complex structure of the nO–H band also finds its confirmation in the temperature-induced changes in the band (see Figs. 5 and 6). The temperature affects the two spectral branches, however, in a different way. For the longer-wavelength branch of the nO–H band a ca. two-fold increase of intensity can be observed, compared to the shorter-wavelength branch integral intensity. For the shorter-wavelength branch, some relatively weak temperature effects can be noticed, indicating a more complex nature of this spectral branch. 6.3. Linear dichroic effects The linear dichroic effects, as observed in the nO–H band frequency range of the 3-hydroxypyridine crystal spectra, are very characteristic: this band exhibits a doublet structure, composed of two well-separated spectral branches (their centers of gravity located at 2450 and 1900 cm1, respectively). The splitting magnitude of the nO–H band (see Figs. 5 and 6) suggests the existence of a strong vibrational exciton coupling in the crystal, although the two branches display almost identical polarization properties. The dichroic properties of these bands simply result from the hydrogen bond orientations in the crystalline lattice: the integral intensity ratio of the polarized component bands in the solid-state spectra
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12.00
3000
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Intensity
III
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0.00 8.00
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_
hω O---N Fig. 8. Theoretical reconstitution of the most intense nO–H band component from the low-temperature spectra of 3-hydroxypyridine crystals. (I) The ‘‘plus’’ dimeric band reconstituting the symmetry-allowed transition; (II) the ‘‘minus’’ dimeric band reproducing the forbidden transition; and (III) the superposition of the ‘‘plus’’ and ‘‘minus’’ bands taken with their statistical weight parameters Fþ and F. The coupling parameter values: bH ¼ 1:8, C0 ¼ 3:0, C1 ¼ þ0:3, F þ ¼ 1:0, F ¼ 0:9, OOO ¼ 90 cm1. The experimental spectrum is presented at the right upper edge of the picture.
(see Figs. 5 and 6) is approximately proportional to the ratio of squares of the direction cosines of the O N vector, calculated in relation to the electric field E vector direction in the experimental conditions. A lack of differences of the polarization properties between the two branches of the nO–H band makes difficult a detailed analysis of the crystal spectra. The peculiar dichroic properties of the spectra directly result from the chain arrangement of the hydrogen bonds in the lattice and from the chain geometry. To a good approximation, neighboring hydrogen bonds from one chain are parallel (the angle between the O N vectors is 38). This fact may be helpful in
explanation of identical vibrational transition moment directions, characterizing the longer- and shorterwavelength branches of the nO–H band in the 3-hydroxypyridine crystal spectra. In our discussion of the dichroic effects, a solution of the two-branch spectrum generation mechanism for the nO–H band seems to be crucial. In this task, the parallel arrangement of all hydrogen bonds is a substantial problem. A considerable difference of the branch frequencies, approximately equal to 550 cm1, is most probably connected with the strongest hydrogen bond exciton interactions, concerning the most closely spaced hydrogen bonds from one unit cell.
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10.00
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II
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_
hωO---N Fig. 9. Theoretical prediction of the most intense nO–D band component from the low-temperature spectra of 3-hydroxypyridine crystals. (I) The ‘‘plus’’ dimeric band; (II) the ‘‘minus’’ dimeric band reproducing the forbidden transition. The coupling parameter values: bD ¼ 1:3, C0 ¼ 3:0, C1 ¼ þ0:3, F þ ¼ 1:0, F ¼ 0:0, ONO ¼ 90 cm1. The experimental spectrum is presented at the right upper edge of the picture.
In this approach, the influence on the spectra of the weak and non-conventional hydrogen bonds CH O (see Table 1) seems to be of second order and, therefore, negligible in our considerations. At this point, it must be decided whether the strongest exciton interaction concerns the neighboring hydrogen bonds from one chain, or connects hydrogen bonds belonging to two different chains (see Fig. 3). The temperature effects in the spectra strongly support the latter option. The intensity of the longer-wavelength branch of the nO–H band increased when the temperature decreased, whereas the intensity of the shorter-wavelength branch did not undergo such strong changes with the decreasing temperature. Such behavior is typical of centrosymmetric hydrogen bond dimers with a side-to-side placement of the hydrogen bonds in a dimer [8,9,11,17,18]. In the case of strongest interactions between adjacent hydrogen bonds in one chain, the temperature effect on the spectra would be of opposite character; the shorter-wavelength branch, ascribed to the symmetry forbidden transition
for non-totally symmetric proton vibrations in a chain (the hydrogen bonds in one chain are almost parallel), would be most temperature-affected. Therefore, the longer-wavelength branch of the 3-hydroxypyridine crystal spectrum is most probably associated with the symmetry-forbidden transition in the IR. It is connected with the excitation of the totally symmetric proton vibrations in centrosymmetric hydrogen bond dimers, formed by hydrogen bonds belonging to adjacent chains. Such a transition would become allowed thanks to a vibronic promotion mechanism, considered as a reversal of the familiar Herzberg-Teller effect from the electronic molecular spectroscopy [26]. The above-presented interpretation is supported by the isotope effect in the solid-state spectra (see Fig. 7). The nO–D band in the spectra of the deuterium derivative of 3-hydroxypyridine is located at ca. 1900 cm1, overlapping with the longer-wavelength branch of the ‘‘residual’’ nO–H band. This nO–D band pffiffiffi frequency is about 2 times lower than the nO–H band
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higher-energy branch frequency (see Figs. 5 and 6), which represents a regular H/D isotopic effect, usually observed in the vibrational spectroscopy. However, the nO–D band is a singlet, with no second spectral branch corresponding to the lower-frequency branch of the nO–H band; in the nO–D band the longer-wavelength branch is extinguished. For centrosymmetric hydrogen bond dimers, the lower-energy transition in the spectra is forbidden by symmetry rules. The probability of this transition and also the longer-wavelength branch intensity for each band, namely the nO–H and nO–H bands, depends on the electronic properties of hydrogen bonds in the dimers. The vibronic mechanism responsible for promotion of the forbidden transition will also strongly depend on the hydrogen atom masses in the hydrogen bonds [25]. This latter factor can explain the strong diminution of the forbidden transition intensity in the dimer system spectra and in some cases it explains its total extinction [11,18], as observed in the nO–D band frequency range for 3-hydroxypyridine (see Fig. 7). 6.4. Isotopic dilution effects in the crystalline spectra The H/D isotope effects in the solid-state spectra of 3-hydroxypyridine are relatively complex. Besides, the above-mentioned relation between the nO–D and nO–H band structures, a very substantial isotope effect concerns the nO–H ‘‘residual’’ band properties, measured for samples characterized by high deuterium content (see Fig. 7). Clearly, the loss of the protons in a sample by deuteration is responsible for proportional diminution of the nO–H ‘‘residual’’ band intensity. This relation is valid for the shorter-wavelength branch of the band. However, the nO–H and nO–D bands meet at a frequency near 1900 cm1; in this frequency range the spectrum intensity is not simply proportional to the proton concentration, as protons and deuterons introduce opposite trends. An increasing concentration of deuterons in a crystalline sample is accompanied by the appearance of a relatively narrow band at ca. 2130 cm1. This new band is characterized by identical polarization properties to those of the nO–H band. Its intensity follows the proton concentrations in a sample. Furthermore, the pffiffiffinew band frequency is not related by the factor 2 to any of the nO–H band spectral branch frequencies. The narrow band
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frequency corresponds almost to the average frequency of the gravity centers of the nO–H band spectral branches. Thus, exciton interactions between the OH N hydrogen bonds play no essential role in generating this band, as these bonds are effectively separated by the deuterium bonds. 6.5. Distribution of protons and deuterons in the lattice The spectra of isotopically diluted samples of 3hydroxypyridine indicate that distribution of protons and deuterons within the chain system of the hydrogen bonds is totally random. A similar conclusion is valid for the hydrogen isotope distribution over the hydrogen bonds of the adjacent chains in the lattice. This means that in this case no so-called isotopic ‘‘selforganization’’ effects exist, in contrast to some other dimeric hydrogen bond systems [7,17,18], or for some crystals with chains of hydrogen bonds in their lattices, e.g. for 4-mercaptopyridine [27], pyrazole [28], imidazole [29], or for the high-temperature form of 4-hydroxypyridine [29]. It was suggested recently that ‘‘self-organization’’ effects in the IR spectra of the hydrogen bond are most probably a result of a mechanism that generates additional attracting forces between identical hydrogen isotope atoms, in systems of mutually coupled hydrogen bonds. The new kind of co-operative interaction mechanism in the hydrogen bond area seems to be generally of a dynamic, vibronic nature [16]. This statement is supported by the observed strong hydrogen atom mass-dependence of this effect. The ‘‘self-organization’’ energy effects are not accessible even using the most sophisticated calculation methods of contemporary quantum chemistry. They find no counterpart even in the latest monographs concerning hydrogen bonds [1–5,30,31]. In the spectra of 3-hydroxypyridine crystals, the isotopic ‘‘self-organization’’ effects are not observed. With an increasing deuterium content, the doublet structure of the nO–H band transforms towards a singlet band, located exactly in the center of the doublet. This line correlates with isolated OH N hydrogen bonds, not coupled with other, similar hydrogen bonds in the lattice. This isotope effect is very similar to those observed in the case of the ethanol crystal spectra, where those effects were ascribed to a fully random distribution of protons and deuterons in chains
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of OH O hydrogen bonds [19,20]. Also, the spectra of the low-temperature crystalline phase of 4-hydroxypyridine indicate no isotopic ‘‘self-organization’’ effects. These results will be published elsewhere. 6.6. Possible nature of the ‘‘self-organization’’ mechanisms for chain systems Detailed analysis of the all cases where the ‘‘selforganization’’ H/D isotopic effects were detected in the crystalline spectra [26–28], in contrast to the 3hydroxypyridine and the ethanol crystal [19,20] cases, might facilitate proposing a hypothesis concerning the mechanisms of these processes. It seems that structural factor is crucial in these mechanisms, which are most probably of a vibronic nature. In the ‘‘self-organization’’ mechanisms, a prominent role is played by couplings between the proton stretching vibrations and electronic motions in the hydrogen bonds, p-electrons of the associated molecules included [25]. When the proton-donor atoms are directly connected to a p-electronic system, such coupling may be effective, in consequence becoming a source of the ‘‘self-organization’’ effects in the spectra. In such a case, a totally symmetric and coordinated movement of protons, coupled with electron systems of the hydrogen bonds in a chain, might be responsible for appearance of some relatively weak attraction forces between identical hydrogen isotope atoms from the neighboring hydrogen bonds [16]. In the case of solid-state 3-hydroxypyridine, the protons of the hydrogen bonds are separated from the pyridine aromatic rings by oxygen atoms. Thus, the vibronic mechanisms become much less effective. For ethanol crystals, with no electrons in p-orbitals, such mechanism is also expected to be ineffective. All these analyzed spectral effects may be helpful in elaboration in the future of a general theory of ‘‘selforganization’’ effects in the IR spectroscopy of hydrogen bonded crystalline systems.
7. Conclusion Our investigations of the IR spectra of the hydrogen bond in 3-hydroxypyridine crystals suggest a lack of ‘‘self-organization’’ effects in the spectra of samples
characterized by relatively high deuterium content in the hydrogen bonds. Such effects were previously found for some solid-state systems with a chain arrangement of hydrogen bonds in their lattices. The spectra of deuterium-substituted 3-hydroxypyridine crystals demonstrate with high probability a random distribution of protons and deuterons among the hydrogen bonds. The search for a correlation between spectral properties of a number of crystalline systems (namely, the ‘‘self-organization’’ effects in the spectra), with molecular structures of the hydrogen-bonded molecules suggests that such a correlation is valid in terms of a simple vibronic model. These effects seem to be an attribute of this particular kind of hydrogen bonds, in which the proton-donor atoms are directly attached to p-electronic systems and the proton-acceptor atoms also belong to the same system.
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