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Polarization IR spectra of hydrogen bonded 1-naphthoic acid and 2-naphthoic acid crystals: electronic effects in the spectra
Journal of Molecular Structure 659 (2003) 103–117 www.elsevier.com/locate/molstruc
Polarization IR spectra of hydrogen bonded 1-naphthoic acid and 2-naphthoic acid crystals: electronic effects in the spectra Henryk T. Flakus*, Michal⁄ Chel⁄mecki Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl 40-006 Katowice, Poland Received 18 March 2003; revised 31 July 2003; accepted 5 August 2003
Abstract This article deals with measurements and a theoretical interpretation of the polarization IR spectra of the hydrogen bond in h8-1-naphthoic and h8-2-naphthoic acid crystals and crystals of their deuterium isotopomers: d7-1-naphthoic acid and d7-2naphthoic acid. Polarization spectra were measured at the room temperature and at 77 K. Similarly as for other carboxylic acid crystal cases, the intensity distribution in the bands may be discussed on the basis of the strong-coupling model, when assuming that the isolated (COOH)2 and (COOD)2 cycles determine basic spectral properties of hydrogen bonds in the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. Another band shaping mechanism, i.e. a vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations, in the centrosymmetric dimers of hydrogen bonds, was also considered. It was shown that for 1-naphthoic acid crystal spectra the promotion mechanism is ca. two times weaker than in the case of 2-naphthoic acid crystal. This relation was found independent of the hydrogen isotopes linked with the aromatic rings. A new kind of the long-range isotopic effects H/D in the spectra was indicated, depending on the influence of the aromatic ring hydrogen atoms in the nO – H band fine structure patterns. The role of the aromatic ring electronic properties was also discussed, in order to explain reasons of extremely effective promotion of the forbidden transition, as well as of the Fermi resonance impact on the crystalline spectra. q 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen bond; Molecular crystals; Polarization spectra in the IR; Strong-coupling model; Band shape analysis; Linear dichroism effects; Temperature effects; Fermi resonance effect; Long-range H/D isotopic effects
1. Introduction The vibrational spectroscopy in the IR is still considered to be a basic tool in studies of the unique dynamics of the hydrogen bond forming atoms [1 – 6]. The most spectacular spectral effects attributed to creation of the X –H· · ·Y hydrogen bonds include: (i) * Corresponding author. Fax: þ 48-32-2599-978. E-mail address: [email protected] (H.T. Flakus).
an increase of the X – H bond stretching vibration band integral intensity (the vibration symbol nX – H ); (ii) the low-frequency shift of the band and (iii) a considerable increase of the band width. Sometimes the nX – H bands exhibit well-developed fine structure patterns [1 –4]. These band fine structure patterns are generated by the strong anharmonic coupling mechanism, in which the most prominent role is played by the high frequency proton stretching vibrations nX – H
0022-2860/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.08.007
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and by the low frequency, X –H· · ·Y hydrogen bond stretching vibrations nX· · ·Y [1 – 4]. Over recent 60 years these spectral phenomena were subject of interest for a number of generations of researchers: experimentalists and theoreticians. Most of the proposed theoretical models were only qualitative models, including the Fermi resonance model, the most popular in that group of theories [7]. An only exception seems to be a 30-years old, fully quantitative model, the so called strong-coupling model, which appeared to be in power to successfully describe spectral properties of not only single hydrogen bond systems, but also of mutually interacting hydrogen bond systems [8 – 11]. Such an approach allowed quantitatively interpreting not only the spectra of simple molecular aggregates like the carboxylic acid dimers, formed in the gaseous phase [12 –14], but also the spectra of large and complex systems like hydrogen bonded molecular crystals [15 – 19]. For these latter systems, on the basis of the strong-coupling model, even complex effects, such as isotopic, temperature and polarization effects in their spectra, were successfully analyzed [17,19]. The latest quantitative model, the so-called relaxation, or the linear response theory of IR spectra of the hydrogen bond [20], has also incorporated the strong-coupling mechanism in its most recent version of the formalism, which allowed to formulate a general theoretical model of spectra of hydrogenbonded systems [21 –23]. Unfortunately, so far in the literature no works have been reported, based on the relaxation theory and devoted to a quantitative interpretation of IR experimental spectra of particular molecular crystals. Probably, this fact might be partially explained by a relatively small number of spectral studies reported, presenting results of polarization spectra measurements, performed for hydrogen bonded solid-state systems [24 – 29]. Among experimental methods of the hydrogen bond research, the IR spectroscopy of hydrogen bonded crystals in the polarized light, seems to be of a particularly great importance. This way of measuring of the spectra is potentially able to deliver experimental data, allowing for a deepest insight into the mechanisms of dynamical interactions, considered by modern theoretical models as the essence of the hydrogen bond nature. On the other hand, this kind of the advanced spectral experiment is a challenge for
the theory, stimulating its further development. Nevertheless, the strong-coupling mechanism cannot be treated as the only nX – H band shaping mechanism. Recent studies of the crystalline spectra indicated more fine mechanism influences onto the hydrogen bond spectra in the IR, including the vibronic couplings in the hydrogen bond systems. When investigating the spectral properties of hydrogen bonded systems, the carboxylic acids, the mono- as well as of the dicarboxylic ones, were frequently chosen as proper model systems [9,12 –14, 17,19,28,29]. The IR spectra of the associated carboxylic acid systems used to exhibit well-developed fine structures of the nO – H and nO – D bands, strongly susceptible to the influence of temperature and of intermolecular interactions attributed to the condensation states of the matter. As spectral properties of the hydrogen bonds in the (COOH)2 and (COOD)2 cycles are relatively well known, carboxylic acid crystals can be treated as proper model systems for studies of more fine spectral phenomena, accompanying the basic nO – H and nO – D band generation mechanisms. Most of theoretical models, elaborated for describing of the hydrogen bond IR spectra, had treated this problem as purely vibrational. However, at this moment such approach seems fairly non-sufficient, as recent studies have proved that vibronic couplings in hydrogen bonded systems influence in a nonnegligible degree their IR spectral properties. This statement is based on a number of newly identified spectral effects, such as: (I) vibrational selection rules breaking effects for centrosymmetric dimeric systems [30]; (II) some long-range H/D isotopic effects in the hydrogen bond IR bond spectra [31]; or (III) some socalled self-organization effect in hydrogen and deuteron bond systems [32]. It seems therefore, that the problem of relation between electronic properties of associating molecules and onto the spectral properties of their hydrogen bonds, is being far from a full understanding, also due to lack of systematic experimental studies in this area. In this article we present the results of experimental studies, as well as of theoretical analyses, of the 1naphthoic acid and 2-naphthoic acid crystal polarization IR spectra. In crystals of the 1-naphthoic acid and 2-naphthoic acid molecules form dimers, linked by the (COOH)2 or the (COOD)2 cycles [33 –35].
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These crystalline systems provide spectra in the nO – H and nO – D band frequency ranges, considerably differing by the intensity distribution from the spectra of other carboxylic acid crystals, especially of those with saturated hydrocarbon chains in their molecules (e.g. the adipic [28,29] and glutaric acid crystals [19]). Undoubtedly, in this way an influence of the aromatic naphthyl rings on the spectral properties of the crystals is manifested. It would seem that the two isomeric molecular systems should only slightly differ with regard to IR spectral properties attributed to their hydrogen bonds. However, such suggestion is simply based on our chemical intuition. Differences in the spectral properties might be a result of difference magnitudes in electronic coupling effect magnitudes for these two systems, occurring in the associated carboxyl groups and naphthalene rings, substituted in the positions ‘1’ or ‘2’. Experimental studies of polarization IR spectra for 1-naphthoic acid and 2-naphthoic acid crystals should provide most complete data, allowing estimation of the electronic coupling effects in the IR spectra of aromatic carboxylic acid crystals. Some elements of coupling mechanisms might be recognized on such basis, concerning coupling between proton stretching vibrations and aromatic ring electrons. Such approach has found no proper counterpart in the literature.
2. Crystal structures of 1-naphthoic acid and 2-naphthoic acid 2.1. 1-Naphthoic acid crystal Crystals of 1-naphthoic acid are monoclinic, space 5 b¼ symmetry group is P21 =a ¼ C2h ; a ¼ 31:12ð10Þ A; 3:87ð1Þ A; c ¼ 6:92ð2Þ A and b ¼ 92:2ð2Þ8; ðZ ¼4Þ [33,35]. In each unit cell there are two centrosym˚ metric hydrogen-bonded dimers, linked with 2.58 A long hydrogen bonds. Crystals form colourless plates elongated along the a-axis, with the ac (010) plane developed. Melting point is equal to 161 8C. 2.2. 2-Naphthoic acid crystal Crystals of 2-naphthoic acid are monoclinic, space5 b¼ symmetry group P21 =n ¼ C2h ; a ¼ 30:59ð10Þ A;
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c ¼ 5:63ð2Þ A and the angle b ¼ 92:6ð2Þ8; 5:00ð2Þ A; ðZ ¼ 4Þ [34,35]. In each unit cell there are two centrosymmetric hydrogen bonded dimers, linked ˚ long hydrogen bonds. Crystals form with 2.54 A oblong plates, with the ab (001) plane developed. Crystalline plates elongate along the b-axis. Melting point is equal to 185 8C.
3. Experimental Samples of h8-1-naphthoic acid and h8-2naphthoic acid were taken as commercial chemicals (Aldrich). Their deuterium d7 -isotopomers (C10D7COOH) were synthesized starting from naphthalene C10D8 (Sigma-Aldrich), by utilizing familiar methods, developed in the past for synthesis of 1-naphthoic acid and 2-naphthoic acid [36 –40] and then purified by re-crystallization. The purity of samples of d8-1-naphthoic acid and of d8-2-naphthoic acid obtained in such a way was estimated as being approximately equal to 95% in the individual case of each isomer. It was also estimated that monocrystalline fragments obtained from melt were of a higher purity and no single crystals containing the both naphthoic acid isomers have been found. Deuterium bonded samples were obtained by evaporation of D2O solution of each compound under reduced pressure. Single crystals proper for the spectral studies were obtained by crystallization of molten substances between CaF2 windows. Other experimental details were described earlier [19]. For 1-naphthoic acid crystals we found using a diffraction method, that they develop the ac (010) crystalline face in the experimental conditions. In similar circumstances of the crystal growth 2naphthoic acid crystals develop the ab (001) plane. For 1-naphthoic acid crystals the spectra were measured for E parallel to the cp -axis and also for E parallel to the a-axis. In the case of 2-naphthoic acid crystals the spectra were measured for E parallel to the a-axis, as well as for E parallel to the b-axis (E is the electric field vector of the incident light and cp symbol denotes a vector in the reciprocal lattice).
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4. Results Spectra of h8-1-naphthoic and h8-2-naphthoic acid samples measured in the CCl4 solution are shown in Fig. 1. Spectra of the polycrystalline samples of the two isomers of naphthoic acid, dispersed in the KBr pellets, measured at the two different temperatures are shown in Fig. 2. One can find considerably strong temperature induced evolution of the nO – H band contour shapes. To identify the nC – H bands
and to estimate the impact of these bands on the crystalline nO – H bands, the Raman spectra of the polycrystalline samples of h8-1-naphthoic and h8-2-naphthoic acid were measured at the room temperature. It allows recognition of the nC – H band influence as rather small, restricted to a narrow range close to the frequency of 3070 cm21, at the short-wave edge of the nO – H band. Fig. 3 presents the polarization spectra of h8-1naphthoic and h8-2-naphthoic acid crystals, measured at room temperature and in Fig. 4 one can see
Fig. 1. The nO – H bands in the IR spectra of h8-1-naphthoic acid and h8-2-naphthoic acid dissolved in CCl4 : a. h8-1-naphthoic acid, b. h8-2naphthoic acid.
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Fig. 2. The nO – H bands in the IR spectra of the polycrystalline samples of h8-1-naphthoic acid and h8-2-naphthoic acid, 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: a. h8-1-naphthoic acid, b. h8-2-naphthoic acid.
the corresponding spectra, measured at 77 K. The spectra show noticeably strong linear dichroic effects, in the nO – H band frequency range. Low temperature polarization spectra of solid-state d7h-1-naphthoic and d7h-2-naphthoic acids in the nO – H band frequency range are presented in Fig. 5. In Fig. 6 the polarization spectra of the deuterium bonded h7d-1-naphthoic and h7d-2naphthoic acid crystal are shown for the nO – D frequency range and in Fig. 7 the low-temperature polarization spectra are presented. In Fig. 8 the low-temperature polarization spectra of the deuterium bonded d8-1-naphthoic
and d8-2-naphthoic acid crystals were shown for the nO – D frequency range. In Fig. 9 the low-temperature polarization crystalline spectra of h8-1-naphthoic and h8-2naphthoic acid are shown, obtained for highly deuterated samples (ca. 90% of the D-atoms), measured for the frequency range of the residual nO – H bands. These studies aimed to find changes in the nO – H band shapes, accompanying the isotopic dilution with deuterium of the hydrogen bonds. The changes might be ascribed to disappearance of the exciton couplings between the (COOH)2 cycles in each unit cell.
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Fig. 3. The polarized spectra of h8-1-naphthoic acid and h8-2-naphthoic acid, crystals measured at room temperature in the nO – H band frequency range: a. Spectra of the h8-1-naphthoic acid crystal. 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 a-axis; (II) E perpendicular to the a-axis ( parallel cp -axis); (III) Spectrum (II) re-normalized to full scale with spectrum (I). b. Spectra of the h8-2-naphthoic acid crystal. The beam of normal incidence with respect to the ab crystalline face. (I) E parallel to the a-axis; (II) E perpendicular to the a-axis (parallel to the b-axis); (III) Spectrum (II) re-normalized to full scale with spectrum (I). Spectra (I) and (II) were drawn on a common scale.
5. Discussion 5.1. Differences in spectral properties of 1-naphthoic acid and 2-naphthoic acid crystals The presented above IR solid-state spectra of the two isomeric molecular systems, i.e. 1-naphthoic and 2-naphthoic acid, exhibit some noticeable differences, concerning nO – H band shapes, temperature,
polarization and H/D isotopic effects. These differences seem to be too large, to be connected solely with purely vibrational mechanisms of spectra generation. The differences between the spectra of two crystalline systems also do not result directly from their different structures, as centrosymmetric dimeric molecular units in the lattices most probably are the source of the basic crystal spectral properties, similarly as for other carboxylic acid crystals. Therefore, let us
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Fig. 4. The n O –H bands in the polarized spectra of h8-1-naphthoic acid and h8-2-naphthoic acid crystals measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
analyze in detail similarities and differences in spectral properties of the two systems. Our discussion must be related to the polarization spectra of the benzoic acid crystals, that were measured recently [41], also to the available polarization spectra of aliphatic dicarboxylic acid crystals [17,19,28,29]. In all these cases identification and confirmation of the main mechanism, responsible for generation of the nO – H and nO – D band fine structures, based on a model taking into the account a strong anharmonic coupling of the proton (or the deuteron) stretching vibrations, with the low-frequency hydrogen bond stretching vibrations nO· · ·O [2 – 4,8,9]. It was
also shown recently, that generation of a two-branch structure of nO – H and nO – D bands results from activation of spectrally non-active totally symmetric proton vibrations for centrosymmetric dimers, composed with O – H· · ·O hydrogen bonds in the associated carboxyl groups. These mechanisms were considered responsible for polarization and temperature effects, differentiating properties of the nO – H band spectral branches [30]. Comparison of the solid-state spectra, with the corresponding spectra of the compounds dissolved in CCl4 suggest, that centrosymmetric dimers of the hydrogen bonds are the structural units in the crystals,
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Fig. 5. The nO – H bands in the polarized spectra of d7h-1-naphthoic acid and d7h-2-naphthoic acid crystals measured at 77 K: d7a. d7h-1naphthoic acid, d7b. d7h-2-naphthoic acid. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
responsible for the crystal spectral properties (Figs. 1 and 2). Similarly as for the benzoic acid crystal case [41], the nO – H and the nO – D band contour shapes may be quantitatively reproduced by the model calculations, performed within the limits of the strongcoupling model [8,9,16] in a dimer approximation. In the case of the crystal spectra generation mechanisms, vibrational exciton couplings between (COOH)2, or (COOD)2 groups, are most probably only secondary interactions, responsible for very weak Davydowsplitting effects in the nO – H and the nO – D bands.
Within the strong-coupling theory, each experimental nO – H or nO – D band may be considered as a superposition of two component bands of a different origin. Each component band forms one separate branch in each analyzed band. In the spectra of centrosymmetric hydrogen bond dimers, the shorterwave branch of each band corresponds to the excitation of the non-totally symmetric proton vibrations in the dimers [9,16,30]. The longer-wave branches in the spectra correspond to the totally symmetric proton vibrations. However, these
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Fig. 6. The polarized spectra of h7d-1-naphthoic acid and h7d-2-naphthoic acid, crystals measured at room temperature in the nO – D band frequency range. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
vibrations should be spectrally non-active by the symmetry rules. Nevertheless, these become active via a promotion mechanism, which is basically a vibronic one [30]. This promotion mechanism, considered as a kind of reversion of the familiar ‘Herzberg –Teller’ effect, known from the electronic spectroscopy of aromatic molecular systems, is responsible for temperature, polarization and also for some H/D isotopic effects in spectra of carboxylic acid crystals [17,19,30]. The observed differences in the spectral properties of the two isomeric crystalline systems mainly depend
on different relative integral intensities of the component bands, forming the longer- and the shorter-wave branches of the corresponding nO – H and nO – D bands. In the case of the h8- and d8-2naphthoic acid crystals, the longer-wave branches, generated by the forbidden transitions, are of generally higher relative intensity, in comparison with the corresponding component band properties from the spectra of the h8- and d8-1-naphthoic acid crystals. For the 2-naphthoic acid crystal spectra a strong temperature induced growth of the lowerfrequency branch of the nO – H band can also be
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Fig. 7. The nO – D bands in the polarized spectra of h7d-1-naphthoic acid and h7d-2-naphthoic acid crystals measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
noticed. With regard to the spectral properties the 2naphthoic acid crystals fairly resemble spectral properties of benzoic acid crystals [41]. One might ask about the magnitudes of the promotion effect for the two naphthoic acid crystals. A most probable cause of the discussed spectral effects is connected with different electronic properties of the 1- and 2- substituted naphthalene rings. Different electron populations of the naphthalene ring system at the 1- and 2- positions are considered to be responsible for differentiation of the chemical reactivity at the two places of naphthalene rings. It might be therefore expected that electronic properties of
the 1- and 2- substituted naphthalene rings influence the promotion mechanism of the forbidden vibrational transitions for centrosymmetric hydrogen bond dimers. One can expect that carboxyl groups effectively withdraw electronic charge from the aromatic rings. For the naphthalene ring system this charge withdrawal in a different way concerns the position 1, in comparison to the position 2. This effect used to be stronger for the position 2. This statement was supported by ab initio calculations aiming to estimate charges on the atoms in hydrogen-bonded molecules of 1-naphthoic acid and of 2-naphthoic acid.
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Fig. 8. The nO – D bands in the polarized spectra of d8-1-naphthoic acid and d8-2-naphthoic acid crystals measured at 77 K: d7a. D8-1-naphthoic acid, d7b. D8-2-naphthoic acid. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
The calculations were performed using the GAUSSIAN 98 series of programs [42] at the SCF and at the DFT level, for the 6-31G basis. A full optimization of the dimer geometry was assumed. Our calculations have shown that in the case of 2-naphthoic acid the carboxyl group ca. 40% more effectively withdrew electrons from the naphthalene ring, in comparison with the 1-naphthoic acid case. This effects find its illustration in the calculated Mulliken charges concerning the carboxyl group and the naphthyl ring atoms. At the SCF level, for 1-naphthoic acid an excessive negative charge of 0.012 elementary electron charge has been estimated,
when for 2-naphthoic acid the corresponding value was 0.016. At the DFT level the corresponding values were equal to 0.089 and 0.096, respectively. When the electron charge from the aromatic ring is being strongly shifted towards carboxyl group, vibronic coupling mechanisms between the proton stretching vibrations and the hydrogen bond electrons, coupled with electrons in aromatic rings, may be partially strengthened. In this case a stronger coupling can influence a more effective promotion mechanisms of the forbidden transitions in the IR [30]. The selection rule breaking effect is expected to be stronger for the stronger electron withdrawal case,
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Fig. 9. The residual nO – H bands from the crystalline spectra of the partially deuterated samples of h8-1-naphthoic acid and h8-2-naphthoic acid (In each case the deuterium substitution rate was equal to ca. 90%). Other experimental conditions were identical to those from Fig. 3.
i.e. for 2-naphthoic acid crystal. In such a way different relative intensities of the longer-wave branches of the nO – H and nO – D bands may result for the spectra of the two crystalline systems. Therefore, in the spectra of 1-naphthoic acid the forbidden transition effect is considered as the weaker one, as the relative longer-wave nO – H band branch intensity is ca. two times lower when compared with the 2-naphthoic acid crystal spectrum case. Spectral properties of 1-naphthoic acid crystals fairly resemble corresponding spectral properties of the recently investigated dicarboxylic acid crystals. In the spectra of the latter systems the intensities of
the nO – H and nO – D band longer-wave branches are ca. two times lower in comparison with the shorter-wave branches [17,19]. Generally similar relation between the corresponding nO – H and nO – D band branch intensities are valid for the d7h-1-naphthoic and d7h-2-naphthoic acid crystals. 5.2. Temperature effects in the spectra Temperature effects in the spectra of 1-naphthoic and 2-naphthoic acid crystals basically depend on a growth of the relative intensity of the nO – H and
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the nO – D band lower-frequency branches. This effect is stronger for 2-naphthoic acid crystals. Similar temperature effects were observed in spectra of dicarboxylic acid crystals [17,19] and also in the spectra of benzoic acid crystals [41]. The temperature effects were previously ascribed to defects in crystal lattices, which were generated when obtaining crystals in non-equilibrium conditions, during crystallization from melt [17,19]. 5.3. Polarization effects in the spectra In the two cases (i.e. of 1-naphthoic and 2naphthoic acid crystals) the relative integral intensities of the polarized components of the nO – H and nO – D bands for each crystal remain in a good correlation with the crystal lattice geometry. The component band integral intensity ratio is approximately equal to the ratio of squares of the O – H bond direction cosines in associated COOH groups (dichroic effects of the first kind). In the spectra of the naphthoic acid crystals also another kind polarization effects can be observed (effects of the second kind). These latter effects depend on a slight diversification of the longer- and the shorter-wave branch polarization properties of the nO – H or nO – D bands for each isomer and for each isotopomer of naphthoic acid. These effects were observed in the spectra of hydrogen bonded dicarboxylic acid crystals [19,28,29]. In the low temperature spectra of benzoic acid crystals such effects were only slightly pronounced [41]. From our studies, it results that these effects are strongest in the lowtemperature spectra of 1-naphthoic acid crystals (see Figs. 4 and 8). The polarization effects of the second kind we ascribed to defects in the lattices, most likely generated during a relatively fast crystallization of molten substances when obtaining the solid-state samples [17,19]. 5.4. Fermi resonance influence on the spectra It is a very characteristic property of the nO – H bands that the fine structure patterns are irregular in their longer-wave parts and this irregularity takes a form reminding the so-called Evans holes [2,3], in the frequency ranges being close to 2700 cm21. In each nO – H band contour there are two better or worse
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pronounced Evans holes at frequencies equal to ca. 2750 and 2625 cm21. These effects are most easily observed in the dense fine structure pattern spectra of d7h-1-naphthoic, h8-2-naphthoic and d7h-2-naphthoic acid crystals. This effect does not disappear in the residual nO – H bands, measured for partially deuterated samples. In these all cases band deformations concern the longer-wave part of the band, generated by the symmetry forbidden excitation of the totally symmetric proton stretching motions in the hydrogen bond dimers [41]. Most probably, similarly as like in the case of benzoic acid crystal spectra [41], these effects result from the Fermi resonance between the proton stretching motions with the bending-in-plane proton vibration in their overtone states in the dimers (vibration symbol dO – H ). The Fermi resonance falls just on to the activated forbidden transition subband frequency range for the proton stretching vibrations. This resonance seems possible thanks to the Agsymmetry of the proton vibration excited state, which is responsible for the longer-wave nO – H band branch generation, where the Evans holes effects used to appear. The dO – H vibrations in their overtone state are also of the Ag-symmetry and thus the Fermi resonance becomes possible. Similar approach was proposed to quantitatively explain similar effects in the spectra of the benzoic acid crystals, when these effects were quantitatively reproduced in the frames of the strong-coupling model, incorporating between the proton stretching and bending motions in the dimers [41]. In the case of the spectra of the naphthoic acid crystals, Fermi resonance effects are much stronger, when compared with results of similar effects in the spectra of aliphatic acid crystals [17,19,28,29]. The Fermi resonance effects seem to be a common property of spectra of aromatic acid crystals. In these cases the Fermi resonance mechanisms seem to be very effective, which may be connected with stabilization of aromatic acid dimer structures due to the couplings of hydrogen bonds from the (COOH)2 cycles with aromatic rings, via the P-electrons. At this point it must be added that no such Evans holes effects were observed in the spectra or arylacetic acid crystals, e.g. phenylacetic [43], 1-naphthylacetic and 2-naphthylacetic acid crystals [44]. In these latter cases the (COOH) 2 cycles are separated from
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the aromatic by CH2 groups, which probably makes the coupling much less effective. Therefore, the Fermi resonance mechanisms for arylacetic acid dimers seem to be much weaker in comparison with the arylcarboxylic acid cases. It also seems worth to add that Evans hole effects were observed in spectra of hydrogen bonded trans-cinnamic acid crystals [45], that acts in support for the vibronic coupling source of these effects. 5.5. Isotopic dilution effects on the nO – H bands Residual nO – H bands, measured for isotopically diluted samples of the naphthoic acids, are characterized with fairly similar band shapes as the corresponding bands of the pure, non-deuterated compound crystals do. Some slight differences between the spectra compared are restricted to poorer fine structure of the residual nO – H bands and most probably result from vanishing of the correlation field interactions between dimers of naphthoic acids, due to the isotopic dilution by deuterium [32,46]. However, the isotopic dilution does not destroy a general scheme of the nO – H bands for the investigated crystals. The band structure still remains twobranched, with almost unchanged relation between integral intensities of the component bands, obtained for different polarization of the electric field vector of the beam used for measurement of spectra, with respect to the crystal lattice. The residual nO – H band fine structure patterns prove, that even for high deuterium substitution rates in the solid-state samples, centrosymmetric dimers of hydrogen bonds, with two protons in each (COOH)2 cycles, still remain the source of the crystal spectral properties. This non-traditional H/D isotopic effect, i.e. the self-organization effect in mixture systems of protons and deuterons, seems to be common for cyclic dimeric systems [32]. It depends on a non-random distribution of protons and deuterons, between the dimeric hydrogen bonds in partially deuterated samples. Most probably it results from more stable symmetric dimers, containing identical hydrogen isotopes in one dimer, when compared with the nonsymmetric dimer (i.e. containing simultaneously one hydrogen and one deuterium bond) properties [32,47]. This problem is still being intensively investigated.
6. Conclusion It results from our studies that some noticeable differences in spectral properties of the hydrogen bonds in the crystals of 1-naphthoic and 2-naphthoic acid can be identified. Generally, the crystals of 2naphthoic acid, of the h8 and of the d7h-types, exhibit considerably more intense longer-wave branches of their nO – H bands, when compared with the characteristics of the analogous spectral branches in the spectra of h8- and d7h-1-naphthoic acid crystals. This part of the crystal spectra corresponds to the activated by a vibronic mechanism, symmetry-forbidden excitation of totally symmetric proton stretching vibrations in centrosymmetric dimers of naphthoic acid. The observed differences in the forbidden band intensities may be ascribed to the efficiency of the promotion mechanism, for the forbidden vibrational transition activation mechanism. This is connected with differences in coupling mechanism of the vibrating hydrogen bond protons with electrons of the naphthalene rings through the carbon atoms in the 1 and 2 positions. Thus, the differences in spectral properties of 1- and 2-naphthoic acid crystals generally result from electronic properties of the 1- and 2-substituted naphthalene rings. Replacement of the hydrogen atoms in aromatic rings by the deuterium atoms does not change the relative intensity of the longer- to the shorter-wave branch of the nO – H bands in spectra of each naphthoic acid isomer. The only change in the nO – H band structure due to deuteration of aromatic rings, the long-range H/D isotopic effect, depends on a more dense band fine structure pattern for d7h-1- and d7h-2naphthoic acid crystal spectra. This effect corresponds with diminution of the nO· · ·O low-frequency hydrogen bond stretching vibration quanta. These effects are similar to those measured in the spectra of benzoic acid crystals [41]. In the nO – H band frequency ranges, in the all four investigated compound crystals, spectral effects appear, resembling so-called Evans holes, which may be ascribed to the Fermi resonance mechanisms. These resonances concern the coupling of the proton stretching vibrations in the 1- and 2-naphthoic acid dimers, with the proton bending-in-plane vibrations in their first overtone state. This resonance was considered recently as a main nX – H band fine structure
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modifying mechanism in the IR spectra of hydrogen bonded systems [48,49]. The extreme magnitude of the Evans hole effects inclines us to consider them as an attribute of the arylcarboxyl acid crystals.
References [1] G.C. Pimentel, A.L. McClellan, The Hydrogen Bond, W.H. Freeman, San Francisco, 1960. [2] P. Schuster, G. Zundel, C. Sandorfy (Eds.), The Hydrogen Bond, Recent Developments in the Theory and Experiment, Parts I, II and III, North-Holland, Amsterdam, 1976. [3] H. Ratajczak, W.J. Orville-Thomas (Eds.), Molecular Interactions, vol. I, Wiley, New York, 1980. [4] D. Hadz˙i (Eds.), Theoretical Treatments of Hydrogen Bonding, Wiley, New York, 1997. [5] P. Schuster, W. Mikenda (Eds.), Hydrogen Bond Research, Monatshefte fuˆr Chemie, Chemical Monthly, vol. 130/8, Springer, New York, 1999. [6] S.S. Xantheas (Eds.), 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. [7] S. Bratoz˙, D. Hadz˙i, J. Chem. Phys. 27 (1957) 991. [8] A. Witkowski, J. Chem. Phys. 47 (1967) 3645. [9] Y. Marechal, A. Witkowski, J. Chem. Phys. 48 (1968) 3697. [10] S.F. Fischer, G.L. Hofacker, M.A. Ratner, J. Chem. Phys. 52 (1970) 1943. [11] 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. [12] J.L. Leviel, Y. Marechal, J. Chem. Phys. 54 (1971) 1104. [13] J. Bournay, Y. Marechal, J. Chem. Phys. 55 (1971) 1230. [14] P. Excoffon, Y. Marechal, Spectrochim. Acta A 28 (1972) 269. [15] M.J. Wo´jcik, Int. J. Quantum Chem. 10 (1976) 747. [16] H.T. Flakus, Chem. Phys. 62 (1981) 103. [17] H.T. Flakus, J. Mol. Struct. (Theochem) 285 (1993) 281. [18] H.T. Flakus, A. Bryk, J. Mol. Struct. 372 (1995) 229. [19] H.T. Flakus, A. Miros, J. Mol. Struct. 484 (1999) 103. [20] O. Henri-Rousseau, P. Blaise, The infrared spectral 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, p. 1. [21] D. Chamma, O. Henri-Rousseau, Chem. Phys. 248 (1999) 53. [22] D. Chamma, O. Henri-Rousseau, Chem. Phys. 248 (1999) 71. [23] D. Chamma, O. Henri-Rousseau, Chem. Phys. 248 (1999) 91.
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Polarization IR spectra of hydrogen bonded 1-naphthoic acid and 2-naphthoic acid crystals: electronic effects in the spectra Henryk T. Flakus*, Michal⁄ Chel⁄mecki Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl 40-006 Katowice, Poland Received 18 March 2003; revised 31 July 2003; accepted 5 August 2003
Abstract This article deals with measurements and a theoretical interpretation of the polarization IR spectra of the hydrogen bond in h8-1-naphthoic and h8-2-naphthoic acid crystals and crystals of their deuterium isotopomers: d7-1-naphthoic acid and d7-2naphthoic acid. Polarization spectra were measured at the room temperature and at 77 K. Similarly as for other carboxylic acid crystal cases, the intensity distribution in the bands may be discussed on the basis of the strong-coupling model, when assuming that the isolated (COOH)2 and (COOD)2 cycles determine basic spectral properties of hydrogen bonds in the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. Another band shaping mechanism, i.e. a vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations, in the centrosymmetric dimers of hydrogen bonds, was also considered. It was shown that for 1-naphthoic acid crystal spectra the promotion mechanism is ca. two times weaker than in the case of 2-naphthoic acid crystal. This relation was found independent of the hydrogen isotopes linked with the aromatic rings. A new kind of the long-range isotopic effects H/D in the spectra was indicated, depending on the influence of the aromatic ring hydrogen atoms in the nO – H band fine structure patterns. The role of the aromatic ring electronic properties was also discussed, in order to explain reasons of extremely effective promotion of the forbidden transition, as well as of the Fermi resonance impact on the crystalline spectra. q 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen bond; Molecular crystals; Polarization spectra in the IR; Strong-coupling model; Band shape analysis; Linear dichroism effects; Temperature effects; Fermi resonance effect; Long-range H/D isotopic effects
1. Introduction The vibrational spectroscopy in the IR is still considered to be a basic tool in studies of the unique dynamics of the hydrogen bond forming atoms [1 – 6]. The most spectacular spectral effects attributed to creation of the X –H· · ·Y hydrogen bonds include: (i) * Corresponding author. Fax: þ 48-32-2599-978. E-mail address: [email protected] (H.T. Flakus).
an increase of the X – H bond stretching vibration band integral intensity (the vibration symbol nX – H ); (ii) the low-frequency shift of the band and (iii) a considerable increase of the band width. Sometimes the nX – H bands exhibit well-developed fine structure patterns [1 –4]. These band fine structure patterns are generated by the strong anharmonic coupling mechanism, in which the most prominent role is played by the high frequency proton stretching vibrations nX – H
0022-2860/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.08.007
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and by the low frequency, X –H· · ·Y hydrogen bond stretching vibrations nX· · ·Y [1 – 4]. Over recent 60 years these spectral phenomena were subject of interest for a number of generations of researchers: experimentalists and theoreticians. Most of the proposed theoretical models were only qualitative models, including the Fermi resonance model, the most popular in that group of theories [7]. An only exception seems to be a 30-years old, fully quantitative model, the so called strong-coupling model, which appeared to be in power to successfully describe spectral properties of not only single hydrogen bond systems, but also of mutually interacting hydrogen bond systems [8 – 11]. Such an approach allowed quantitatively interpreting not only the spectra of simple molecular aggregates like the carboxylic acid dimers, formed in the gaseous phase [12 –14], but also the spectra of large and complex systems like hydrogen bonded molecular crystals [15 – 19]. For these latter systems, on the basis of the strong-coupling model, even complex effects, such as isotopic, temperature and polarization effects in their spectra, were successfully analyzed [17,19]. The latest quantitative model, the so-called relaxation, or the linear response theory of IR spectra of the hydrogen bond [20], has also incorporated the strong-coupling mechanism in its most recent version of the formalism, which allowed to formulate a general theoretical model of spectra of hydrogenbonded systems [21 –23]. Unfortunately, so far in the literature no works have been reported, based on the relaxation theory and devoted to a quantitative interpretation of IR experimental spectra of particular molecular crystals. Probably, this fact might be partially explained by a relatively small number of spectral studies reported, presenting results of polarization spectra measurements, performed for hydrogen bonded solid-state systems [24 – 29]. Among experimental methods of the hydrogen bond research, the IR spectroscopy of hydrogen bonded crystals in the polarized light, seems to be of a particularly great importance. This way of measuring of the spectra is potentially able to deliver experimental data, allowing for a deepest insight into the mechanisms of dynamical interactions, considered by modern theoretical models as the essence of the hydrogen bond nature. On the other hand, this kind of the advanced spectral experiment is a challenge for
the theory, stimulating its further development. Nevertheless, the strong-coupling mechanism cannot be treated as the only nX – H band shaping mechanism. Recent studies of the crystalline spectra indicated more fine mechanism influences onto the hydrogen bond spectra in the IR, including the vibronic couplings in the hydrogen bond systems. When investigating the spectral properties of hydrogen bonded systems, the carboxylic acids, the mono- as well as of the dicarboxylic ones, were frequently chosen as proper model systems [9,12 –14, 17,19,28,29]. The IR spectra of the associated carboxylic acid systems used to exhibit well-developed fine structures of the nO – H and nO – D bands, strongly susceptible to the influence of temperature and of intermolecular interactions attributed to the condensation states of the matter. As spectral properties of the hydrogen bonds in the (COOH)2 and (COOD)2 cycles are relatively well known, carboxylic acid crystals can be treated as proper model systems for studies of more fine spectral phenomena, accompanying the basic nO – H and nO – D band generation mechanisms. Most of theoretical models, elaborated for describing of the hydrogen bond IR spectra, had treated this problem as purely vibrational. However, at this moment such approach seems fairly non-sufficient, as recent studies have proved that vibronic couplings in hydrogen bonded systems influence in a nonnegligible degree their IR spectral properties. This statement is based on a number of newly identified spectral effects, such as: (I) vibrational selection rules breaking effects for centrosymmetric dimeric systems [30]; (II) some long-range H/D isotopic effects in the hydrogen bond IR bond spectra [31]; or (III) some socalled self-organization effect in hydrogen and deuteron bond systems [32]. It seems therefore, that the problem of relation between electronic properties of associating molecules and onto the spectral properties of their hydrogen bonds, is being far from a full understanding, also due to lack of systematic experimental studies in this area. In this article we present the results of experimental studies, as well as of theoretical analyses, of the 1naphthoic acid and 2-naphthoic acid crystal polarization IR spectra. In crystals of the 1-naphthoic acid and 2-naphthoic acid molecules form dimers, linked by the (COOH)2 or the (COOD)2 cycles [33 –35].
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These crystalline systems provide spectra in the nO – H and nO – D band frequency ranges, considerably differing by the intensity distribution from the spectra of other carboxylic acid crystals, especially of those with saturated hydrocarbon chains in their molecules (e.g. the adipic [28,29] and glutaric acid crystals [19]). Undoubtedly, in this way an influence of the aromatic naphthyl rings on the spectral properties of the crystals is manifested. It would seem that the two isomeric molecular systems should only slightly differ with regard to IR spectral properties attributed to their hydrogen bonds. However, such suggestion is simply based on our chemical intuition. Differences in the spectral properties might be a result of difference magnitudes in electronic coupling effect magnitudes for these two systems, occurring in the associated carboxyl groups and naphthalene rings, substituted in the positions ‘1’ or ‘2’. Experimental studies of polarization IR spectra for 1-naphthoic acid and 2-naphthoic acid crystals should provide most complete data, allowing estimation of the electronic coupling effects in the IR spectra of aromatic carboxylic acid crystals. Some elements of coupling mechanisms might be recognized on such basis, concerning coupling between proton stretching vibrations and aromatic ring electrons. Such approach has found no proper counterpart in the literature.
2. Crystal structures of 1-naphthoic acid and 2-naphthoic acid 2.1. 1-Naphthoic acid crystal Crystals of 1-naphthoic acid are monoclinic, space 5 b¼ symmetry group is P21 =a ¼ C2h ; a ¼ 31:12ð10Þ A; 3:87ð1Þ A; c ¼ 6:92ð2Þ A and b ¼ 92:2ð2Þ8; ðZ ¼4Þ [33,35]. In each unit cell there are two centrosym˚ metric hydrogen-bonded dimers, linked with 2.58 A long hydrogen bonds. Crystals form colourless plates elongated along the a-axis, with the ac (010) plane developed. Melting point is equal to 161 8C. 2.2. 2-Naphthoic acid crystal Crystals of 2-naphthoic acid are monoclinic, space5 b¼ symmetry group P21 =n ¼ C2h ; a ¼ 30:59ð10Þ A;
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c ¼ 5:63ð2Þ A and the angle b ¼ 92:6ð2Þ8; 5:00ð2Þ A; ðZ ¼ 4Þ [34,35]. In each unit cell there are two centrosymmetric hydrogen bonded dimers, linked ˚ long hydrogen bonds. Crystals form with 2.54 A oblong plates, with the ab (001) plane developed. Crystalline plates elongate along the b-axis. Melting point is equal to 185 8C.
3. Experimental Samples of h8-1-naphthoic acid and h8-2naphthoic acid were taken as commercial chemicals (Aldrich). Their deuterium d7 -isotopomers (C10D7COOH) were synthesized starting from naphthalene C10D8 (Sigma-Aldrich), by utilizing familiar methods, developed in the past for synthesis of 1-naphthoic acid and 2-naphthoic acid [36 –40] and then purified by re-crystallization. The purity of samples of d8-1-naphthoic acid and of d8-2-naphthoic acid obtained in such a way was estimated as being approximately equal to 95% in the individual case of each isomer. It was also estimated that monocrystalline fragments obtained from melt were of a higher purity and no single crystals containing the both naphthoic acid isomers have been found. Deuterium bonded samples were obtained by evaporation of D2O solution of each compound under reduced pressure. Single crystals proper for the spectral studies were obtained by crystallization of molten substances between CaF2 windows. Other experimental details were described earlier [19]. For 1-naphthoic acid crystals we found using a diffraction method, that they develop the ac (010) crystalline face in the experimental conditions. In similar circumstances of the crystal growth 2naphthoic acid crystals develop the ab (001) plane. For 1-naphthoic acid crystals the spectra were measured for E parallel to the cp -axis and also for E parallel to the a-axis. In the case of 2-naphthoic acid crystals the spectra were measured for E parallel to the a-axis, as well as for E parallel to the b-axis (E is the electric field vector of the incident light and cp symbol denotes a vector in the reciprocal lattice).
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4. Results Spectra of h8-1-naphthoic and h8-2-naphthoic acid samples measured in the CCl4 solution are shown in Fig. 1. Spectra of the polycrystalline samples of the two isomers of naphthoic acid, dispersed in the KBr pellets, measured at the two different temperatures are shown in Fig. 2. One can find considerably strong temperature induced evolution of the nO – H band contour shapes. To identify the nC – H bands
and to estimate the impact of these bands on the crystalline nO – H bands, the Raman spectra of the polycrystalline samples of h8-1-naphthoic and h8-2-naphthoic acid were measured at the room temperature. It allows recognition of the nC – H band influence as rather small, restricted to a narrow range close to the frequency of 3070 cm21, at the short-wave edge of the nO – H band. Fig. 3 presents the polarization spectra of h8-1naphthoic and h8-2-naphthoic acid crystals, measured at room temperature and in Fig. 4 one can see
Fig. 1. The nO – H bands in the IR spectra of h8-1-naphthoic acid and h8-2-naphthoic acid dissolved in CCl4 : a. h8-1-naphthoic acid, b. h8-2naphthoic acid.
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Fig. 2. The nO – H bands in the IR spectra of the polycrystalline samples of h8-1-naphthoic acid and h8-2-naphthoic acid, 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: a. h8-1-naphthoic acid, b. h8-2-naphthoic acid.
the corresponding spectra, measured at 77 K. The spectra show noticeably strong linear dichroic effects, in the nO – H band frequency range. Low temperature polarization spectra of solid-state d7h-1-naphthoic and d7h-2-naphthoic acids in the nO – H band frequency range are presented in Fig. 5. In Fig. 6 the polarization spectra of the deuterium bonded h7d-1-naphthoic and h7d-2naphthoic acid crystal are shown for the nO – D frequency range and in Fig. 7 the low-temperature polarization spectra are presented. In Fig. 8 the low-temperature polarization spectra of the deuterium bonded d8-1-naphthoic
and d8-2-naphthoic acid crystals were shown for the nO – D frequency range. In Fig. 9 the low-temperature polarization crystalline spectra of h8-1-naphthoic and h8-2naphthoic acid are shown, obtained for highly deuterated samples (ca. 90% of the D-atoms), measured for the frequency range of the residual nO – H bands. These studies aimed to find changes in the nO – H band shapes, accompanying the isotopic dilution with deuterium of the hydrogen bonds. The changes might be ascribed to disappearance of the exciton couplings between the (COOH)2 cycles in each unit cell.
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Fig. 3. The polarized spectra of h8-1-naphthoic acid and h8-2-naphthoic acid, crystals measured at room temperature in the nO – H band frequency range: a. Spectra of the h8-1-naphthoic acid crystal. 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 a-axis; (II) E perpendicular to the a-axis ( parallel cp -axis); (III) Spectrum (II) re-normalized to full scale with spectrum (I). b. Spectra of the h8-2-naphthoic acid crystal. The beam of normal incidence with respect to the ab crystalline face. (I) E parallel to the a-axis; (II) E perpendicular to the a-axis (parallel to the b-axis); (III) Spectrum (II) re-normalized to full scale with spectrum (I). Spectra (I) and (II) were drawn on a common scale.
5. Discussion 5.1. Differences in spectral properties of 1-naphthoic acid and 2-naphthoic acid crystals The presented above IR solid-state spectra of the two isomeric molecular systems, i.e. 1-naphthoic and 2-naphthoic acid, exhibit some noticeable differences, concerning nO – H band shapes, temperature,
polarization and H/D isotopic effects. These differences seem to be too large, to be connected solely with purely vibrational mechanisms of spectra generation. The differences between the spectra of two crystalline systems also do not result directly from their different structures, as centrosymmetric dimeric molecular units in the lattices most probably are the source of the basic crystal spectral properties, similarly as for other carboxylic acid crystals. Therefore, let us
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Fig. 4. The n O –H bands in the polarized spectra of h8-1-naphthoic acid and h8-2-naphthoic acid crystals measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
analyze in detail similarities and differences in spectral properties of the two systems. Our discussion must be related to the polarization spectra of the benzoic acid crystals, that were measured recently [41], also to the available polarization spectra of aliphatic dicarboxylic acid crystals [17,19,28,29]. In all these cases identification and confirmation of the main mechanism, responsible for generation of the nO – H and nO – D band fine structures, based on a model taking into the account a strong anharmonic coupling of the proton (or the deuteron) stretching vibrations, with the low-frequency hydrogen bond stretching vibrations nO· · ·O [2 – 4,8,9]. It was
also shown recently, that generation of a two-branch structure of nO – H and nO – D bands results from activation of spectrally non-active totally symmetric proton vibrations for centrosymmetric dimers, composed with O – H· · ·O hydrogen bonds in the associated carboxyl groups. These mechanisms were considered responsible for polarization and temperature effects, differentiating properties of the nO – H band spectral branches [30]. Comparison of the solid-state spectra, with the corresponding spectra of the compounds dissolved in CCl4 suggest, that centrosymmetric dimers of the hydrogen bonds are the structural units in the crystals,
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Fig. 5. The nO – H bands in the polarized spectra of d7h-1-naphthoic acid and d7h-2-naphthoic acid crystals measured at 77 K: d7a. d7h-1naphthoic acid, d7b. d7h-2-naphthoic acid. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
responsible for the crystal spectral properties (Figs. 1 and 2). Similarly as for the benzoic acid crystal case [41], the nO – H and the nO – D band contour shapes may be quantitatively reproduced by the model calculations, performed within the limits of the strongcoupling model [8,9,16] in a dimer approximation. In the case of the crystal spectra generation mechanisms, vibrational exciton couplings between (COOH)2, or (COOD)2 groups, are most probably only secondary interactions, responsible for very weak Davydowsplitting effects in the nO – H and the nO – D bands.
Within the strong-coupling theory, each experimental nO – H or nO – D band may be considered as a superposition of two component bands of a different origin. Each component band forms one separate branch in each analyzed band. In the spectra of centrosymmetric hydrogen bond dimers, the shorterwave branch of each band corresponds to the excitation of the non-totally symmetric proton vibrations in the dimers [9,16,30]. The longer-wave branches in the spectra correspond to the totally symmetric proton vibrations. However, these
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Fig. 6. The polarized spectra of h7d-1-naphthoic acid and h7d-2-naphthoic acid, crystals measured at room temperature in the nO – D band frequency range. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
vibrations should be spectrally non-active by the symmetry rules. Nevertheless, these become active via a promotion mechanism, which is basically a vibronic one [30]. This promotion mechanism, considered as a kind of reversion of the familiar ‘Herzberg –Teller’ effect, known from the electronic spectroscopy of aromatic molecular systems, is responsible for temperature, polarization and also for some H/D isotopic effects in spectra of carboxylic acid crystals [17,19,30]. The observed differences in the spectral properties of the two isomeric crystalline systems mainly depend
on different relative integral intensities of the component bands, forming the longer- and the shorter-wave branches of the corresponding nO – H and nO – D bands. In the case of the h8- and d8-2naphthoic acid crystals, the longer-wave branches, generated by the forbidden transitions, are of generally higher relative intensity, in comparison with the corresponding component band properties from the spectra of the h8- and d8-1-naphthoic acid crystals. For the 2-naphthoic acid crystal spectra a strong temperature induced growth of the lowerfrequency branch of the nO – H band can also be
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Fig. 7. The nO – D bands in the polarized spectra of h7d-1-naphthoic acid and h7d-2-naphthoic acid crystals measured at 77 K. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
noticed. With regard to the spectral properties the 2naphthoic acid crystals fairly resemble spectral properties of benzoic acid crystals [41]. One might ask about the magnitudes of the promotion effect for the two naphthoic acid crystals. A most probable cause of the discussed spectral effects is connected with different electronic properties of the 1- and 2- substituted naphthalene rings. Different electron populations of the naphthalene ring system at the 1- and 2- positions are considered to be responsible for differentiation of the chemical reactivity at the two places of naphthalene rings. It might be therefore expected that electronic properties of
the 1- and 2- substituted naphthalene rings influence the promotion mechanism of the forbidden vibrational transitions for centrosymmetric hydrogen bond dimers. One can expect that carboxyl groups effectively withdraw electronic charge from the aromatic rings. For the naphthalene ring system this charge withdrawal in a different way concerns the position 1, in comparison to the position 2. This effect used to be stronger for the position 2. This statement was supported by ab initio calculations aiming to estimate charges on the atoms in hydrogen-bonded molecules of 1-naphthoic acid and of 2-naphthoic acid.
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Fig. 8. The nO – D bands in the polarized spectra of d8-1-naphthoic acid and d8-2-naphthoic acid crystals measured at 77 K: d7a. D8-1-naphthoic acid, d7b. D8-2-naphthoic acid. Other experimental conditions and the presentation of the spectra are identical to those given for Fig. 3.
The calculations were performed using the GAUSSIAN 98 series of programs [42] at the SCF and at the DFT level, for the 6-31G basis. A full optimization of the dimer geometry was assumed. Our calculations have shown that in the case of 2-naphthoic acid the carboxyl group ca. 40% more effectively withdrew electrons from the naphthalene ring, in comparison with the 1-naphthoic acid case. This effects find its illustration in the calculated Mulliken charges concerning the carboxyl group and the naphthyl ring atoms. At the SCF level, for 1-naphthoic acid an excessive negative charge of 0.012 elementary electron charge has been estimated,
when for 2-naphthoic acid the corresponding value was 0.016. At the DFT level the corresponding values were equal to 0.089 and 0.096, respectively. When the electron charge from the aromatic ring is being strongly shifted towards carboxyl group, vibronic coupling mechanisms between the proton stretching vibrations and the hydrogen bond electrons, coupled with electrons in aromatic rings, may be partially strengthened. In this case a stronger coupling can influence a more effective promotion mechanisms of the forbidden transitions in the IR [30]. The selection rule breaking effect is expected to be stronger for the stronger electron withdrawal case,
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Fig. 9. The residual nO – H bands from the crystalline spectra of the partially deuterated samples of h8-1-naphthoic acid and h8-2-naphthoic acid (In each case the deuterium substitution rate was equal to ca. 90%). Other experimental conditions were identical to those from Fig. 3.
i.e. for 2-naphthoic acid crystal. In such a way different relative intensities of the longer-wave branches of the nO – H and nO – D bands may result for the spectra of the two crystalline systems. Therefore, in the spectra of 1-naphthoic acid the forbidden transition effect is considered as the weaker one, as the relative longer-wave nO – H band branch intensity is ca. two times lower when compared with the 2-naphthoic acid crystal spectrum case. Spectral properties of 1-naphthoic acid crystals fairly resemble corresponding spectral properties of the recently investigated dicarboxylic acid crystals. In the spectra of the latter systems the intensities of
the nO – H and nO – D band longer-wave branches are ca. two times lower in comparison with the shorter-wave branches [17,19]. Generally similar relation between the corresponding nO – H and nO – D band branch intensities are valid for the d7h-1-naphthoic and d7h-2-naphthoic acid crystals. 5.2. Temperature effects in the spectra Temperature effects in the spectra of 1-naphthoic and 2-naphthoic acid crystals basically depend on a growth of the relative intensity of the nO – H and
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the nO – D band lower-frequency branches. This effect is stronger for 2-naphthoic acid crystals. Similar temperature effects were observed in spectra of dicarboxylic acid crystals [17,19] and also in the spectra of benzoic acid crystals [41]. The temperature effects were previously ascribed to defects in crystal lattices, which were generated when obtaining crystals in non-equilibrium conditions, during crystallization from melt [17,19]. 5.3. Polarization effects in the spectra In the two cases (i.e. of 1-naphthoic and 2naphthoic acid crystals) the relative integral intensities of the polarized components of the nO – H and nO – D bands for each crystal remain in a good correlation with the crystal lattice geometry. The component band integral intensity ratio is approximately equal to the ratio of squares of the O – H bond direction cosines in associated COOH groups (dichroic effects of the first kind). In the spectra of the naphthoic acid crystals also another kind polarization effects can be observed (effects of the second kind). These latter effects depend on a slight diversification of the longer- and the shorter-wave branch polarization properties of the nO – H or nO – D bands for each isomer and for each isotopomer of naphthoic acid. These effects were observed in the spectra of hydrogen bonded dicarboxylic acid crystals [19,28,29]. In the low temperature spectra of benzoic acid crystals such effects were only slightly pronounced [41]. From our studies, it results that these effects are strongest in the lowtemperature spectra of 1-naphthoic acid crystals (see Figs. 4 and 8). The polarization effects of the second kind we ascribed to defects in the lattices, most likely generated during a relatively fast crystallization of molten substances when obtaining the solid-state samples [17,19]. 5.4. Fermi resonance influence on the spectra It is a very characteristic property of the nO – H bands that the fine structure patterns are irregular in their longer-wave parts and this irregularity takes a form reminding the so-called Evans holes [2,3], in the frequency ranges being close to 2700 cm21. In each nO – H band contour there are two better or worse
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pronounced Evans holes at frequencies equal to ca. 2750 and 2625 cm21. These effects are most easily observed in the dense fine structure pattern spectra of d7h-1-naphthoic, h8-2-naphthoic and d7h-2-naphthoic acid crystals. This effect does not disappear in the residual nO – H bands, measured for partially deuterated samples. In these all cases band deformations concern the longer-wave part of the band, generated by the symmetry forbidden excitation of the totally symmetric proton stretching motions in the hydrogen bond dimers [41]. Most probably, similarly as like in the case of benzoic acid crystal spectra [41], these effects result from the Fermi resonance between the proton stretching motions with the bending-in-plane proton vibration in their overtone states in the dimers (vibration symbol dO – H ). The Fermi resonance falls just on to the activated forbidden transition subband frequency range for the proton stretching vibrations. This resonance seems possible thanks to the Agsymmetry of the proton vibration excited state, which is responsible for the longer-wave nO – H band branch generation, where the Evans holes effects used to appear. The dO – H vibrations in their overtone state are also of the Ag-symmetry and thus the Fermi resonance becomes possible. Similar approach was proposed to quantitatively explain similar effects in the spectra of the benzoic acid crystals, when these effects were quantitatively reproduced in the frames of the strong-coupling model, incorporating between the proton stretching and bending motions in the dimers [41]. In the case of the spectra of the naphthoic acid crystals, Fermi resonance effects are much stronger, when compared with results of similar effects in the spectra of aliphatic acid crystals [17,19,28,29]. The Fermi resonance effects seem to be a common property of spectra of aromatic acid crystals. In these cases the Fermi resonance mechanisms seem to be very effective, which may be connected with stabilization of aromatic acid dimer structures due to the couplings of hydrogen bonds from the (COOH)2 cycles with aromatic rings, via the P-electrons. At this point it must be added that no such Evans holes effects were observed in the spectra or arylacetic acid crystals, e.g. phenylacetic [43], 1-naphthylacetic and 2-naphthylacetic acid crystals [44]. In these latter cases the (COOH) 2 cycles are separated from
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the aromatic by CH2 groups, which probably makes the coupling much less effective. Therefore, the Fermi resonance mechanisms for arylacetic acid dimers seem to be much weaker in comparison with the arylcarboxylic acid cases. It also seems worth to add that Evans hole effects were observed in spectra of hydrogen bonded trans-cinnamic acid crystals [45], that acts in support for the vibronic coupling source of these effects. 5.5. Isotopic dilution effects on the nO – H bands Residual nO – H bands, measured for isotopically diluted samples of the naphthoic acids, are characterized with fairly similar band shapes as the corresponding bands of the pure, non-deuterated compound crystals do. Some slight differences between the spectra compared are restricted to poorer fine structure of the residual nO – H bands and most probably result from vanishing of the correlation field interactions between dimers of naphthoic acids, due to the isotopic dilution by deuterium [32,46]. However, the isotopic dilution does not destroy a general scheme of the nO – H bands for the investigated crystals. The band structure still remains twobranched, with almost unchanged relation between integral intensities of the component bands, obtained for different polarization of the electric field vector of the beam used for measurement of spectra, with respect to the crystal lattice. The residual nO – H band fine structure patterns prove, that even for high deuterium substitution rates in the solid-state samples, centrosymmetric dimers of hydrogen bonds, with two protons in each (COOH)2 cycles, still remain the source of the crystal spectral properties. This non-traditional H/D isotopic effect, i.e. the self-organization effect in mixture systems of protons and deuterons, seems to be common for cyclic dimeric systems [32]. It depends on a non-random distribution of protons and deuterons, between the dimeric hydrogen bonds in partially deuterated samples. Most probably it results from more stable symmetric dimers, containing identical hydrogen isotopes in one dimer, when compared with the nonsymmetric dimer (i.e. containing simultaneously one hydrogen and one deuterium bond) properties [32,47]. This problem is still being intensively investigated.
6. Conclusion It results from our studies that some noticeable differences in spectral properties of the hydrogen bonds in the crystals of 1-naphthoic and 2-naphthoic acid can be identified. Generally, the crystals of 2naphthoic acid, of the h8 and of the d7h-types, exhibit considerably more intense longer-wave branches of their nO – H bands, when compared with the characteristics of the analogous spectral branches in the spectra of h8- and d7h-1-naphthoic acid crystals. This part of the crystal spectra corresponds to the activated by a vibronic mechanism, symmetry-forbidden excitation of totally symmetric proton stretching vibrations in centrosymmetric dimers of naphthoic acid. The observed differences in the forbidden band intensities may be ascribed to the efficiency of the promotion mechanism, for the forbidden vibrational transition activation mechanism. This is connected with differences in coupling mechanism of the vibrating hydrogen bond protons with electrons of the naphthalene rings through the carbon atoms in the 1 and 2 positions. Thus, the differences in spectral properties of 1- and 2-naphthoic acid crystals generally result from electronic properties of the 1- and 2-substituted naphthalene rings. Replacement of the hydrogen atoms in aromatic rings by the deuterium atoms does not change the relative intensity of the longer- to the shorter-wave branch of the nO – H bands in spectra of each naphthoic acid isomer. The only change in the nO – H band structure due to deuteration of aromatic rings, the long-range H/D isotopic effect, depends on a more dense band fine structure pattern for d7h-1- and d7h-2naphthoic acid crystal spectra. This effect corresponds with diminution of the nO· · ·O low-frequency hydrogen bond stretching vibration quanta. These effects are similar to those measured in the spectra of benzoic acid crystals [41]. In the nO – H band frequency ranges, in the all four investigated compound crystals, spectral effects appear, resembling so-called Evans holes, which may be ascribed to the Fermi resonance mechanisms. These resonances concern the coupling of the proton stretching vibrations in the 1- and 2-naphthoic acid dimers, with the proton bending-in-plane vibrations in their first overtone state. This resonance was considered recently as a main nX – H band fine structure
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modifying mechanism in the IR spectra of hydrogen bonded systems [48,49]. The extreme magnitude of the Evans hole effects inclines us to consider them as an attribute of the arylcarboxyl acid crystals.
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