Polarized IR spectra of the hydrogen bond in 2-thiopheneacetic acid and 2-thiopheneacrylic acid crystals: H/D isotopic and temperature effects

Polarized IR spectra of 2-thiopheneacetic acid and of 2-thiopheneacrylic acid crystals were measured at 293 and 77 K in the υ(O-H) and υ(O-D) band frequency ranges. The corresponding spectra of the two individual systems strongly differ, one from the other, by the corresponding band shapes as well as by the temperature effect characterizing the bands. The crystal spectral properties remain in close relation with the electronic structure of the two different molecular systems. We show that a vibronic coupling mechanism involving the hydrogen bond protons and the electrons on the π- electronic systems in the molecules determines the way in which the vibrational exciton coupling between the hydrogen bonds in the carboxylic acid dimers occurs. Strong coupling in 2-thiopheneacrylic acid dimers prefers a "tail-to-head"-type Davydov coupling widespread by the π- electrons. A weak through-space coupling in 2-thiopheneacetic acid dimers, of a van der Waals type, is responsible for a "side-to-side"-type coupling. The relative contribution of each exciton coupling mechanism in the dimer spectra generation is temperature and the molecular electronic structure dependent. This explains the observed difference in the temperature- induced evolution of the compared spectra.     

H/D isotopic recognition in hydrogen bonded systems: H/D isotopic self-organization effects in the IR spectra of the hydrogen bond in 2-methylimidazole crystals

Polarized IR spectra of H12(3)45 2-methylimidazole and of its H1D2(3)45, D1H2(3)45 and D12(3)45 deuterium derivative crystals are reported and interpreted within the limits of the "strong-coupling" theory. The spectra interpretation facilitated the recognition of the H/D isotopic "self-organization" phenomenon, which depends on a non-random distribution of protons and deuterons in the lattices of isotopically diluted crystal samples. The H/D isotopic "self-organization" mechanism engaged all four hydrogen bonds from each unit cell. These effects basically resulted from the dynamical co-operative interactions involving adjacent hydrogen bonds in each hydrogen bond chain. A weaker exciton coupling involved the closely spaced hydrogen bonds; each belonging to a different chain of associated 2-methylimidazole molecules. The high intensity of the narrow band at ca. 1880cm(-1) was interpreted as the result of coupling between the γ(N-H?N) proton bending "out of plane" vibration overtone and the ν(N-H) proton stretching vibration.     

Polarization IR spectra of hydrogen bonded pyrazole crystals: self-organization effects in proton and deuteron mixture systems. Long-range H/D isotopic effects

This paper deals with experimental studies of the polarization IR spectra of solid-state pyrazole H1345, as well as of its H1D345, D1H345 and D1345 deuterium derivatives. Spectra were measured for the vN-H and vN-D band frequency ranges at temperatures of 298 and 77 K. The spectra were found to strongly change their intensity distribution and their polarization properties with the decrease of temperature. These effects were ascribed to some temperature-induced conformational changes in the hydrogen bond lattices. The studies reported allowed the finding of new kind of isotopic effects H/D in the open-chain hydrogen bond systems, i.e. the self-organization effects. It was found that the spectrally active aggregates of hydrogen bonds remain unchanged despite the growing isotope H/D exchange rate. This statement was supported by analysis of the residual polarized vN-H and vN-D band properties, measured for the isotopically diluted crystalline samples. Analysis of the band shapes of the four hydrogen isotope derivative crystals proved the existence of another kind of H/D isotopic effect, i.e. the long-range isotopic effect. It depends on an influence of the pyrazole ring hydrogen atoms onto the vN-H and vN-D band widths and onto the band fine structures.     

Infrared spectra of the hydrogen bond in pimelic acid crystals: polarization and temperature effects.

This paper deals with the polarized IR spectra of the hydrogen bond in pimelic acid crystals and their quantitative interpretation. The spectra were measured for the vO-H and vO-D band frequency regions, at temperatures of 298 and 77 K, for the 'alpha' crystalline form of pimelic acid. Two kinds of transmission spectra were obtained, for the beam perpendicular to the two different crystalline faces: 'ac' and 'ab'. The spectra of the hydrogen and the deuterium bonded systems were quantitatively reproduced, with sufficient accuracy, on the basis of the 'strong-coupling' model, assuming the (COOH)2 cycles to be the structural units responsible for the basic spectral properties. It was found that the spectra could be reproduced only, when assuming spectral activity in the IR of the totally symmetric proton stretching vibrations in centrosymmetric cyclic dimers of hydrogen bonds. The polarization effects in the crystal spectra were interpreted as directly connected with the hydrogen bond orientation in the crystal. However, in the pimelic acid crystalline spectra there were not observed another polarization effects characteristic for another carboxylic acid crystals, depending on differentiation of the long- and of the short-wave branch properties of each of the vO-H and vO-D bands. The temperature variation only affect the intensity ratio between the lower- and the higher-frequency branches of the vO-H and vO-D bands. These spectral effects were ascribed to less strained hydrogen bond structures in the pimelic acid lattices of the alpha' crystalline form and a relatively low concentration of defects in the lattices.     

Infrared spectra of the hydrogen bond in benzoic acid crystals: temperature and polarization effects.

This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline spectra of benzoic acid and of its deuterium isotopomers: C6H5COOD, C6D5COOH and C6D5COOD. The spectra were measured in the vO-H and in the vO-D band frequency ranges at temperatures of 298 and 77 K. The intensity distribution in the bands was quantitatively reproduced on the basis of the 'strong-coupling' model, when assuming that the isolated (COOH)2 and (COOD)2 cycles were the source of the spectral properties of 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. 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 onto the vO-H band fine structure patterns. The role of the aromatic rings was also discussed, in order to explain mechanisms of extremely effective promotion of the forbidden transition, as well as of the Fermi resonance impact on the crystalline spectra.     

Polarized IR spectra of the hydrogen bond in acetic acid crystals

The paper presents the results of our investigations of the polarized IR spectra of the hydrogen bond in crystals of acetic acid, CH₃COOH, as well as in crystals of three deuterium isotopomers of the compound: CH₃COOD, CD₃COOH and CD₃COOD. The spectra were measured at 283 K and at 77 K by a transmission method using polarized light. Theoretical analysis of the results concerned the linear dichroic effects, together with the H/D isotopic and temperature effects observed in the solid-state IR spectra of the hydrogen and of the deuterium bond at the frequency ranges of the ν_O–H and the ν_O–D bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of one of the quantitative theories of the IR spectra of the hydrogen bond, i.e. the “strong-coupling” theory or the “relaxation” theory when a hydrogen bond dimer model is used. From the spectra obtained it resulted that the strongest exciton coupling involved the closely spaced hydrogen bonds, belonging to different chains of associated acetic acid molecules. These results contradict the former explanation of the spectra within a model, which assumed a strong vibrational exciton coupling between four hydrogen bonds in a unit cell. On analyzing the spectra of isotopically diluted crystalline samples of acetic acid it has been proved that a non-random distribution of the protons and deuterons takes place in the hydrogen bond lattices. This non-conventional isotopic effect is a result of dynamical co-operative interactions involving hydrogen bonds in the system. Simultaneously it has been also found that in an individual hydrogen bonded chain in the crystals, distribution of the hydrogen isotope atoms H and D was fully random. The H/D isotopic “self-organization” mechanism most probably involves a pair of hydrogen bonds from each unit cell where each hydrogen bond belongs to a different chain.     

Study of polarized IR spectra of the hydrogen bond system in crystals of styrylacetic acid

We have investigated the polarized IR spectra of the hydrogen bond system in crystals of trans-styrylacetic acid C(6)H(5)CHCHCH(2)COOH, and also in crystals of the following three deuterium isotopomers of the compound: C(6)H(5)CHCHCH(2)COOD, C(6)H(5)CHCHCD(2)COOH and C(6)H(5)CHCHCD(2)COOD. The spectra were measured at room temperature and at 77K by a transmission method. The spectral studies were preceded by determination of the X-ray crystal structure. Theoretical analysis of the results concerned linear dichroic effects, the H/D isotopic and temperature effects, observed in the solid-state IR spectra of the hydrogen and of the deuterium bond, at the frequency ranges of the nu(OH) and the nu(OD) bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of the "strong-coupling" theory, when based on a hydrogen bond dimer model. This model sufficiently explained not only a two-branch structure of the nu(OH) and the nu(OD) bands, and temperature-induced evolution of the crystalline spectra, but also the linear dichroic effects observed in the band frequency ranges. A vibronic mechanism was analyzed, responsible for promotion of the symmetry-forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers. It was found to be of minor importance, when compared with analogous spectral properties of arylcarboxylic acid, or of cinnamic acid crystals. These effects were ascribed to a substantial weakening of electronic couplings between the hydrogen bonds of the associated carboxyl groups and the styryl radicals, associated with the separation of these groups in styrylacetic acid molecules by methylene groups in the molecules.     

H/D isotopic recognition in hydrogen-bonded systems: strong dynamical coupling effects in the polarized IR spectra of 3-methylthioacetanilide and 4-methylthioacetanilide crystals

This paper presents the investigation results of the polarized IR spectra of the hydrogen bond in crystals of 3- and 4-methylthioacetanilide. The spectra were measured at 293 and 77 K by a transmission method, with the use of polarized light. The main spectral properties of the crystals can be interpreted satisfactorily in terms of the "strong-coupling" theory, on the basis of the hydrogen bond centrosymmetric dimer model. The spectra revealed that the strongest vibrational exciton coupling involved the closely spaced hydrogen bonds, each belonging to a different chain of associated 3- and 4-methylthioacetanilide molecules. A weaker exciton coupling involved the adjacent hydrogen bonds in each individual chain. It was proven that a nonrandom distribution of the protons and deuterons took place in the lattices of isotopically diluted crystalline samples of 3- and 4-methylthioacetanilide. In each case, the H/D isotopic "self-organization" mechanism involved all four hydrogen bonds from each unit cell.     

Strong vibrational exciton coupling effects in polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals

This paper presents the results of investigation on polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals. The spectra were measured in the frequency range of the NH and ND bond stretching vibrations, for two different crystalline forms, having developed ab or bc crystal faces. The spectra exhibited extremely strong vibrational exciton coupling effects characterized by a large Davydov-splitting (correlation field splitting), whose existence was confirmed by a strong difference between the polarized spectra of the two forms of 2-thiopyridone crystals. It was shown that extremely strong exciton interactions involving the translationally non-equivalent hydrogen bonds in the unit cell are responsible for these effects. Isotopic dilution in the crystals caused the disappearance of the spectral effects, ascribed to the inter-dimer exciton couplings, and the simultaneous retaining of the dimeric character of the “residual” ν_NH and ν_ND bands. This spectral behavior of the isotopically diluted crystals was interpreted as the result of the dynamical co-operative interactions involving the hydrogen bonds in the lattice. These interactions lead to a non-random distribution of the protons and deuterons in the cyclic hydrogen bond dimeric systems and in consequence to the so-called H/D isotopic “self-organization” effects in the crystal spectra. The fine structure of the “residual” ν_NH and ν_ND bands is also influenced by such non-conventional spectral effects as the selection rule breaking for IR transitions, as well as the “reversal” exciton coupling effect for centrosymmetric hydrogen bonded dimers. This statement is supported by model calculations of the analyzed band shapes. They are performed in terms of the “strong-coupling” theory which assumes a strong anharmonic coupling involving different frequency hydrogen bond normal vibrations in the dimers, namely the high-frequency NH stretching and the low-frequency ν_N⋯S hydrogen bond stretching vibrations.     

'Self-organization' processes in proton and deuteron mixtures in open-chain hydrogen bond systems: polarization IR spectra of 4-mercaptopyridine crystals.

This paper is devoted to IR spectroscopic studies in polarized light of 4-mercaptopyridine (4-MPD) hydrogen-bonded crystals. These studies were preceded by determination of the 4-mercaptopyridine X-ray structure. Polarization spectra of 4-mercaptopyridine crystals were measured in the frequency ranges of V(N-H) and V(N-D) bands at room temperature, and also at the temperature of liquid nitrogen, for the two different crystalline faces: 'bc' and 'ac'. When investigating 'residual' v(N-H) band shapes for crystals that were diluted by deuterium, strong dichroic effects in the spectra were still observed, providing evidence for the existence in the hydrogen bond chains of domains containing exclusively protons or deuterons. This phenomenon proves the existence of a new kind of H/D isotopic effects in open chain hydrogen bond systems, namely 'self-organization' effects. Such effects, however, were not observed for other open chain hydrogen bond systems, e.g. alcohol crystals. Solid-state spectra of 4-mercaptopyridine were quantitatively interpreted, along with the strong polarization and the isotopic effects, when based on the 'strong-coupling' theory for linearly arranged hydrogen bond dimers.     

Effects of dynamical couplings in IR spectra of the hydrogen bond in N-phenylacrylamide crystals

This article presents the investigation results of the polarized IR spectra of the hydrogen bond in N-phenylacrylamide crystals measured in the frequency range of the proton and deuteron, ν(N-H) and ν(N-D), stretching vibration bands. The basic spectral properties of the crystals were interpreted quantitatively in terms of the "strong-coupling" theory. The proposed model of the centrosymmetric dimer of hydrogen bonds facilitated the explanation of the well-developed, two-branch structure of the ν(N-H) and ν(N-D) bands as well as the isotopic dilution effects in the spectra. The vibronic mechanism of the generation of the long-wave branch of the ν(N-H) band ascribed to the excitation of the totally symmetric proton vibration was elucidated. The complex fine structure pattern of ν(N-H) and ν(N-D) bands in N-phenylacrylamide spectra in comparison with the spectra of other secondary amide crystals (e.g., N-methylacetamide and acetanilide) can be accounted for in terms of the vibronic model for the forbidden transition breaking in the dimers. On the basis of the linear dichroic and temperature effects in the polarized IR spectra of N-phenylacrylamide crystals, the H/D isotopic "self-organization" effects were revealed.     

Inter-hydrogen bond dynamical coupling effects in polarized IR spectra of N-methylacetamide single crystals

This article presents the investigation results of polarized IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals measured in the frequency range of the proton and deuteron stretching vibration bands, ν_N–H and ν_N–D. A similar study was also performed for crystals of the deuterium isotopomers of the compound, D₇-NMA (CD₃CONDCD₃) and D₆-NMA (CD₃CONHCD₃). On the basis of the analysis of the linear dichroic and temperature effects, the two-branch structure of the ν_N–H bands in the spectra was ascribed to centrosymmetric hydrogen bond pairs in the lattice. Each hydrogen bond in such a dimeric system belonged to another chain of the associated molecules. The exciton interactions involving the dimer hydrogen bonds were considered to be responsible for the band shape generation. For the deuterium-bonded crystals the exciton interactions were found to be weaker since the ν_N–D bands were less split. Within an individual hydrogen or deuterium bond chain the in-chain exciton couplings involving hydrogen bonds were estimated as considerably weaker than the inter-chain ones. The exciton dilution retains the two-branch fine structure pattern of the “residual” ν_N–H and ν_N–D bands. This means that the inter-chain couplings involving hydrogen bonds do not change, when the in-chain couplings vanish. These results are the evidence of the influence of non-conventional co-operative interactions occurring in the hydrogen bond systems on the spectra. These co-operative interactions are responsible for the non-random distribution of the hydrogen isotope atoms in the hydrogen bridge lattices, namely for the grouping of identical hydrogen isotope atoms in the dimers. The proposed interpretation of the IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals casts light on the spectra generation mechanisms and gives a new meaning to the traditional nomenclature applied for describing the ν_N–H band structure pattern in IR spectra of amides.     

Effects of strong inter-hydrogen bond dynamical couplings in the polarized IR spectra of adipic acid crystals

This paper presents the results of the re-investigation of polarized IR spectra of adipic acid and of its d₂, d₈ and d₁₀ deuterium derivative crystals. The spectra were measured at 77 K by a transmission method using polarized light for two different crystalline faces. Theoretical analysis concerned linear dichroic effects and H/D isotopic effects observed in the spectra of the hydrogen and deuterium bonds in adipic acid crystals at the frequency ranges of the ν_O–H and the ν_O–D bands. The two-branch fine structure pattern of the ν_O–H and ν_O–D bands and the basic linear dichroic effects characterizing them were ascribed to the vibronic mechanism of vibrational dipole selection rule breaking for IR transitions in centrosymmetric hydrogen bond dimers. It was proved that for isotopically diluted crystalline samples of adipic acid, a non-random distribution of protons and deuterons occurs in the dimers (H/D isotopic “self-organization” effect). This effect results from the dynamical co-operative interactions involving the dimeric hydrogen bonds.     

Theoretical study of the νO–H IR spectra for the hydrogen bond dimers from the polarized spectra of glutaric and 1-naphthoic acid crystals: Fermi resonances effects

A full quantum theoretical model is proposed to study the ν_O–H experimental IR line shapes of polarized crystalline glutaric and 1-naphthoic acid dimer crystals at room and liquid nitrogen temperatures. This work is an application of a previous model [M. E-A. Benmalti, D. Chamma, P. Blaise, and O. Henri-Rousseau, J. Mol. Struct. 785 (2006) 27–31] by accounting for Fermi resonances. The approach is dealing with the strong anharmonic coupling, Davydov coupling, multiple Fermi resonances between the first harmonics of some bending modes and the first excited state of the symmetric combination of the two ν_O–H modes and the quantum direct and indirect relaxation.Numerical results show that mixing of all these effects allows to reproduce satisfactorily the main features of the experimental IR line shapes of crystalline hydrogenated and deuterated glutaric and 1-naphthoic acid crystals and are expected to provide efficient of Fermi resonances effects.     

Effects of dynamical couplings in hydrogen bond systems in the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals

This paper presents the investigation results of the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals measured at 293 and 77 K. Analysis of the results concerned the linear dichroic, H/D isotopic and temperature effects observed in the spectra of the hydrogen and deuterium bond at the frequency ranges of the ν_O–H and the ν_O–D bands, respectively. The main spectral properties of the crystals were interpreted in terms of the “strong-coupling” theory on the basis of the hydrogen bond dimer model. The spectra revealed that the strongest vibrational exciton coupling involved the closely-spaced hydrogen bonds, each belonging to a different chain of associated molecules. The reason for two different crystalline systems, are characterized by almost identical ν_O–H and ν_O–D band shapes, is explained. It was proved that a random distribution of the protons and deuterons took place in the lattices of the isotopically diluted crystals.     

Influence of molecular electronic properties on the IR spectra of dimeric hydrogen bond systems: polarized spectra of 2-hydroxybenzothiazole and 2-mercaptobenzothiazole crystals

We have studied the polarized IR spectra of the hydrogen-bonded molecular crystals of 2-hydroxybenzothiazole (HBT) and 2-mercaptobenzothiazole (MBT). The crystal structure of 2-hydroxybenzothiazole was determined by X-ray diffraction. The polarized spectra of the crystals were measured, in the frequency ranges of the ν_N-H and ν_N-D bands, at room temperature, and at 77 K. In both systems an extremely strong H/D isotopic effect in the spectra was observed, involving reduction of the well-developed ν_N-H band fine structure to a single prominent ν_N-D line only. The two ν_N-H bands were also shown to exhibit almost identical properties, band shapes, temperature and dichroic properties included. The spectra were quantitatively reconstituted, along with the strong isotopic effect, when calculated using the ‘strong-coupling’ theory, assuming the centrosymmetric dimers of HBT or MBT to be the structural units responsible for the crystalline spectral properties. The similarity of the spectra of the two crystalline systems was considered to be a result of longer-distance couplings between the proton vibrations in the dimers, via the aromatic ring electrons. When investigating the ‘residual’ ν_N-H band shapes for crystals isotopically diluted by deuterium, we observed some ‘self-organization’ effects in the spectra, indicating the energetically favored presence of two identical hydrogen isotopes in each hydrogen bond dimer.     

Theory of the infrared spectra of the hydrogen bond in molecular crystals

A general model for the vibrational interactions of the hydrogen bonds in molecular crystals is presented. The energy and the intensity distributions of the IR spectra of the 1-methylthymine and uracil crystals are correctly reproduced.     

Effect of the resonance of the C-H and O-H bond stretching vibrations on the IR spectra of the hydrogen bond in formic and acetic acid

It is shown that the resonance of the O-H and C-H bond stretching vibrations is responsible for a noticeable intensity redistribution effect in the IR spectra of associated formic acid molecules in the gaseous phase. This effect is manifested by a considerably high growth in intensity of the νC-H band, which overlaps the νO-H band contour in the spectra. A vibronic coupling of the Herzberg-Teller-type expressed by the second order term in the perturbation theory is the most probable source of these spectral effects. The presented mechanism explains the variation of the effect magnitude accompanying the phase transitions. The proposed model also facilitates the understanding of the H/D isotopic effects in the spectra as well as the essential difference in the corresponding spectral properties between the formic and the acetic acid.     

Solid-liquid equilibrium and the hydrogen bond in crystals

The types of hydrogen-bonded associates formed in phenol and aromatic acid crystals are discussed. A hypothesis is advanced that dissolution of a self-associated solid substance in a nonpolar solvent consists in transferring the associates existing in the crystal phase into saturated solution. The mechanism has been confirmed for several aromatic acids and phenols.