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.     

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.     

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.     

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.     

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.     

The source of similarity of the IR spectra of acetic acid in the liquid and solid-state phases

This paper explains the problem of the fair similarity of the ν_O-H band contour shapes in the spectra of liquid and crystal samples of hydrogen-bonded acetic acid, in contrast to the corresponding spectral properties of formic acid. It has been proven that regardless of the relative arrangement of hydrogen bonds in each phase, the hydrogen-bonded dimeric systems decide about the profile of the bands. Moreover, the model dimers of different symmetry, i.e., the C_i or C_2v one of the “side-to-side” type arrangement of moieties, yield nearly identical spectra. Qualitatively similar spectral properties in the ν_O-D band frequency range were found for deuterium-bonded formic and acetic acid isotopologues.     

Vibrational spectra and vibrational states of simple gas-phase hydrogen-bonded dimers

The article aims to outline the growth of evidence and ideas about infrared band broadening for simple, gaseous, moderately strong, hydrogen bonded dimers B…HA, to draw attention to areas in need of further development and to collect together experimental information available at the present time about vibrational states associated with such dimers. The band associated with the modified HA stretching mode, ν_s, is observed for several dimers not only to be broad, but to have sub-band structure, which is satisfactorily interpreted as arising from combination bands of ν_s with the low frequency stretch-mode ν_σ, giving a progression ν_s ± nν_σ as a result of strong anharmonic coupling. For weaker dimers with lower values of ν_σ the sub-band structure is less evident, and may appear only as shoulders, while for still weaker dimers, the sub-bands may be merged into a featureless broad band. A major factor contributing to the breadth of the individual sub-bands in the band structure is the presence of hot-bands, especially a long series based on successive excited states of the low-frequency bending mode ν_β. A link is indicated with the interpretation of band-broadening for moderately strong hydrogen-bonded complexes in the liquid state. The anharmonic coupling of ν_s and ν_σ is again a central feature but there is a new factor, namely the coupling of ν_σ through a fluctuating potential with the surroundings, which has the result that the ν_s mode rapidly loses phase coherence resulting in a broad structureless band.     

Spectral evidence of solid state interactions in mixed dimethyl sulfone-water ices

Infrared spectra of solid mixtures of dimethyl sulfone and ice were studied. A comparison of the spectra to those of the pure solids indicates bands attributable to two solid-solid interactions-a stronger and a weaker one—both centered on the OH?OS moieties. The bonded ν(OH) stretch assigned to the stronger interaction is part of the broad ‘3μ’ ice band and its wavenumber value is close to that reported previously for the 1:1 ((CH₃)₂SO₂)·H₂O) complex isolated in rare gas matrices. The weaker interaction is discerned at higher wavenumbers as a very low intensity feature, only slightly shifted from the free ν(OH) band positions.     

'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.     

Theoretical study of the polarized infrared spectra of the hydrogen bond in 2‐furoic acid crystal dimer

This work presents a theoretical simulation of ν_{O? H} and ν_{O? D} band shapes in the polarized infrared spectra of 2‐furoic acid dimer crystals measured at liquid‐nitrogen temperature. The line shapes are studied theoretically within the framework of the anharmonic couplings between low‐frequency hydrogen‐bond vibrations and degenerate excited states of high‐frequency hydrogen vibrations in hydrogen‐bonded dimers and the anharmonic coupling between the first excited state of the fast mode and the harmonics or band combinations of some low‐frequency bending modes, which lead to Fermi resonances.This approach takes into account the adiabatic approximation, the intrinsic anharmonicity of the low‐frequency mode through a Morse potential, Davydov coupling triggered by resonance exchange between the excited states of the fast modes of the two hydrogen bonds involved in the cyclic dimer, and the direct and indirect damping of the fast‐stretching modes of the hydrogen bonds and of the bending modes. The infrared spectral density was calculated within the linear response theory by Fourier transform of the autocorrelation function of the transition dipole moment operator of the fast mode. Numerical results show that mixing of all these effects allows satisfactory reproduction of the main features of the experimental IR line shapes of crystalline H‐ and D‐bonded 2‐furoic acid at liquid‐nitrogen temperature and for different polarizations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

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.     

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.     

Polarization IR spectra of the hydrogen bond in phenylacetic acid crystals: H/D isotopic effects temperature and polarization effects

This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline spectra of phenylacetic acid and of its deuterium isotopomers d2 and d7. The spectra were measured in the v(O-H) and in the v(O-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. A vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers, was found to be of a considerably minor importance, when compared with analogous properties of arylcarboxylic acid crystals. The spectra of phenylacetic acid crystals, unlike the spectra of arylacetic acid crystals do not exhibit the so-called H/D long-range isotopic effects, depending on an influence of the aromatic ring hydrogen atoms on the v(O-H) band fine structure patterns. Also no Fermi resonance impact on the v(O-H) band shape was identified in the phenylacetic acid crystal spectra. These effects were ascribed to weakening of electronic couplings between the hydrogen bonds and the phenyl rings, due to the separation of these groups in phenylacetic acid molecules by methylene groups.     

The vibrational spectra of the elementoorganic starburst dendrimers

The main features of vibrational spectra of starburst dendrimers have been analyzed for the first time. Their spectral pattern, in general, is determined by the ratio of a number of terminal groups to a number of repeating units. This ratio tends to m_r−1 (m_r — branching functionality of repeating unit), and becomes constant, when the generation number of the starburst dendrimer increases higher than 3-5. IR and Raman spectra of twelve generations of the phosphorus-containing dendrimers are represented and interpreted on the basis of the calculation of frequencies of the normal vibrations and band intensities in the IR spectra of ‘molecules’ terminated by dangling methyl groups, which are the fragments of the dendrimer molecule. Tailored spectra of these fragments are then compared with experimental spectra and satisfactory similarity has been obtained. Experimental spectra of generations higher than 4 are very similar, according to the theoretical approach. The results can be used for the analysis of the chemical and physical transformations in starburst dendrimers.     

Vibrational spectra of transition metal hexafluoride crystals. II. Two-particle and mixed crystal spectra as techniques for determination of densities

In part II of this series two techniques for obtaining densities of exciton states are discussed: heavily doped crystals and two-particle overtone and combination bands. It is demonstrated through Raman spectra and calculations that (ν_i + ν₁) combination bands yield very nearly true density of states functions for ν_i in the case for which ν₁ is essentially dispersionless. The mixed crystal method for density of states determinations is compared to the combination band technique and approximate mixed crystal concentrations appropriate for such studies can be calibrated for individual bands. It is pointed out that the overtone method, whenever applicable, is both simpler and more accurate for exciton state studies. Detailed analyses of ν₁ and 2ν₁ show that the major contribution to overtone intensity comes from the second order polarizability derivative and not anharmonic contributions.     

Fermi resonances in the Raman spectra of alkali halide/BH4−

Raman spectra of a series of alkali-halide/BH^?₄ (and BD^?₄ crystals have been obtained. These spectra show some interesting examples of Fermi resonance type interactions between the stretching mode levels and overtone and combination band levels of the bending modes. Two resonances will be considered: (i) that between ν₁ and 2ν₄(A₁), and (ii) that between ν₃, 2ν₄ (F₂) and (ν₂+ν₄) (F₂).The F₂ resonance between ν₃, 2ν₄ and ν₂+ν₄ appears in the infrared spectrum and it has been studied on several occasions. However the equivalent Raman spectrum is of interest because the relative intensities of the bands are significantly different to those shown by the infrared spectrum.In the A₁ (and E) Raman spectrum of the stretching mode region there are two strong bands for each for the ¹⁰B and ¹¹B isotopes. The ν₁ would not be expected to show any ¹⁰B and ¹¹B splitting, but the observed bands are both closely resonating mixtures of ν₁ and 2ν₄(A₁). In fact the analysis shows that the stronger band has the higher proportion of 2ν₄ character, and the larger isotopic shift of the more intense band can then be seen to be reasonable.     

Conformation and vibrational spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6-d2

The infrared spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6-d₂ as vapours, liquids, as solutes in various solvents and as crystalline solids at low temperatures and at high pressures have been recorded. Raman spectra were obtained for the liquids, including semiquantitative polarization measurements, and for the low temperature crystals.The data were interpreted in terms of two conformers, anti and gauche, in the vapour and liquid state and one, the anti, in the crystalline forms. A phase transition for 1,5-hexadiyne was observed at ca. 240 K. Both the high and low temperature crystals had molecules in the anti conformer.Interpretation of the spectra in terms of conformational equilibria was facilitated by a thorough vapour phase band contour analysis. With a few exceptions, all the vibrational fundamentals for both conformers were assigned and found to be in good agreement with results from normal coordinate calculations.     

Experimental and theoretical study of the polarized infrared spectra of the hydrogen bond in 3‐thiophenic acid crystal

This article presents the results of experimental and theoretical studies of the v_{O H} and v_{O D} band shapes in the polarized infrared spectra of 3‐thiophenic acid crystals measured at room temperature and at 77 K. The line shapes are studied theoretically within the framework of the anharmonic coupling theory, Davydov coupling, Fermi resonance, direct and indirect damping, as well as the selection rule breaking mechanism for forbidden transitions. The adiabatic approximation allowing to separate the high‐frequency motion from the slow one of the H‐bond bridge is performed for each separate H‐bond bridge of the dimer and a strong nonadiabatic correction is introduced via the resonant exchange between the fast‐mode excited states of the two moieties. The spectral density is obtained within the linear response theory by Fourier transform of the damped autocorrelation functions. The approach correctly fits the experimental line shape of the hydrogenated compound and predicts satisfactorily the evolution in the line shapes with temperature and the change in the line shape with isotopic substitution. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010