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In the present work, we reported a combined experimental and theoretical study on molecular structure and vibrational spectra of 2-furanacrylic acid (abbreviated as 2FAA). The FT-IR and FT-Raman spectra of 2FAA have been recorded in the regions 4000–400 and 4000–100 cm −1 . The spectra were interpreted in terms of fundamentals modes, combination and overtone bands. The monomer and dimer structures of the title molecule have been obtained from Hartree–Fock (HF) and density functional theory (DFT) B3LYP methods with 6-311++G(d,p) as basis set calculations. The vibrational frequencies were calculated by DFT method and compared with the experimental frequencies, which yield good agreement between observed and calculated frequencies. Intermolecular O H⋯O hydrogen bonds are discussed in dimer structure of the molecule. The infrared and Raman spectra were also predicted from the calculated intensities. The polarizability and first order hyperpolarizabilty of the title molecule were calculated and interpreted. A study on the electronic properties, such as excitation energies, oscillator strength, wavelengths, HOMO and LUMO energies, are performed by time-dependent DFT (TD-DFT) approach. In addition, Milliken atomic charges, possible charge transfer, natural bond orbital (NBO) and AIM topological analysis were performed. Moreover, molecular electrostatic potential (MEP) and the thermodynamic properties (heat capacity, entropy, and enthalpy) of the title compound at different temperatures were calculated in gas phase.

This paper presents the results of our investigation with the polarized IR spectra of the hydrogen bond in the 3-pyridinealdoxime and 4-pyridnealdoxime crystals, and also in the crystals of the deuterium derivatives of these two compounds. In each case these spectra were measured for the two different crystalline faces at room temperature and at 77 K by a transmission method, using the polarized light. Theoretical analysis of the results concerned linear dichroic effects, and the H/D isotopic and temperature effects, observed in the solid-state IR spectra of the hydrogen and the deuterium bond, at the frequency ranges of the ν O–H and the ν O–D bands. Basic spectral properties of the crystals can be interpreted in a satisfactory quantitative way for a hydrogen bond linear dimer model in terms of the ‘ strong-coupling ’ theory. This model explains not only a two-branch structure of the ν O–H and the ν O–D bands in the crystalline spectra, but also some essential linear dichroic effects in the band frequency ranges, for isotopically diluted crystals. Within the scope of our studies, the mechanism of the H/D isotopic self-organization processes (which take place in the crystalline hydrogen bond lattices) was also recognized. It was proved that for isotopically diluted crystalline samples of 3-pyridinealdoxime and 4-pyridnealdoxime, a non-random distribution of the protons and deuterons occurs exclusively in some restricted fragments (i.e. domains) of open chains of the hydrogen bonded molecules. Nevertheless, these co-operative interactions between the hydrogen bonds do not concern the adjacent fragments of the neighboring hydrogen bond chains in the lattice. The electronic structure of these compounds was considered as responsible for such co-operative effects of the hydrogen bonds.

This paper presents the investigation results of the polarized IR spectra of H1245 imidazole crystals and of D1H245, D1245 and H1D245 imidazole deuterium derivative crystals. The spectra were measured using polarized light at the room temperature and at 77 K by a transmission method, for two different crystalline faces. Theoretical analysis of the results concerned linear dichroic effects, H/D isotopic and temperature effects, observed in the spectra of the hydrogen and of the deuterium bonds in imidazole crystals, at the frequency ranges of ν N–H and ν N–D bands. The basic crystal spectral properties can be satisfactorily interpreted in a quantitative way for a hydrogen bond linear dimer model. Such a model explains not only a two-branch structure of the ν N–H and ν N–D bands in crystalline spectra, but also some essential linear dichroic effects in the band frequency ranges, for isotopically diluted crystals. Model calculations, performed within the limits of the strong-coupling model, allowed for quantitative interpretation and for understanding of the basic properties of the hydrogen bond IR spectra of imidazole crystals, H/D isotopic, temperature and dichroic effects included. The results allowed verification of theoretical models proposed recently for the imidazole crystal spectra generation mechanisms. In the scope of our studies, the mechanism of H/D isotopic self-organization processes, taking place in the crystal hydrogen bond lattices, was also recognized. It was proved that for isotopically diluted crystalline samples of imidazole, a non-random distribution of protons and deuterons exclusively occurs in some restricted fragments (domains) of open chains of the hydrogen-bonded molecules. Nevertheless, these co-operative interactions between the hydrogen bonds do not concern adjacent fragments of neighboring hydrogen bond chains in the lattice. Analysis of the isotopic self-organization effects in the spectra of imidazole crystals delivered crucial arguments for understanding of the nature of the hydrogen bond spectra generation mechanisms.

This paper presents the results of investigation of the polarized IR spectra of cinnamic acid and of its deuterium derivative crystals. The spectra were measured by a transmission method, using polarized light, at the room temperature and at 77 K, for two different crystalline faces. Theoretical analysis of the results concerned linear dichroic effects, H/D isotopic and temperature effects, observed in the spectra of the hydrogen and of the deuterium bonds in cinnamic acid crystals, at the frequency ranges of the ν O–H and the ν O–D bands. The basic crystal spectral properties could be satisfactorily interpreted in a quantitative way for a centrosymmetric cyclic hydrogen bond dimer model. Such a model explains not only a two-branch structure of the ν O–H and ν O–D bands in crystalline spectra, but also some essential linear dichroic effects in the band frequency ranges, measured for isotopically diluted crystals. Model calculations, performed within the limits of the ‘strong-coupling’ model, allowed for quantitative interpretation and for understanding of the basic properties of the hydrogen bond IR spectra of cinnamic acid crystals, H/D isotopic, temperature and dichroic effects included. In the scope of our studies the mechanism of H/D isotopic ‘self-organization’ processes, taking place in the crystal hydrogen bond lattices, was also recognized. It was proved that for isotopically diluted crystalline samples of cinnamic acid, a non-random distribution of protons and deuterons occurs exclusively in the hydrogen bond dimers. Nevertheless, these co-operative interactions between the hydrogen bonds do not involve the adjacent hydrogen bond dimers in each unit cell. The two-branch fine structure pattern of the ν O–H and ν O–D bands was ascribed to the vibronic mechanism of vibrational dipole selection rule breaking in centrosymmetric hydrogen bond dimers. The observed in the spectra very high intensity of the forbidden transition sub-band in the analyzed ν O–H and ν O–D bands is a manifestation of an extremely effective symmetry rule breaking mechanism. It correlates with a relatively large excess electron charge on the cinnamic aid dimer carboxyl groups. This effect is a result of a partial withdrawal of the electron charge, from the conjugated π-bond systems of the styryl substituents, by the carboxyl groups. This statement has been supported by ab initio calculations.

This paper deals with experimental studies and with the quantitative interpretation of the polarized IR crystalline spectra of 1-naphthylacetic and 2-naphthylacetic acid. The spectra were measured in the ν O–H and in the ν O–D band frequency ranges, at the 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 spectral properties of arylcarboxylic acid crystals. Also no Fermi resonance impact on the ν O–H band shape was identified in the 1-naphthylacetic and 2-naphthylacetic 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 molecular fragments by methylene groups in 1-naphthylacetic and 2-naphthylacetic acid molecules.

This article deals with measurements and a theoretical interpretation of the polarization IR spectra of the hydrogen bond in h 8 -1-naphthoic and h 8 -2-naphthoic acid crystals and crystals of their deuterium isotopomers: d 7 -1-naphthoic acid and d 7 -2-naphthoic 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 ν O–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.

In this paper a theoretical model has been proposed aiming to explain a new kind of H/D isotopic effects, i.e. the isotopic ‘self-organization’ effects, recently deduced from IR spectra of hydrogen bonded molecular crystals. The existence of the new kind of co-operative effects in cyclic dimeric systems of hydrogen bonds was explained in the limits of a vibronic model in the Herzberg–Teller approximation, which assumed a strong coupling between the proton stretching vibrations in a dimer with the electronic motions. It was shown that additional ‘attracting’ forces, responsible for an additional stabilization energy of a dimer, concerning identical hydrogen isotope atoms, appear in the ground vibrational and the ground electronic state, when the proton totally symmetric vibrations in a dimer couple with the electronic movement. These self-organization effects were deduced to be absent in the case of non-symmetric ‘HD’ -type dimers, containing both X–H⋯Y and X–D⋯Y bonds in one dimer. Therefore, the symmetric dimers of the ‘HH’ and the ‘DD’ -type should be more stable when compared with the ‘HD’ -type dimer properties. This would explain characteristic spectral effects, registered in the spectra of partially deuterated molecular crystals containing cyclic dimers of hydrogen bonds in their lattices, depending on a fair invariance of the proton ‘residual’ ν X–H bands in their IR spectra.

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 νN–H and νN–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 νN–H band fine structure to a single prominent νN–D line only. The two νN–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’ νN–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.

This paper deals with experimental studies and with quantitative interpretation of the polarized IR spectra of hydrogen bonded glutaric acid crystals. The spectra were measured for the νO–H and νO–D band frequency regions at temperatures of 298 and 77 K, for the “beta” form of the glutaric acid crystal. The spectra were quantitatively reproduced with a sufficient accuracy on the basis of the “strong-coupling” model, when assuming the centrosymmetric dimer approximation for the crystalline spectral properties. The experimentally proved differences in the polarization properties between the long- and the short-wave branches of the νO–H and νO–D bands were interpreted as a result of the two simultaneously acting mechanisms: (i) promotion of the forbidden transition in the IR region for the totally symmetric protonic vibrations in the (COOH)2 and (COOD)2 cycles of the crystalline lattices, and (ii) the influence of the static “out-of-plane ” deformations of the hydrogen bonds in the cycles on the dimer selection rules. The same mechanisms are responsible for temperature effects in the spectra.

In this paper we discuss the problem of a stable coexistence of the X–H…Y and X–D…Y bonds in one cyclic dimer of the hydrogen bonds. Comparing the spectra of the partially deuterated dimeric systems with those of the pure hydrogen bonded systems, and considering the results of calculations of the dimeric IR spectra, based on the `strong-coupling' model, we found no convincing evidence for the existence of the nonsymmetrical dimers in mixtures of the hydrogen isotope derivative cyclic dimers, although such spectra should easily be distinguished from those of the symmetric dimers of the hydrogen bonds. This isotopic effect seems to apply to the different hydrogen bond cyclic dimeric systems and to be generally widespread in nature. We suggest that the mechanism, which restricts simultaneous existence of the two different hydrogen isotope hydrogen bonds in one dimer, is of a vibronic nature and results from the collective properties of hydrogen bonds. The nonsymmetrical, different hydrogen isotope dimers, most likely characterized by a higher energy, when compared with the symmetrical ones, are considerably rarer than predicted by a statistical approach. We roughly estimated the excess energy value for the mixed hydrogen isotope dimers as equal to 1.5–2.0 kcal mol−1 of the dimers.

In this paper a mechanism is analysed of a long-distance influence of the hydrogen atoms of C-H groups (peripherial with respect to the hydrogen bonds) on the spectral properties of the centrosymmetric dimers of hydrogen bonds, along with the so called ‘long-range’ isotopic effects measured in the frequency range of the ν X-H and ν X-D bands. For this purpose a model of the vibronic coupling for the dimeric system was proposed, within the Herzberg-Teller approximation describing a resonance between the proton stretching vibrations ν X-H and the C-H group stretching vibrations ν C-H , which occur via the electrons of the systems. We derived the purely electronic matrix Hamiltonian of the dimer, responsible for its spectral properties. The consequence of the model is enhancement of an uncommon spectral phenomenon depending on breaking of the vibrational selection rules for the dimer in the IR range, due to promotion of the symmetry forbidden excitation of the totally symmetric protonic vibrations. This effect was found to be mass-dependent on all the hydrogen atoms in the model system. From this model it also seen that the strongest ‘long-range’ isotopic effects correspond with the case when all the hydrogen isotopes in the dimer are identical.

In this paper we propose a new explanation for the abnormal isotopic H/D effects concerning the ν XH and ν XD band integral intensities in the infrared for cyclic systems of hydrogen bonds. In our approach we take into account two different but parallelly acting mechanisms of generating the band contours. One of them is governed by the symmetry-allowed transition to the A u state of the nontotally symmetric protonic vibrations, while the other corresponds to the vibronically activated forbidden transition to the A g state of the totally symmetric vibration of the protons. As the latter mechanism was found to be much more dependent on hydrogen atom mass compared with the allowed transition mechanism, the ν XH to ν XD band intensity ratio could noticeably exceed the expected value of 1.41 (which characterizes the symmetry-allowed substransitions forming the ν XH and ν XD band contours) and approach 1.9. No similar situation takes place for chain systems of hydrogen bonds, for which the forbidden band promotion mechanism cannot play a dominant role, with the isotopic effect being more regular and the band intensity ratio being close to 1.41. The proposed hypothesis was verified experimentally by investigating the isotopic effect for NH⋯S bonded cyclic dimeric systems of 2-thiapyridone and mercaptobenzothiazole. For the IR spectra of these two compounds the forbidden components of their dimeric ν ND bands in the infrared are practically absent, so that the ν NH to ν ND band intensity ratios were expected to exceed 1.9. For both cases the measured band intensity ratios were equal to 2.6 ± 0.3, which generally remains in good agreement with the predictions of the assumed model of the spectral phenomenon.

In this paper we attempt to explain spectral effects measured in the IR spectra of a number of the H-bonded crystals, which appear in the frequency range corresponding to the overtone bands of the γ X − H + … Y (“bending-out-of-plane”) vibrations and are connected with considerable intensities of the overtone bands. We also attempt to explain the observed fine structure of these bands and a strong temperature effect. We consider a simple model of coupling between the two proton vibrations in the H-bonds of the crystalline lattices of imidazole and adipic acid, formally similar to the Fermi resonance model. However, we rather exclude anharmonicity of the protonic vibrations in the H-bonds as a source of the spectral effects analyzed, ascribing spectral phenomena to the effective geometry change (i.e. bending) of the H-bonds in the crystals. Such an approach allows us to explain temperature effects in the crystalline spectra, i.e. to predict the decrease of the coupling energy values with growing temperatures. Model calculations demonstrate that the appearance of the overtone band fine structure is related to the vX-H band fine structure and the γ X − H + … Y band shape depends on the intensity in each individual vX-H band. The analyzed mechanism is a kind of transfer of the “strong-coupling” mechanism onto the proton bending vibrations in the H-bonds, which—due to their highly harmonic nature— do not participate in this sort of interaction.

In this paper we attempt to reproduce quantitatively the IR spectra of 2-mercaptoquinoline in the ν NH and ν ND band frequency regions. The investigated spectra were quantitatively interpreted within the limits of the “strong-coupling” theory for the hydrogen bond vibrations in centrosymmetric H-bonded dimers, including the observed isotopic effect. The observed, very nonregular, spectral properties of 2-mercaptoquinoline were found to be connected with the “strong-coupling” mechanism as well as with the electronic properties of the H-bonds in the molecular associates.

Infrared spectra of the hydrogen bond in acetic acid crystals are interpreted on the basis of strong-coupling theory. A satisfactory numerical reconstitution of the CH 3 COOH, CD 3 COOH, CH 3 COOD and CD 3 COOD crystalline spectra is achieved. A regular isotopic effect was recognised, depending on decrease in distortion parameter value, after replacing hydrogen atoms by deuterium in the H-bonds. The selection rules for vibrational transitions in the crystalline H-bonds are considered, taking into account the calculation results.

In this paper we consider problems concerning the effectiveness of iterational calculations on vibrational analysis. Instead of the widespread least-squares method of force field refinement, another method is proposed based on the steepest-descent algorithm for calculation of the functional minimum of many variables. The new method does not require inverting of a large, poorly conditioned matrix or the introduction of experimental damping coefficients for improving the convergency of the calculations. Test calculations performed for several molecules exhibit a significant “elasticity” of the proposed algorithm allowing more precise reproduction of the molecular vibration frequencies than can be found in recently published works.