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In this work we have attempted to provide a new interpretation of some complex spectral effects, recently observed in the high frequency region of the IR spectra of hydrogen bonded imidazole crystals. The subjects of our interest were effects corresponding to the intensity distribution, the linear dichroism, the influence of temperature and the isotopic substitution of deuterium in imidazole molecules, measured in the v NH and the v ND band ranges in the crystalline spectra. Based on model calculations it was shown that the “strong-coupling” model was able to quantitatively reproduce the v NH and v ND band shapes in the spectra of the imidazole crystal. This fact can be considered as a basis for an explanation of the isotopic, polarization and temperature effects in the spectra. The Fermi resonance effects have been found to be of a minor importance. The key property of the crystal responsible for the abnormal spectral properties of its H-bonds seems to be the statistical disorder in the crystalline lattice, due to bending of the H-bonds. This structural effect, along with the very specific selection rules in the IR for the centrosymmetric H-bonded dimers and the correlation field interactions in the crystal, were considered to be the main source of the spectral effects analyzed.

In this paper we present a theory of the spectral properties of hydrogen bonded imidazole-type monoclinic molecular crystals in the IR, where Z = 4, the site symmetry is C i , and infinite open chains of H-bonds are considered. For the model crystal restricted to a single unit cell, a vibrational hamiltonian was derived, describing a strong anharmonic coupling between the two normal modes of the H-bonds with different frequencies: the high-frequency proton stretching vibration v XH and the low-frequency H-bond stretching vibration v X…Y . The proposed model emphasizes an extremely strong coupling between these H-bonds in the unit cell, connected by the inversion centre operation. This coupling occurs not only due to the non-totally symmetric vibrations, but also via the totally symmetric H-bond vibrations in the two centrosymmetric dimers which occupy the unit cell. The coupling is mathematically expressed, for the vibrationally excited H-bonds in each dimer, by a dependence of the resonance interaction integral upon the H-bond low-energy vibration coordinates. Thus a specific coupling between the normal vibrations of the H-bonds in the model system occurs and in consequence a new mathematical treatment of the problem has to be worked out. In this paper we present a method for numerical solution of the vibrational eigenvalue problem for the assumed “strong-coupling” model of the crystal. The vibrational dipole selection rules for the optical transitions in the crystal were also derived, along with the formulas for the integral properties of the crystalline spectra. On the basis of this method, numerical simulation of the crystalline spectra for the v XH band region was finally proposed.

This paper attempts to provide a new interpretation of some complex spectral effects observed in the IR spectra of the hydrogen bond in the adipic acid crystal. The subjects of our interest are the effects corresponding to the abnormal linear dichroism, the influence of temperature, and the isotopic substitution of deuterium in the H-bonds, measured in the vOH and the vOH band ranges in the crystalline spectra. In this paper a critical review of some recent solutions to the problem is given, and a new “strong coupling” approach for the crystalline spectral properties proposed. Based on the model calculations it is shown that the “strong coupling” model is able to quantitatively reproduce the vOH and vOD band shapes in the spectra of the adipic acid crystal, a fact which can be considered as a basis for an explanation of the isotopic, polarization and temperature effects in the spectra. The key property of the crystal responsible for the abnormal spectral properties of the hydrogen bonds seems to be the statistical disorder in the crystalline lattice. The H-bonds bent up or down with respect to the ring formed by the two linked COOH groups are considered to be the main source of the spectral effects analyzed, along with the very specific selection rules in the IR for the centrosymmetric H-bonded dimers.

The concept of conjugation between formal double bonds separated by a formal single bond is frequently used to describe qualitatively the electron delocalization in π-electron systems. Whereas in carbon compounds the prerequisite for optimum conjugation is a rather strict, geometrical condition of exact or near coplanarity of the participating carbon atoms, the geometrical conditions are much more relaxed for third-row atoms if valence-shell d orbitals are involved in the bonding. In a family of sulfur compounds, N-sulfonylsulfilimines, the conjugation is virtually unaffected by large distortions of bond-rotation angles, leading to rotation-independent conjugation. Ab initio SCF-HF calculations, using an STO-3G∗ basis set that includes d orbitals for the S atoms, provide evidence for such “flexible conjugation” and give an explanation for the experimentally observed interconversion of chiral conformers of molecules of the above family. The results are in agreement with experimental data and the qualitative conclusions of earlier, semiempirical calculations.

An attempt is made at explaining the nature of the mechanism of intervibrational interactions occurring in bifurcated hydrogen bonds, which are responsible for the appearance of doublet structures of the vN-H bands in infrared (IR) spectroscopy. The model employed assumes strong anharmonicity of the proton stretching vibration in the hydrogen bonds, corresponding with the three minima of the potential-energy surface. In the first order of the perturbation theory the excited state of the proton vibration can be treated as doubly degenerate. For the description of the coupling mechanism between the protonic motions and the hydrogen-bond low-frequency vibrations an approach similar to that from the “strong coupling” theory of the hydrogen-bond spectra was used. Namely, a matrix vibrational Hamiltonian of the model bifurcated hydrogen bond, describing coupling between vibrations taking place in the system, was derived. Starting from this Hamiltonian a numerical simulation of the vN-H band shapes was done for the hydrogen bond spectra. Good agreement between the calculated and the measured band shapes was achieved, along with a good reproduction of the band fine-structure elements. Nevertheless, the model failed to explain the vN-D band shapes for the deuterated analogues of the hydrogen bonds. This discrepancy is most probably connected with the relation between the proton vibration quantum magnitude and the potential-energy surface parameters.

The abnormal IR spectral properties of the hydrogen bond in 2-thiapyridone have been investigated together with the complex effect of deuteration on the NH⋯S hydrogen bonds. The hydrogen-bond spectra of the deuterated form of 2-thiapyridone have been remeasured in order to confirm the results of some recent investigations. The spectra have also been theoretically constructed based on the strong coupling model of the vibrational spectra of the hydrogen-bonded dimers. In addition, a dependence of the resonance interaction energy of the dimeric νNH vibrations on the normal coordinates has been assumed thus describing the low-energy vibrations of the hydrogen bridges of the dimer.

This paper attempts to give a theoretical interpretation of the anomalies observed in the spectra of the hydrogen-bonded centrosymmetric dimers. The analyzed effects were observed in the numerous recently published dimeric spectra and ascribed to a deviation from the dipole selection rules for the centrosymmetric dimers; they depend on activation of the forbidden transition in IR to the A g excited state of the proton stretching vibrations in the hydrogen bonds. A vibronic model of interactions in the hydrogen bonds was assumed, which allowed one to derive a purely electronic matrix Hamiltonian of a dimer. It described mixing between the dimeric vibrational states, belonging to different irreducible representations of the C i group. In our model two kinds of interactions, i.e., the vibronic coupling in the Herzberg-Teller approximation and proton vibration anharmonicity, played an essential role in the coupling mechanism and determined the shape of the Hamiltonian. The analysis of some Hamiltonian properties allowed one to conclude that in some cases the vibrational transition to the A g state may be allowed for the dimers as a result of a modified Herzberg-Teller mechanism, originally established for the molecular electronic spectra to explain the vibrational spectral phenomena. The deviation from the selection rule is connected with a dynamic asymmetry of the electron charge distribution and with a dynamic asymmetry of the protonic positions in the A g excited state of the proton vibrations. The model also predicts diminution of the forbidden transition intensity after replacing the H-atoms by the D-atoms in the dimeric hydrogen bonds.

Electronic population changes and the evolution of the contour maps of the charge density and its Laplacian are used to study the polarizability of hydrogen bonds as a function of the position of the proton within the hydrogen bond. Standard ab initio molecular orbital theory at the 4–31G level (44-31G* for sulphur) has been used to study the complexes between the acceptors, formaldehyde and thioformaldehyde, and the donors, hydrogen fluoride and the ammonium cation. A number of important conclusions for the interpretation of infrared spectra are discussed.