Marek J. Wójcik

Infrared spectra of hydrogen bond a general theoretical model

Abstract For the IR spectra of the hydrogen bond a general theoretical model is presented which puts on equal footing the Fermi resonance and the coupling between the high and low frequency modes. The fundamental hamiltonian is derived and its spectral consequences studied in detail. The theoretical IR spectra of the hydrogen bond are obtained and the relative roles of both mechanisms are quantitatively studied and discussed.

Fermi resonance in dimers : a model study

A theoretical model is proposed for symmetric dimers of carboxylic acids which accounts for three effects on an equal footing : (i) coupling between the high and low-frequency modes ; (ii) Davydov-type splitting of the degenerate vibrational states ; and (iii) Fermi resonance between the hydrogen-stretching fundamental and the first overtone of the hydrogen bending. These three mechanisms contribute to the infra-red spectra which have been computed for different values of an anharmonic parameter.

Infrared spectra of ice surfaces and assignment of surface‐localized modes from simulated spectra of cubic ice

The use of a new method of preparing micron‐thick deposits of nanocrystals of ice for Fourier transform infrared sampling, with the nanocrystals supported on a vertical infrared window, has greatly improved the signal‐to‐noise levels of the spectra available for large ice clusters. High quality spectra of modes of the surface molecules are reported, even for regions that underlie the intense bands of the bulk ice modes. These experimental features are most clearly displayed through the use of difference spectra. For example, the difference between spectra obtained for nanocrystals, before and after an annealing cycle that significantly increases the average cluster size, reflects the decrease in number of surface groups and the corresponding increase in number of interior molecules. Similarly, differences between spectra of bare and adsorbate‐covered nanocrystals, obtained at the same temperature for the same ice sample, show the significant shifts of ‘‘surface‐localized’’ ice modes caused by the adsorbat...

Experimental and simulated vibrational spectra of H2 absorbed in amorphous ice: Surface structures, energetics, and relaxations

Infrared spectra are reported for thin films of deuterated microporous amorphous ice formed at 12 K and saturated with absorbed molecular hydrogen. This paper focuses on both the influence of the surface‐bound H2 on the absorption bands of the OD groups that dangle from the micropore surfaces and the behavior of the induced infrared bands of the stretching mode of H2 itself. Both structural changes and the relaxation of ortho‐H2 to para‐H2 are apparent from variations in the observed spectra with time and temperature. A reasonably detailed interpretation of the complex spectral behavior has been possible through simulation of spectra for H2 interacting with the surface of amorphous ice clusters generated previously in a classical trajectory study of the cluster growth through the accumulation and relaxation of individual water molecules. Potential minima were calculated with respect to H2 coordinates on the cluster surface and a qualitative interpretation of adsorbate–surface bonding provided via the part...

Theoretical modeling of infrared spectra of aspirin and its deuterated derivative

Abstract Theoretical simulation of the ν s stretching band is presented for aspirin (acetylsalicylic acid) and its OD derivative at 300 and 77 K. The simulation takes into account an adiabatic coupling between the high-frequency O–H(D) stretching and the low-frequency intermolecular O⋯O stretching modes, linear and quadratic distortions of the potential energy for the low-frequency vibrations in the excited state of the O–H(D) stretching vibration, resonance interaction between two hydrogen bonds in the dimer, and Fermi resonance between the O–H(D) stretching and the overtone of the O–H(D) bending vibrations. The effect of deuteration and the temperature has been successfully reproduced by our model calculations. Infrared, far-infrared, Raman and low-frequency Raman spectra of the polycrystalline aspirin have been measured. The geometry and experimental frequencies are compared with the results of our B3LYP/6-31++G** calculations.

Car–Parrinello Simulation of the Vibrational Spectrum of a Medium Strong Hydrogen Bond by Two-Dimensional Quantization of the Nuclear Motion: Application to 2-Hydroxy-5-nitrobenzamide

The nature of medium strong intra- and intermolecular hydrogen bonding in 2-hydroxy-5-nitrobenzamide in the crystal phase was examined by infrared spectroscopy and Car-Parrinello molecular dynamics simulation. The focal point of our study was the part of the infrared spectra associated with the O-H and N-H stretching modes that are very sensitive to the strength of hydrogen bonding. For spectra calculations we used an isolated dimer and the fully periodic crystal environment. We calculated the spectra by using harmonic approximation, the time course of the dipole moment function as obtained from the Car-Parrinello simulation, and the quantization of the nuclear motion of the proton for an instantaneous snapshot of the structures in one and two dimensions. Although quantitative assessment of the agreement between the computed and experimental band contour is difficult due to the fact that the experimental band is very broad, we feel that the most reasonable qualitative agreement with the experiment is obtained from snapshot structures and two-dimensional quantization of the proton motion. We have also critically examined the methods of constructing the one-dimensional proton potential. Perspectives are given for the treatment of nuclear quantum effects in biocatalysis.

Spectra of isotopic ice mixtures

Studies of vibrational spectra of ice I and amorphous ice in the stretching mode frequency range were extended to include (a) the observed effect of a full range of isotopic dilution on the infrared spectra and (b) computational modeling of the observed influence of each dilution step on the properties of vibrationally excited states and on infrared and Raman spectra. The quantum‐mechanical computational scheme included effects of frequency lowering due to hydrogen bonding, and of intra‐ and intermolecular coupling between bonds. The H2O/D2O mixtures can be viewed as a collection of clusters of one X2O isotopomer embedded in a matrix of the other isotopomer. The properties of the vibrationally excited states and of the spectra are determined by complex interplay between the size distribution of the embedded clusters, and the inter‐ and intramolecular coupling. Vibrational excitations are delocalized over large portions of the embedded clusters. In the limit of a pure crystalline isotopomer, the excitation...

Note on a theoretical interpretation of the isotopic ratio vOH/vOD versus vOH in isotopically diluted HDO molecules in solid hydrates

Abstract A simple intra- and inter-molecular potential for water molecules engaged in hydrogen bonding has been used in a one-dimensional approximation for a theoretical interpretation of the experimental correlation v OH / v OD versus v OH in solid hydrates. The variation in the equilibrium OH(D) distance has also been considered.

Dynamic interactions of hydrogen bonds in oxalic acid crystals

Abstract A theoretical interpretation is presented of the IR spectra of the hydrogen bonds in three different crystalline forms of oxalic acid (α, β and dihydrate). The calculated spectra are in quantitative agreement with the experimental one for both the frequency and intensity distribution of the vibrational fine structure of the hydrogen bond absorption spectrum. The drastic changes in the IR spectra introduced by the deuterium substitution in the hydrogen bonds are quantitatively predicted by the theory.

Theoretical model for a tetrad of hydrogen bonds and its application to interpretation of infrared spectra of salicylic acid

Theoretical model of vibrational interactions in hydrogen-bonded salicylic acid dimer is presented which takes into account the adiabatic couplings between high- and low-frequency O-H and O...O stretching vibrations, resonance interactions between both intermolecular hydrogen bonds and between inter- and intramolecular hydrogen bonds, and Fermi resonance between the O-H stretching fundamental and the first overtone of the O-H in-plane bending vibrations. The model is used for theoretical simulation of the nu(s) stretching bands of salicylic acid and its OD derivative at 300 K. The effect of deuteration is successfully reproduced by our model. Infrared, far infrared, Raman, and low-frequency Raman spectra of the polycrystalline salicylic acid and its deuterated derivative have been measured. The geometry and experimental frequencies are compared with the results of density-functional theory calculations performed at the B3LYP6-31 ++ G**, B3LYP/cc-pVTZ, B3PW916-31 ++ G**, and B3PW91/cc-pVTZ levels. O-H, O-D, and O...O stretching frequencies are used in theoretical simulation of the nu(s) stretching bands.