Spectra of H-bonded Crystals IR versus INS

Due to relatively small changes in the dipole moment associated with O -H -O bendings, these fundamentals often have relatively small intensities in the IR spectra. However, these bands correspond to large vibrational displacements of the proton, and they are usually clearly seen in the INS spectrum [30, 49]. Summarizing, the number of bands corresponding to the fundamental vibrations of the O -H -O fragment, and their frequencies in the considered crystals, may be different in the IR and INS spectra. 

As a result, a large number of a relatively intensive nonfim-damental transitions may appear in the IR spectra. In particular, the broad IR band associated with the asymmetric stretch of the O H O fragment is due to combinations with the O- O stretch. The change in the dipole moment function plays no role in the INS spectra and this is why the Vjs(OHO) bands in INS spectra are usually narrow and contain no combinations. 

A possible way to detect strong coupling between the asymmetric and symmetric stretches of the 0- -H- -0 fragment is to compare the low-temperature INS spectrum of the crystal with that obtained at relatively high temperature, that is when the first excited state of the O -O stretch is sufficiently populated. To our knowledge, such a comparison has not been done yet. 

At present three different codes are widely used for calculations of the structural and spectroscopic properties of H-bonded crystals, for example see Refs. [82-85]. The Car-Parrinello molecular dynamics (CPMD) program [86] and the Vienna ab initio simulation program (VASP) [87, 88] use a plane wave basis set, while an atom centered set is used with periodic boundary conditions in the CRYSTAL 

In the first step the positions of all atoms in the cell are optimized. Cell parameters are usually borrowed from experiment. In some cases they are optimized [84] and in some cases not [85]. Harmonic frequency calculations verify that the computed structure corresponds to the global PES minimum. In the second step the anharmonic OH stretching [83, 84] frequency is estimated using ID potential curves calculated as a function of the displacement for the hydrogen atom. In the third step classical molecular dynamics (MD) simulations are performed. The IR [85] or vibrational spectrum [82, 83] of the crystal is computed from the Fourier transform of the corresponding time correlation function (see Section 9.3.1). 

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