Raman Spectra of Hydrogen and Deuterium Halide Crystals

Abstract

The crystal Raman Spectra of HCl, DCl, HBr, and DBr have been observed for the lattice and intramolecular vibrational regions. The results for the intramolecular region indicate that the crystal structure of the low‐temperature orthorhombic phase is $C_{\text{2v}}^9{\mathrm{\hspace{0.17em}-\hspace{0.17em}P}}_{{\text{n2}}_{1}a}$ with four molecules in a unit cell, containing nonplanar hydrogen‐bonded zigzag chains. However, the nonplanarity of the chain is so small that it is not sensitively reflected to the spectra of lattice vibrational region. As a result, the lattice vibrational bands may be assigned adequately based on a planar structure $C_{\text{2v}}^{12}{\mathrm{\hspace{0.17em}-\hspace{0.17em}B}}_{{\text{b2}}_{1}m}$ , which is the structure determined by neutron diffraction measurement. Normal coordinate calculations have been made by using several kinds of intermolecular potentials. A central force field which assumed dispersion and repulsion forces and dipole–dipole forces failed to explain the observed lattice frequencies. A potential which assumed hydrogen bonding explicitly gave a reasonable fit of the calculated frequencies with the observed ones. The calculations showed that the hydrogen bond involved is relatively weak and high frequencies of the rotational lattice vibrations in spite of the weakness of the hydrogen bond are due simply to a small value of the moment of inertia of the molecule. The Raman spectra of the high‐temperature cubic phase showed no strong band in the lattice region and very broad bands in the stretching region. From the results obtained, a model for the molecular reorientational motions in the cubic crystals has been proposed, in which the molecule in the crystal is bound to one of the equilibrium orientational positions, but reorients at random from one equilibrium position to the other before the molecule as a whole completes one cycle of translational oscillation. This model seems to explain the experimental data available.