Hydrogen-Bond Symmetrization and Molecular Dissociation in Hydrogen Halides

Hydrogen chloride is a simple diatomic molecule forming a planar zigzag chain of molecules connected by hydrogen bonding in the solid phase. Raman spectra were measured for solid HCl to 60 GPa, at room temperature. The molecular stretching frequency falls toward zero at about 51 GPa, whereas the molecular vibrational peaks disappear and the lattice peaks remain. The spectral changes are very similar to those observed for HBr at about 42 GPa and are interpreted as hydrogen bond symmetrization. Molecular dissociation into diatomic halogen molecules, which has been observed for HBr, does not occur in HCl.