Relaxations of molecular charge distributions and the vibrational force constants in diatomic hydrides

Abstract

The force constants for LiH, HF, NaH, and HCl are calculated from Hartree–Fock wavefunctions by a polynomial fit of the forces exerted on the nuclei as a function of the internuclear separation. The magnitude of the force constant is interpreted in terms of the relaxation of the molecular charge distribution induced by the nuclear displacement. In LiH or NaH, for which the molecular charge distribution exhibits the characteristics of ionic binding, two distinct relaxations are evident: a relaxation in the region of the cationic core and a relaxation of the density localized on the proton. The relaxation of the charge density in the vicinity of the Li⁺ or Na⁺ core opposes the motion of either nucleus while the relaxation of the density localized on the proton facilitates the displacement of the nuclei. In HF or HCl the relaxation of the molecular charge distribution is dominated by one continuous region of charge increase (for bond contraction) or decrease (for bond extension) over the whole of the binding region, a relaxation which facilitates the motion of the nuclei. Thus the relaxation of a molecular charge distribution and its effect in determining the magnitude of the force constant is dominated by the same features of the static charge distribution which serve to distinguish ionic from covalent binding.