Corrections to the Born-Oppenheimer approximation and electronic effects on isotopic exchange equilibria. II

Abstract

If nonadiabatic effects are neglected (the adiabatic approximation), the potential energy for the motions of the nuclei of a molecule is the sum of the Born‐Oppenheimer electronic energy and the adiabatic correction (diagonal nuclear motion correction). This adiabatic correction has been evaluated at the experimental equilibrium internuclear separation for a number of first‐ and second‐row diatomic hydrides and deuterides. The electronic wavefunctions which have been used in these calculations are LCAO MO SCF functions which range in quality from so‐called minimum basis set functions to so‐called near‐Hartree‐Fock functions. The explicit R dependence of the LCAO coefficients has been taken into account. The adiabatic corrections to the electronic energy are functions of the nuclear masses. With the assumption of the distance independence of the adiabatic correction, the calculations here can be used to evaluate the effect of the adiabatic correction on the equilibrium constants for the isotopic exchange reactions HX+HD=DX+H₂. For the HX systems on which calculations were done here, these correction factors range, with good single configuration wavefunctions, from 0.963 to 1.113 at 300°K.