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

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Abstract
Corrections to the Born‐Oppenheimer electronic energy are calculated for several first and second row diatomic molecules. These corrections are evaluated in the adiabatic approximation at the experimental internuclear distance and thereby constitute the leading terms in the non‐Born‐Oppenheimer molecular energy. The corrections to the electronic energy are functions of the nuclear masses and are found to produce significant electronic isotope effects on the equilibrium constants of certain exchange reactions. In particular, at 300 °K the electronic isotope effect on the equilibrium constant of the reaction HX + HD = DX + H2 where X is Li, B, N, or F is found to range from 1.029 for Li to 1.101 for B. By partitioning the correction to the Born‐Oppenheimer electronic energy into a sum of atomic and overlap terms, the electronic isotope effect is shown to be dependent upon certain characteristics of the molecules, the single most important of which is the X atom electronic angular momentum.

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