Direct calculation of equilibrium constants for isotopic exchange reactions by ab initio molecular orbital theory

The ability of simple levels of ab initio molecular orbital theory to describe with reasonable accuracy the energetics of isotopic exchange processes is demonstrated. Three levels of ab initio molecular orbital theory have been surveyed. The first two levels are single-determinant Hartree-Foch methods utilizing the 3-21G split-valence and 6-31G* polarization basis sets. The third level, which is computationally the most complex, uses the 6-31G* basis set but allows for partial account of electron correlation by way of Moller-Plesset perturbation theory terminated at second order. Theoretical and spectroscopic equilibrium constants for reactions XH + XD reversible XD + H/sub 2/ where XD is a hydride of the first row of the periodic table are tabulated. (BLM)