Shifts in vibrational constants from corrections to the Born-Oppenheimer approximation: Effects on isotopic exchange equilibria

The adiabatic correction to the Born-Oppenheimer approximation has been evaluated as a function of internuclear separation for the diatomic molecules in the isotopic exchange reaction HX+HD=DX+H2, where X=D, Li, or Cl. The sum of the isotope-dependent adiabatic correction and the isotope-independent Born-Oppenheimer electronic energy is the effective potential energy for nuclear motion if nonadiabatic effects are neglected. The resulting shifts, from the Born-Oppenheimer values, of the equilibrium internuclear separations and of other vibrational constants are calculated. It is found that these shifts are so small that they have negligible effect on calculated values of the isotopic exchange equilibrium constants. The only important contribution of the adiabatic correction to the equilibrium constants arises from the fact that the ΔE of the isotopic exchange reactions between translationless, rotationless, vibrationless reactants and corresponding products no longer equals zero.