DIRECT DYNAMICS STUDY OF INTRAMOLECULAR PROTON-TRANSFER IN HYDROGENOXALATE ANION

We have carried out ab initio calculations for the intramolecular proton-transfer process in hydrogenoxalate anion using Moller-Plesset perturbation theory with a reasonably large basis set. We found that electron correlation is very important in predicting the barrier height as well as the equilibrium and transition-state structures. The classical barrier height calculated at the MP2/6-31++G** level is 3.1 kcal/mol. Including the zero-point energy correction reduces the barrier to only 0.4 kcal/mol for the proton transfer, and to 1.3 and 1.6 kcal/mol for the deuterium and tritium isotope substituted reactions, respectively. We also used these results with transition-state theory and an Eckart semiclassical tunneling method to calculate the rate constants and kinetic isotope effect for this reaction. We found that the tunneling contribution to the rate constant is smaller for the proton transfer than for other heavier isotopes. The calculated kinetic isotope effect is quite large and due mostly to the in-plane hydrogen stretch and bend vibrational modes.