High Level and Dual Level Direct Dynamics in the Intramolecular Proton Transfer of Hydrogenoxalate Anion. Influence of Tunneling and Isotopic Effect

Direct ab initio dynamics methodology was used to investigate intramolecular proton transfer in hydrogenoxalate anion and its deuterated species. The method used is based on the variational theory of the transition state as modified by introducing semiclassical corrections for the estimation of tunneling on the sole basis of electronic structure calculations. Such calculations, which included energies, gradients, and Hessians, both at stationary points and throughout the reaction path, were done by using the MP2/6-31++G** level with barrier height corrections at QCISD/6-31++G** (4.85 kcal/mol). No variational effects were observed at this fairly high computational level over the temperature range studied. Some of the modes of this reaction are highly coupled to the reaction path, so tunneling may be quite substantial. Within the direct ab initio dynamics we used the small curvature approximation (SCT) to assess tunneling; however, because the particle transferred is a light particle, the problem may call for an approximation that considers a more rectilinear path for the proton. Such is the case with the large curvature approximation (LCT). We had calculated the LCT transmission factors as well as the SCT transmission factors within the dual level dynamics, replacing ab initio calculations in the nonstationary points by a semiempirical method, which was previously parametrized for this kind of system. The results of high level and dual level calculations were quite consistent. Also, the SCT approximation was found to describe tunneling more accurately than did the LCT treatment, partly as a result of the low transfer barrier involved. The analysis of contributions to kinetic isotopic effect revealed that, although tunneling contributes significantly, vibration is the single most influential factor in this respect.