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 intramol ecular proton transfer in hydrogenoxalate anion and its deuterated spe cies. The method used is based on the variational theory of the transi tion state as modified by introducing semiclassical corrections for th e estimation of tunneling on the sole basis of electronic structure ca lculations. 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 correc tions 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 t o 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 cons iders a more rectilinear path for the proton. Such is the case with th e large curvature approximation(LCT). We had calculated the LCT transm ission 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 f or this kind of system. The results of high level and dual level calcu lations were quite consistent. Also, the SCT approximation was found t o describe tunneling more accurately than did the LCT treatment, partl y as a result of the low transfer barrier involved. The analysis of co ntributions to kinetic isotopic effect revealed that, although tunneli ng contributes significantly, vibration is the single most influential factor in this respect.