DIRECT DYNAMICS CALCULATION OF THE KINETIC ISOTOPE EFFECT FOR AN ORGANIC HYDROGEN-TRANSFER REACTION, INCLUDING CORNER-CUTTING TUNNELING IN 21 DIMENSIONS

We have calculated the kinetic isotope effect of the reaction CF3 + CD3H by a semiclassical method that gives insight into tunneling paths for hydrogen atom transfer. In particular, tunneling is treated by a new optimized multidimensional semiclassical method which is valid even for large curvature of the reaction path, and should have wide applicability. The method used in the dynamics calculations is variational transition state theory with ground-state transmission coefficients. The torsional vibrational mode of the transition state is treated as a hindered internal rotation. The calculations are made practical for a system with 27 degrees of freedom by employing the direct dynamics approach, i.e., the force field necessary for the dynamics calculations is evaluated ''on the fly'' by using the neglect of diatomic differential overlap (NDDO) molecular orbital theory with semiempirical specific-reaction parameters (SRP), which are based on the standard AM1 parametrization adjusted to improve the agreement between experiment and the calculated quantities such as the vibrational frequencies of reactants and products and the classical barrier height. The kinetic isotope effects are calculated by. using two SRP force fields, and they are in good agreement with the experimental measurements. The picture of the corner cutting tunneling process that emerges is discussed graphically.