DIRECT DYNAMICS CALCULATIONS OF DEUTERIUM AND C-13 KINETIC ISOTOPE EFFECTS FOR THE REACTION CL+CH4

The kinetic isotope effects, KIEs, for hydrogen abstraction from the i sotopologs CH4, (CH4)-C-13, CH3D, and CD4 by chlorine atoms have been studied by the dual-level direct dynamics approach with the MORATE com puter program. Low-level calculations of the potential energy surface were carried out at the NDDO-SRP level (in particular the AM1-SRP leve l), using two different sets of specific reaction parameters labeled S RP4 and SRP13. High-level structural and energetic properties of the r eactants, saddle point, and products were obtained at the MP2-SAC and MP2 levels using the 6-311G(2d,d,p) basis set and were used to interpo late corrections to the low-level calculations. The dual-level calcula tions were carried out using the ICL-Eckart improved interpolated corr ections algorithm. Tunneling was included by the microcanonical optimi zed multidimensional tunneling (mu OMT) method, and we find that large -curvature tunneling paths usually provide the dominant contribution, with significant participation of excited vibrational states. Both rec tilinear and curvilinear coordinates were applied to the unsubstituted reaction. The C-12/C-13 KIEs calculated at the MP2-SAC///SRP4 level u sing MP2 frequencies are in very close agreement with the experimental ones, with values of 1.07 and 1.06 at 243 and 297 K, respectively, as compared to experimental values of 1.07 at both temperatures, For the reaction CH3D + Cl, the calculated H/D KIEs are equal to 1.55 and 1.4 5 at 223 and 296 K, while the measured values are equal to 1.59 and 1. 50, respectively. The H/D KIE for the reaction CD4 + Cl is calculated to decrease from 11 to 4.7 as the temperature increases from 300 K to 450 K, whereas the experimental value decreases from 12 to 3.9, We als o make comparison with previous results for the unsubstituted reaction CH4 + Cl.