The H+D2→HD+D and D+H2→HD+H reactions:: ab initio and rate constants calculations

The ab initio calculations have been carried out on the H + D-2 -> HD + D and D + H-2 -> HD + H reactions. Geometry optimization, vibrational frequency and energy calculations have been performed on all reactants and the transition states using the Gaussian 98 set of quantum chemistry codes at different levels of ab initio theory. The ab initio frequencies and geometries for H2 and D2, calculated at the QCISD/cc-PVTZ and QCISD/aug-cc-PVTZ levels of theory, are in very good agreement with experiment. The ab initio normal mode frequencies and geometries for the transition states are similar to those for the double many-body expansion surface. Based on these ab initio calculations, conventional transition state theory, thermodynamic formulation of transition state theory and semiclassical transition state theory have been used to predict the rate constants as function of temperature (300-1100 K). With QCISD/cc-PVTZ and QCISD/aug-cc-PVTZ levels, the calculated rate constants are in reasonable agreement with their experimental counterparts.