The reactions CHnD4−n+OH→P and CH4+OD→CH3+HOD as a test of current direct dynamics computational methods to determine variational transition-state rate constants. I.

In the present work, we have theoretically calculated the rate constants and their temperature dependence for the reactions CHnD4−n+OH→P, and for the reaction of methane with OD, by means of variational transition-state theory plus multidimensional tunneling corrections, at the MP-SAC2//MP2/cc-pVTZ/// and CCSD(T)//MP2/cc-pVTZ/// electronic levels. Also, the newly developed single-point energy interpolation algorithm has been used at the CCSD(T)/aug-cc-pVTZ//MP2/cc-pVTZ and CCSD(T)-SAC//MP2/cc-pVTZ levels. For reactions with n=1, 2 or 3, the competitive canonical unified statistical theory has been applied as they involve more than one nonequivalent reaction channel. Variational effects and tunneling have been found to be very important. The proton shift classical energy barrier turns out to be 5.83 and 4.97 kcal/mol at the CCSD(T)/aug-cc-pVTZ//MP2/cc-pVTZ and CCSD(T)-SAC//MP2/cc-pVTZ levels, respectively. Even though we have used the highest ab initio electronic level reported up to now for dynamics calcu...

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