Direct ab initio dynamics studies of N+H2⇆NH+H reaction

Kinetics of the N+H-2<----> NH+H reaction have been studied using a direct ab initio dynamics method. Potential energy surface for low electronic states have been explored at the QCISD/cc-pVDZ level of theory. We found the ground-state reaction is N(S-4)+H-2--> NH((3)Sigma(-))+H. Thermal rate constants for this reaction were calculated using the microcanonical variational transition state theory. Reaction path information was calculated at the QCISD/cc-pVDZ level of theory. Energies along the minimum energy path (MEP) were then refined at the QCISD(TQ)/cc-pVTZ level of theory. The forward and reverse barriers of the ground-state reaction are predicted to be 29.60 and 0.53 kcal/mol, respectively. The calculated rate constants for both forward and reverse reactions are in good agreement with available experimental data. They can be expressed as k(T)=2.33x10(14) exp(-30.83 (kcal/mol)/RT) cm(3) mol(-1) s(-1) for the forward reaction and k(T)=5.55x10(8)T(1.44) exp(-0.78(kcal/mol)/RT) cm(3) mol(-1) s(-1) for the reverse reaction in the temperature range 400-2500 K. (C) 2000 American Institute of Physics. [S0021- 9606(00)30139-8].