Quasiclassical trajectory and transition state theory studies of the N(4S)+H2⇆NH(X3Σ-)+H reaction

Detailed forward and reverse quasiclassical trajectory and variational transition state theory studies of the title reaction are presented. The potential energy surface (PES) has been calculated at the MCQDPT2//FORS-MCSCF(7,6)/6-311++G** level of theory using GAMESS. Selection of points and analytical interpolation of the surface was done using the GROW program of Collins, Thompson, Jordan, and Bettens. Broad ranges of translational energies, and ground and excited rovibrational energy levels (v,j) are chosen as initial conditions of the reactant molecules. All three reactions considered in this study (e.g., N + H-2 and NH + H abstraction and exchange) show angular distributions that are backward peaked. Both the forward abstraction and the exchange reactions show product vibrational distributions that are dominated by the ground state, whereas the reverse reaction prefers v' = I for trajectories initiated at low ET. The rate constants for the forward and reverse reactions were found to be in good accord with available experimental and theoretical data and can be expressed in the Arrhenius form as k(T) = 6.85 x 10(-10) exp(-25.9(kcal/mol)/RT) and k(T) = 1.85 x 10(-14)T(1.07) exp(-2.2(kcal/mol)/RT) cm(3) molecule(-1) s(-1), respectively. For ICVT results, these rate constants can be similarly expressed as k(7 = 5.52 x 10(-10) exp(-32.4(kcal/mol)/RT) and k(T) = 1.56 x 10(-16)T(1.64) exp(-0.7(kcal/mol)/RT) cm(3) molecule(-1) s(-1), respectively. The reasonable agreement with the available data substantiates the sufficiency of the computational methods employed in this study.