A quantum reactive scattering study of the spin-forbidden CH(X 2Π)+N2(X 1Σg+)→HCN(X 1Σ+)+N(4S) reaction

Abstract

The dynamics of the spin-forbidden CH(X 2Π)+N2(X 1Σg+)→HCN(X 1Σ+)+N(4S) reaction has been studied theoretically using the reduced dimensionality quantum scattering method. Three degrees of freedom have been considered in the dynamics calculations by treating CH as a united atom. The problem is thus reduced to the usual atom–diatom scattering calculation. Three-dimensional potential energy surfaces for both the doublet and quartet states were constructed using ab initio electronic structure calculations while the spin–orbit coupling element was taken from previous work. Time-independent quantum reactive scattering calculations have been performed using the hyperspherical close-coupling method. The calculated cumulative reaction probabilities show that the reaction dynamics is exclusively resonance-dominated. The thermal rate constants calculated using the reduced dimensionality cumulative reaction probabilities with the energy shifting and J-shifting approximations were found to be much smaller than experimental measurements and previous reduced-dimensionality results of Seideman [J. Chem. Phys. 101, 3662 (1994)] by a factor of more than two orders of magnitude. In order to understand this serious disagreement, we have carried out the scattering calculations with the use of modified potential energy surfaces and spin–orbit couplings but found that the calculated rate constants were still much smaller than experimental data. The present computational study strongly suggests that further experimental studies including direct detection of N(4S) and/or any other mechanism for the “prompt-NO” formation will be necessary.