A theoretical treatment of the intersystem crossing in the spin-forbidden reaction N(4S)+CS2 (X1Σ+g)→NS(X2Π)+CS(X1Σ+)

The mechanism of the spin-forbidden reaction N(S-4) + CS2 (X-1 Sigma(+)(g)) -> NS(X-2 Pi) + CS(X-1 Sigma(+)) on both doublet and quartet potential energy surfaces has been investigated at B3LYP, CCSD(T) and CASSCF levels of theory. The minimum energy point (MEX-1) on the doublet-quartet crossing surface is determined using the complete-active-space-self-consistent field (CASSCF) calculations with (3,4), (5,5), (7,6), and (7,7) active spaces with a weighting of 50%/50% to the doublet and quartet states. The minimum energy point (MEX-1) has a Cs structure resembling the intermediate structure M-2-1. The intersystem crossing at the MEX-1 crossing point is taking place, and it is attributed to the stronger spin-orbit coupling (SOC)(50.2-75.5 cm(-1)) and the barrier (65.01-72.10 kJ/mol) with respect to the ground state reactants N(S-4) + CS2(X-1 Sigma(+)(g)). This is in close agreement with the experimental observation reporting large rate coefficients. (c) 2006 Elsevier B.V. All rights reserved.