A theoretical treatment of the intersystem crossing in the spin-forbidden reaction N(S4)+CS2(X∑g+1)→NS(X∏2)+CS(X1∑+)

The mechanism of the spin-forbidden reaction N(S4)+CS2(X∑g+1)→NS(X∏2)+CS(X1∑+) 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 2M-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(S4)+CS2(X∑g+1). This is in close agreement with the experimental observation reporting large rate coefficients.