A theoretical study of the intersystem crossing channels of gaseous 4H-pyran-4-thione

The intersystem crossing channels of gaseous 4H-pyran-4-thione were investigated using the CASSCF//TD-B3LYP methods and group theory. Calculated results show that phosphorescence and nonradiative decay via intersystem crossing to the S0 state are simultaneous processes occurring at the T1 state. The spin-orbit coupling constant in the S0/T1 intersection is large, 〈T1∣HSO∣S0〉 = 156.0 cm−1 and the computed average phosphorescence radiative lifetime is 136.5 μs. A rapid depletion of the S1 state via intersystem crossing to the T1 state can be mediated by the T2 state, if spin relaxation is fast within the triplet levels. Our calculated results are in close agreement with experimental observations.