Spin-forbidden channels in reactions of unsaturated hydrocarbons with O(3P)
Electronic structure of four prototypical Cvetanovi\'c diradicals, species derived by addition of O(3P) to unsaturated compounds, is investigated by high-level electronic structure calculations and kinetics modeling. The main focus of this study is on the electronic factors controlling the rate of inter-system crossing (ISC), minimal energy crossing points (MECPs) and spin-orbit couplings (SOCs). The calculations illuminate significant differences in the electronic structure of ethylene- and acetylene-derived compounds and a relatively minor effect due to methylation. The computed MECPs heights and SOCs reveal different mechanisms of ISC in ethylene- and acetylene-derived species, thus explaining variations in the observed branching ratios between singlet and triplet products and a puzzling effect of the methyl substitution. In the ethylene- and propylene-derived species, the MECP is very low and the rate is controlled by the SOC variations, whereas in the acetylene- and propyne-derived species the MECP is high and the changes in the ISC rate due to methyl substitutions are driven by the variations in MECP heights.