Spin-forbidden channels in reactions of unsaturated hydrocarbons with O(³P)

P. Pokhilko, R. Shannon, D. Glowacki, H. Wang, and A. I. Krylov
J. Phys. Chem. A  123, 482 – 491 (2019)

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.

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