Spin-Forbidden Channels in Reactions of Unsaturated Hydrocarbons with O(3P)

The electronic structure
of four prototypical Cvetanović
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 intersystem crossing (ISC): minimal
energy crossing points (MECPs) and spin–orbit couplings (SOCs).
The calculations illuminate significant differences in the electronic
structure of ethene- and ethyne-derived compounds and explain the
effect of methylation. The computed MECP heights and SOCs reveal different
mechanisms of ISC in ethene- and ethyne-derived species, thus explaining
variations in the observed branching ratios between singlet and triplet
products and a puzzling effect of methyl substitution. In the ethene-
and propene-derived species, the MECP is very low and the rate is
controlled by variations of SOC, whereas in the ethyne- and propyne-derived
species the MECP is high and the changes in the ISC rate due to methyl
substitutions are driven by the variations of MECP heights.

本研究对四种典型的 Cvetanović 双自由基电子结构进行探究，这些自由基由 O­(3P) 加成到不饱和化合物中衍生而来。通过高级电子结构计算和动力学建模，主要聚焦于控制 intersystem crossing (ISC) 速率的电子因素：最小能量交叉点 (MECPs) 和自旋-轨道耦合 (SOCs)。计算结果揭示了由乙烯和乙炔衍生化合物在电子结构上的显著差异，并阐明了甲基化的影响。计算得到的 MECP 高度和 SOCs 表明，乙烯和乙炔衍生物种的 ISC 机制存在不同，从而解释了单线态与三重态产物之间的分支比变化以及甲基取代的神秘效应。在乙烯和丙烯衍生物种中，MECP 非常低，速率受 SOC 变化的控制；而在乙炔和丙炔衍生物种中，MECP 较高，由于 MECP 高度的变化导致的 ISC 速率变化由甲基取代所驱动。