Abstract
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.
Highlights
Triplet oxygen atom reacting with unsaturated hydrocarbons (RH) can produce Cvetanovic diradicals,[1,2] species in which O(3P) is added to a double or a triple bond
One interesting question is what aspects of electronic structure of the Cvetanovic diradicals derived from different unsaturated hydrocarbons control the rate of intersystem crossing (ISC)
In this work we explore the qualitative differences between the electronic structure of several Cvetanovic diradicals and aG.*Capozza,*E.*Segoloni,*F.*Leonori,*G.G.*Volpi,*and*P.*Casavecchia*(2004);*bP.*Casavecchia,*G.*Capozza,*E.*Segoloni,*F.*Leonori,*N.*Balucani,*and*G.G.*Volpi*(2005);*cJ.D.* evaluate important trends and features relevant to ISC
Summary
Triplet oxygen atom reacting with unsaturated hydrocarbons (RH) can produce Cvetanovic diradicals,[1,2] species in which O(3P) is added to a double or a triple bond. The diradical is formed in the triplet state It can undergo series of reactions on the triplet potential energy surface (PES) as well as intersystem crossing (ISC) to the singlet manifold. One interesting question is what aspects of electronic structure of the Cvetanovic diradicals derived from different unsaturated hydrocarbons control the rate of ISC. The efficiency of ISC in alkenes and alkynes is quite different: while in ethylene the branching ratio for the singlet products is above 50%, in acetylene it is less than 10%. The goal of the present study is to characterize the electronic structure of the prototypical Cvetanovic diradicals derived by O(3P)
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