Among various reaction products, triplet oxygen atom reacting with unsaturated hydrocarbons (RH) can produce Cvetanović diradicals, species in which O( 3 P) is added to a double or a triple bond. Because oxygen is triplet, the diradical is formed in the triplet state. It can then undergo series of reactions on the triplet potential energy surface (PES) as well as intersystem crossing (ISC) to the singlet manifold. The reactions of Cvetanović diradicals are important in hydrocarbon combustion, and, to an extent, in atmospheric and interstellar chemistry. For example, the reaction of ethylene with O( 3 P) can lead to effective secondary chain-branching and thus enhanced fuel oxidation rate. Although numerous studies have investigated the role of Cvetanović diradicals and ISC in the reactions of O( 3 P) with unsaturated hydrocarbons, the key details of ISC are still unresolved. One particularly interesting question is what aspects of electronic structure of the Cvetanović diradicals derived from different unsaturated