The mechanism of the spin-forbidden reaction Ti+(4F, 3d24s1) + C2H4 → TiC2H2+ (2A2) + H2 on both doublet and quartet potential energy surfaces has been investigated at the B3LYP level of theory. Crossing points between the potential energy surfaces and the possible spin inversion process are discussed by means of spin-orbit coupling (SOC) calculations. The strength of the SOC between the low-lying quartet state and the doublet state is 59.3 cm-1 in the intermediate complex IM1-4B2. Thus, the changes of its spin multiplicity may occur from the quartet to the doublet surface to form IM1- 2A1, leading to a sig-nificant decrease in the barrier height on the quartet PES. After the insertion intermediate IM2, two dis-tinct reaction paths on the doublet PES have been found, i.e., a stepwise path and a concerted path. The latter is found to be the lowest energy path on the doublet PES to exothermic TiC2H2+ (2A2) + H2 products, with the active barrier of 4.52 kcal/mol. In other words, this reaction proceeds in the following way: Ti++C2H4 →4IC→IM1-4B2→4,2ISC→IM1- 2A1→[2TSins]→IM2→[2TSMCTS]→IM5→TiC2H2+(2A2)+H2.
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