Possible molecular mechanisms of the gas-phase ion/molecule reaction of VO2+ in its lowest singlet and triplet states (1A1/3A' ') with propyne have been investigated theoretically by density functional theory (DFT) methods. The geometries, energetic values, and bonding features of all stationary and intersystem crossing points involved in the five different reaction pathways (paths 1-5), in both high-spin (triplet) and low-spin (singlet) surfaces, are reported and analyzed. The oxidation reaction starts by a hydrogen transfer from propyne molecule to the vanadyl complex, followed by oxygen migration to the hydrocarbon moiety. A hydride transfer process to the vanadium atom opens four different reaction courses, paths 1-4, while path 5 arises from a hydrogen transfer process to the hydroxyl group. Five crossing points between high- and low-spin states are found: one of them takes place before the first branching point, while the others occur along path 1. Four different exit channels are found: elimination of hydrogen molecule to yield propynaldehyde and VO+ (1Sigma/3Sigma); formation of propynaldehyde and the moiety V-(OH2)+; and two elimination processes of water molecule to yield cationic products, Prod-fc+ and Prod-dc+ where the vanadium atom adopts a four- and di-coordinate structure, respectively.
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