Theoretical study on the reaction of NbS+ (3∑−, 1Γ) with COS in gas phase

The reactions of NbS+ (3∑−, 1Γ) with COS have been studied using density functional theory at the B3LYP level: the O/S exchange reaction (NbS+ + COS → NbO+ + CS2), the S-transfer reaction (NbS+ + COS → NbS2+ + CO) and the O-transfer reaction (NbS+ + COS → ONbS+ + CS). The different mechanisms on the triplet and singlet surfaces have been identified. It is found that the S-transfer reaction has two parallel reaction paths (path A and path B) with respective to two isomeric products (NbS2+-1 and NbS2+-2), which may account, respectively, for the endothermic and the exothermic features of the product NbS2+ cross-section observed in the experiment. The spin-forbidden reaction NbS+ (3∑−) + COS → NbS2+(1-A′) + CO was found to be energetically much more favorable than the spin-allowed reaction NbS+ (3∑−) + COS → NbS2+ (3-A′) + CO. According to the identified reaction mechanisms, a triplet-singlet surface crossing is suggested. The larger activation barriers (23.7 and 59.5 kcal/mol) and the small probability of the formation of the preceding intermediate 3IM1 result in a much lower efficiency of the O/S exchange reaction and the O-transfer reaction of the 3∑− ground state of NbS+ with COS. For the O-transfer reaction, a triplet-singlet surface crossing may also exist.