Using the second-order M?ller-Plesset perturbation theory (MP2) based on 6-31＋＋G(d,p) basis set, the mechanism for the ion-molecule reaction of n-butene anion with nitrous oxide in gas phase has been characterized in detail. The single-point energies have also been refined at the QCISD/6-31＋＋G(d,p) level to get more accurate energies using the MP2/6-31＋＋G(d,p) optimized geometries. As a result, there are cis and trans isomers in n-butene anion, and both the primary carbon and secondary carbon in n-butene anion can react with N₂O, the former contains α-H abstraction pathway, β-H abstraction pathway, IM11 based pathway and O-atom abstraction pathway. The α-H abstraction pathway is the main pathway, whose primary products are cis-propenyl-diazomethyl anion, and trans-propenyl-diazomethyl anion. The latter comprises methyl-H abstraction pathway, vinyl-H abstraction pathway, IM12' based pathway and O-atom abstraction pathway. The methyl-H abstraction pathway is the main pathway, whose primary product is butadienyl anion. In comparison, the latter reaction is more favorable. The O-atom direct abstraction reaction can also compete with the major reaction pathway. And its products can be detected. Furthermore, all the reactive pathways, regardless of the main or minor ones, are strongly exothermic processes.

Key words: n-butene anion, reaction mechanism, second-order M?ller-Plesset perturbation theory (MP2)