High level ab initio calculations have been performed to investigate the mechanism of the ionmolecule reaction NH 3 ϩC 2 H 2 ϩ . Three channels, covalent complex formation ͑CC͒, proton transfer ͑PT͒, and charge transfer ͑CT͒ have been studied. Among the two pathways found for the PT channel, one leads the reactants NH 3 ϩC 2 H 2 ϩ to NH 4 ϩ ϩC 2 H( 2 ⌸) through a moderately bound complex without any barrier, and the other leads NH 3 ϩ ϩC 2 H 2 to the H-atom transferred products NH 4 ϩ ϩC 2 H( 2

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... ⌺ ϩ ) with a modest barrier. These findings support the fast "stripping" mechanism proposed by Anderson et al. As to the CC channel, several isomers of C 2 H 5 N ϩ and the isomerization transition states have been located. No significant barrier relative to the reactants has been found on either the ground or the 2 AЉ excited state. To rationalize the experimental fact that no CC channel products have been observed, it is argued that the reactants NH 3 ϩC 2 H 2 ϩ correlate adiabatically to excited states of covalent C 2 H 5 N ϩ species, whose formation requires significant alternation of the C 2 H 2 ϩ geometry and electronic structure. Therefore, the system is most likely to follow the PT or the CT channel instead of visiting the CC channel. For the CT channel, limited potential energy surface scans of the three electronic states (1,2 2 AЈϩ 2 AЉ) indicate that CT at different approach angles or between electronic states of different symmetries ͑AЈ→AЈ,AЉ→AЈ͒ may produce final products of different characteristics, and might account for the two pathways proposed by Anderson et al.