Isomerization of acetonitrile N-methylide [CH3CNCH2](center dot+) and N-methylketenimine [CH3NCCH2](center dot+) radical cations in the gas phase: Theoretical study of the [C-3,H-5,N](center dot+) potential energy surface

A theoretical study of the [C-3,HS5,N](.+) potential energy surface is pres ented. Ab initio molecular orbital calculations at the QCISD(T)/UMP2 level with the 6-31G(d,p) basis set show that acetonitrile N-methylide [CH3CNCH2] (.+), a(.+), and N-methylketenimine [CH3NCCH2](.+), b(.+), are the most sta ble species among the 15 isomers considered, and a heat of formation of Del ta(f)H degrees(298) = 970 kJ/mol is proposed for both species. Detailed exa mination of the [C-3,H-5,N](.+) potential energy surface indicates that a(. +), b(.+), and related isomers are stable and distinct species in the gas p hase, isolated by energy barriers as high as 300 kJ/mol. Their neutral equi valent, a and b, have also been studied, thus allowing their adiabatic ioni zation energies to be estimated: IEa(a) 7.1 eV and IEa(b) 8.0 eV. Finally, the theoretical study of a(.+) and b(.+) fragmentations provides an explana tion for the similarity in their high-energy CID spectra by showing a possi ble isomerization of these species prior to dissociation.