Methyl loss from conventional and distonic isomers of C₃H₇N·⁺¹

The unimolecular reactions of a number of C₂H₅N·⁺ and C₃H₇N·⁺ isomers have been investigated. Mass-analyzed ion kinetic energy (MIKE) and collision-induced dissociation (CID) experiments as well as ab initio calculations show that the N-methylaziridine molecular ion and the distonic radical cation CH₂=N(CH₃)CH₂/·⁺ can interconvert below the threshold for unimolecular fragmentation, whereas the N-ethylmethylenimine molecular ion is structurally distinct. Likewise, the aziridine molecular ion can undergo ring opening below the threshold for unimolecular fragmentation. Methyl loss from the N-methylaziridine radical cation proceeds via isomerization to the distonic ion, whereas methyl loss from the imine radical cation is a straightforward simple cleavage. The heats of formation of the C₃H₇N·⁺ and C₂H₅N·⁺ isomers determined with four different composite methods, G2(MP2), G3, CBS-Q, and CBS-RAD, agree reasonably well, and reasonably well with the available experimental data. The distonic ions are found to be the more stable isomers on the C₂H₅N·⁺ and C₃H₇N·⁺ potential energy surfaces. (C) 2000 Elsevier Science B.V.