Methyl loss from conventional and distonic isomers of C3H7N center dot+1

The unimolecular reactions of a number of C2H5N+ and C3H7K+ isomers have be en investigated. Mass-analyzed ion kinetic energy (MIKE) and collision-indu ced dissociation (CID) experiments as well as ab initio calculations show t hat the N-methylaziridine molecular ion and the distonic radical cation CH2 =N(CW3)CH2+ can interconvert below the threshold for unimolecular fragmenta tion, whereas the N-ethylmethylenimine molecular ion is structurally distin ct. Likewise, the aziridine molecular ion can undergo ring opening below th e threshold for unimolecular fragmentation. Methyl loss from the N-melhylaz iridine radical cation proceeds via isomerization to the distonic ion, wher eas methyl loss from the imine radical cation is a straightforward simple c leavage. The heats of formation of the C3H7N+ and C2H5N+ isomers determined with four different composite methods, G2(MP2), G3, CBS-Q, and CBS-RAD, ag ree reasonably well, and reasonably well with the available experimental da ta. 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.