How Does Ionized N-Methyl-O-Methyl Carbamate, CH3N(H)COOCH3•+, Lose a Methyl Group? Unimolecular Equilibration with Two Distonic Isomers

The unimolecular chemistry of the N-methyl-O-methyl carbamate radical cation CH3N(H)COOCH3•+, 1 has been studied by a variety of mass spectrometry based techniques [metastable ion (MI), collisional activation (CA) and neutralisation–reionisation (NR) mass spectrometry and multiple collision (MS/MS/MS) experiments] in combination with D, 13C and 18O labelling experiments. The loss of a methyl radical is not a simple bond cleavage process; rather, at high internal energies, the reaction proceeds via a 1,4-hydrogen shift to produce the distonic ion CH2=N(H)–C(OH)OCH3•+ which then sheds the O–CH3 methyl group to produce the N-carboxyliminium ion CH2=N(H)–COOH+, a putatively destabilized iminium ion. The latter step is exothermic (by c. 15 kcal mol−1) and moreover involves a significant barrier for the forward reaction, characteristic of O–C bond cleavages in (distonic) radical cations, rationalizing the large kinetic energy release observed for this dissociation. At low internal energies, the hydrogen atoms of the methyl groups, but not that of the N–H group, exchange to the statistical limit. This is rationalized in terms of the equilibria CH3N(H)COOCH3•+ ↔ CH2=N(H)–C(OH)OCH3•+ ↔ CH3N(H)COOCH3•+ taking place prior to dissociation. The heats of formation of CH3N(H)COOCH3•+ and CH2=N(H)–COOH+ have been estimated as 133 and 82 kcal mol−1, respectively.