HOW DOES IONIZED N-METHYL-O-METHYL CARBAMATE, CH3N(H)COOCH3-CENTER-DOT-GROUP - UNIMOLECULAR EQUILIBRATION WITH 2 DISTONIC ISOMERS(, LOSE A METHYL)

The unimolecular chemistry of the N-methyl-O-methyl carbamate radical cation CH3N(H)COOCH3.+, 1 has been studied by a variety of mass spectr ometry based techniques [metastable ion (MI), collisional activation ( CA) and neutralisation-reionisation (NR) mass spectrometry and multipl e collision (MS/MS/MS) experiments] in combination with D, C-13 and O- 18 labelling experiments. The loss of a methyl radical is not a simple bond cleavage process; rather, at high internal energies, the reactio n 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 iminiu m ion. The latter step is exothermic (by c. 15 kcal mol(-1)) and moreo ver involves a significant barrier for the forward reaction, character istic of O-C bond cleavages in (distonic) radical cations, rationalizi ng the large kinetic energy release observed for this dissociation. At low internal energies, the hydrogen atoms of the methyl groups, but n ot that of the N-H group, exchange to the statistical limit. This is r ationalized 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 est imated as 133 and 82 kcal mol(-1), respectively.