How does ionized

N-methyl-O-methyl carbamate, CH₃N(H)COOCH₃^+", lose a methyl group? Unimolecular equilibration with two distonic isomers

Full-text article (145 kB)
(subscribers only)

Buy article on-line for £11.75
(get immediate access)

Search

Go Back

Eur. J. Mass Spectrom. 1, 261 - 268 (1995)

How does ionized N-methyl-O-methyl carbamate, CH₃N(H)COOCH₃^+", lose a methyl group? Unimolecular equilibration with two distonic isomers

Peter C. Burgers
Hercules European Research Center, Nijverheidsweg 60, 3770 AK Barneveld, The Netherlands
Graham A. McGibbon and Johan K. Terlouw^*
McMaster University, Chemistry Department, 1280 Main St West, Hamilton, ON,Canada L8S 4M1

ABSTRACT:

The unimolecular chemistry of the N-methyl-O-methylcarbamate radical cation CH₃N(H)COOCH₃^+", 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.

Keywords:

You can now buy this paper on-line in PDF format; it costs only £11.75. Just click on the BUY on-line button. You can pay on-line through a secure server and get access immediately.