Isomerization and dissociation in competition: the two-component dissociation rates of methyl acetate ions

Abstract

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of metastable methyl acetate ions, CH 3COOCH 3 .+. The rate of molecular ion fragmentation with the loss of CH 3O . and CH 2OH radicals as a function of ion internal energy was obtained from the coincidence data and used in conjunction with Rice-Ramsperger-Kassel-Markus and ab initio molecular orbital calculations to model the dissociation/isomerization mechanism of the methyl acetate ion (A). The data were found to be consistent with the mechanism involving a hydrogen-bridged complex CH 3CO···H···OCH 2 .+(E) as the direct precursor of the observed fragments CH 3CO + and CH 2OH .. The two-component decay rates were modeled with a three-well-two-product potential energy surface including the distonic ion CH 3C(OH)OCH 2 .+(B) and enol isomer CH 2C(OH)OCH 3 .+(C), which are formed from the methyl acetate ion by two consecutive [1,4]-hydrogen shifts. The 0 K heats of formation of isomers B and C as well as transition states TSAB, TSBC, and TSBE (relative to isomer A) were calculated from Rice-Ramsperger-Kassel-Markus (RRKM) theory.