Completing dissociation channels of nitromethane and methyl nitrite ions and the role of electronic and internal modes of excitation

Abstract

Unimolecular dissociation reactions of the isomeric ions of nitromethane and methyl nitrite have been investigated for various electronic states of the ions up to an ionization energy of 18 eV using a photoelectron-photoion coincidence technique. The principal dissociation channels of the nitromethane ion lead to CH 3 +, NO 2 + and NO +. Of these only NO + is formed from methyl nitrite ion. A comparison of the NO + dissociation data from both precursors indicates that rearrangement of the nitromethane to the more stable methyl nitrite ion structure preceding NO + formation occurs in competition with direct CN bond cleavage dissociation of the nitromethane ion. Although the product kinetic energy distributions, measured as a function of the internal energy of a given parent-ion electronic state, were statistical in nature, evidence of a more specific dependence upon the initial electronic state was found. The formation of excited stated of the neutral NO 2 fragment which accompanies the CH 3 + product from the higher electronic states of the nitromethane ion was identified from the product kinetic energy data. The secondary dissociation of the NO 2 + product to NO + + O was also recognized from the kinetic energy data by a comparison of the reduced, or scaled, forms of the energy distribution functions above and below the thermodynamic limit for the secondary reaction.