Abstract

European Journal of Mass Spectrometry
Volume 15 Issue 2, Pages 157–166 (2009)
doi: 10.1255/ejms.943

Nitro–nitrite isomerization and transition state switching in the dissociation of ionized nitromethane: a threshold photoelectron–photoion coincidence spectroscopy study

Brandon Ferrier,^a Anne-Marie Boulanger,^a David M.P. Holland,^b David A. Shaw^b and Paul M. Mayer^a,*
aDepartment of Chemistry, University of Ottawa, 10 Marie-Curie, Ottawa, Ontario, Canada K1N 6N5. E-mail: pmmayer@uottawa.ca
^bDaresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD UK

Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been employed to investigate the competition between bond cleavage and rearrangement reactions in the dissociation of ionized nitromethane, 1. Modeling TPEPICO breakdown diagrams with a combination of RRKM theory and ab initio calculations at the G3 level of theory allowed the derivation of the activation energy for the isomerisation of 1 to ionized methyl nitrite, 2, 82 kJ mol^–1. In addition, evidence was found for a transition state switch in the bond cleavage reaction in 1 leading to CH₃^• + NO₂⁺. As internal energy increases, the effective transition state for this reaction becomes tighter (i.e. is characterized by a lower entropy of activation, Δ^‡S). Fitted thresholds for NO⁺ and CH₂OHO⁺ ions, originating from the isomeric methyl nitrite ion, are consistent with G3 level ab initio calculations.

Keywords: nitromethane ion, methylnitrite ion, isomerization, variational transition state theory, computational chemistry, ion dissociation, threshold photoelectron–photoion coincidence spectroscopy

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