Abstract

European Journal of Mass Spectrometry
Volume 10 Issue 6, Pages 869–880 (2004)
doi: 10.1255/ejms.680

Electron-rich radicals by neutralization–reionization mass spectrometry. Generation, dissociations and energetics of the hydrogen atom adduct to acetamide

Francis Tam, Erik A. Syrstad, Xiaohong Chen and Frantiček Tureček^*
Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, WA 98195-1700, USA

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1⁺) and the syn-form (syn-1⁺), ΔG°298(anti→syn) = 10.8 kJ mol^–1. Collisional neutralization of 1⁺ produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 µs. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (~95%) which is accompanied by loss of one of the methyl hydrogen atoms (~3%) and loss of the methyl group (~2%). The most favorable dissociation of the O–H bond is calculated to be only 34 kJ mol^–1 endothermic but requires 88 kJ mol^–1 in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck–Condon effects that result in 80–90 kJ mol^–1 vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, ·CH₂CONH₂ and CH₃CH(OH)NH₂, over the reactants.

Keywords: protonated acetamide, acetamide radicals, neutralization–reionization mass spectrometry, Franck–Condon effects, excited states

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