THE STRUCTURE OF DECOMPOSING [C7H7O]+ IONS - BENZYL VERSUS TROPYLIUM ION STRUCTURES
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The unimolecular dissociation reactions for [C7H7O]+ ions generated by fragmentation of a series of precursor molecules have been investigated. The metastable kinetic energy values and branching ratios associated with decarbonylation and expulsion of a molecule of formaldehyde (CH2O) from the [C7H7O]+ ions are interpreted as the hydroxybenzyl and hydroxytropylium [C7H7O]+ not interconverting to a common structure on the microsecond timescale. In addition, similar measurements on protonated benzaldehyde, methylaryloxy and phenyl methylene ether [C7H7O]+ ions are interpreted as the dominant fraction of these decomposing ions having unique structures on the microsecond timescale. These results are supported by experimental heats of formation calculated from ionization/appearance energy measurements. The experimental heats of formation are determined as: hydroxybenzyl ions, 735 kJ mol1; hydroxytropylium ions, 656 kJ mol1; phenyl methylene ether ions, 640 kJ mol1; methylaryloxy ions 803 kJ mol1. The combination of the results reported in this paper with previously reported experimental data for stable [C7H7O]+ ions (see Ref. 1, C. J. Cassady, B. S. Freiser and D. H. Russell, Org. Mass Spectrom.) is interpreted as evidence that the relative population of benzyl versus tropylium [C7H7O]+ ion structures from a given precursor molecule is determined by isomerization of the parent ion and not by structural equilibration of the [C7H7O]+ ion. Copyright 1983 Wiley Heyden Ltd.
