The structure of decomposing [C7H7O]+ ions: Benzyl versus tropylium ion structures

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 time-scale. 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 time-scale. 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 mol^?1; hydroxytropylium ions, 656 kJ mol^?1; phenyl methylene ether ions, 640 kJ mol^?1; methylaryloxy ions 803 kJ mol^?1. 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.