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The structure of decomposing [C 7 H 7 O] + ions: Benzyl versus tropylium ion structures

1983; Wiley; Volume: 18; Issue: 11 Linguagem: Inglês

10.1002/oms.1210181105

2376-3884

David H. Russell, Ben S. Freiser, Eddy H. McBay, D.C. Canada,

Advanced Physical and Chemical Molecular Interactions

Abstract The unimolecular dissociation reactions for [C 7 H 7 O] + 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 (CH 2 O) from the [C 7 H 7 O] + ions are interpreted as the hydroxybenzyl and hydroxytropylium [C 7 H 7 O] + not interconverting to a common structure on the microsecond time‐scale. In addition, similar measurements on protonated benzaldehyde, methylaryloxy and phenyl methylene ether [C 7 H 7 O] + 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 [C 7 H 7 O] + 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 [C 7 H 7 O] + ion structures from a given precursor molecule is determined by isomerization of the parent ion and not by structural equilibration of the [C 7 H 7 O] + ion.