Thermochemistry of organic and heteroorganic species. Part V. Photoionization studies of isomerization and fragmentation of polysubstituted cyclopropenes: 3-methyl-3-acetyl- and related compounds

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Eur. J. Mass Spectrom. 5, 169 - 182 (1999)

Thermochemistry of organic and heteroorganic species. Part V. Photoionization studies of isomerization and fragmentation of polysubstituted cyclopropenes: 3-methyl-3-acetyl- and related compounds

I.N. Domnin
Department of Chemistry, St Petersburg University,198904 St Petersburg, Peterhof, University pr. 2, Russia.
V.V. TakhistovCentre of Ecological Safety of Russian Academy of Sciences, 197042 St Petersburg, Russia.
D.A. Ponomarev^*Department of Chemical Engineering, St Petersburg Forest Technical Academy, 194021 St Petersburg, Russia.

ABSTRACT:

Appearance energies for molecular and some fragment ions from 3-methyl-3-acetyl-, 1,2-dimethyl-3-acetyl-, 1,2-dimethyl-3-hydroxycarbonyl- and 1,3-dimethyl-2-(1-methyl)vinylcyclopropenes and model compounds 2,3-dimethylfuran and 1-methylindene were measured by photoionization mass spectrometry. It was shown that, in all cases, the structure of the [M Me]⁺ ion does not correspond to the expected structures of substituted cyclopropenium ions. Possible schemes for formation of [M Me]⁺ ions are given. Isodesmic reactions were systematically applied for calculation of the heats of formation for 35 possible C₅H₅O⁺ isomers from 3-methyl-3-acetyl-cyclopropene. In addition, the heats of formation for more than 30 other unsaturated ions and free radicals were estimated with the help of series of isodesmic reactions. A new value of 160 kJ mol¹, as compared with the currently used value of 215.5 kJ mol^{1,/SUP>, for the cyclopropylmethyl free radical is suggested. The publication represents the further development of the methodology for the heat of formation estimation procedure for even-electron ions and free radicals.}

Keywords: photoionization, thermochemistry, aromatic ions

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