Abstract: | The Unimolecular mass spectrometric fragmentations of the molecular ions of 1,3-diphenylpropane, 1-(7-cycloheptatrienyl)-2-phenylethane and the 1-phenyl-2-tolylethanes and their d5]phenyl analogues have been investigated by metastable ion techniques and measurements of ionization and appearance energies. By comparing the formation of C7H7]+, C7H8]+?, C8H8]+? and C8H9]+ it is shown that the molecular ions of the four diaryl isomers do not undergo ring expansion reactions of the aromatic nuclei prior to these fragmentations. Conversely, the molecular ions of the cycloheptatrienyl isomer suffer in part a contraction of the 7-membered ring. From these results and from the measured ionization and appearance energies lower limits to the activation energies of these skeletal isomerizations have been estimated yielding E > 33±5 kcal mol?1 formonoalkylbenzene, E > 20 2±5 kc mol?1 for 7-alkylcycloheptatriene and E > 40±5 kcal mol?1 for dialkylvbenzene positive radical ions. Upper limits can be deduced from literature evidence yielding E < 45 kcal mol?1 for monoalkylbenzene and E < 53 kcal 4mol?1 for dialkylbenzene positive radical ions. The activation energy thus estimated for monoalkylbenzene is in excellent agreement with the recently calculated value(s) for the toluene ion. |