Photoionization of the C-1?C-4 monosubstituted alkyl benzenes: Thermochemistry of [C7H7]+ and [C8H9]+ formation

The appearance energies for the C₇H₇]⁺ and C₈H₉]⁺ fragment ions produced in the fragmentation of the C-1? C-4 monosubstituted alkyl benzenes have been measured by photon impact. The mean heat of formation calculated for C₇H₇]⁺ is 205.3 ± 1.9 kcal mol^?1 which is consistent with a threshold tropylium structure. For C₈H₉]⁺ the mean heat of formation is calculated to be 199.2 ± 1.3 kcal mol^?1 which can be equated with either a methyl tropylium or α-phenylethyl structure at threshold. Some evidence is provided for the existence of the α-phenylethyl ion.