A curious regularity in the dissociative photoionization of fluorinated benzenes: why do C6F6+ and C6H6+ dissociate so differently?

Photoelectron-photoion coincidence (PEPICO) spectra at 73.6 nm reveal an interesting fact; from hexafluorobenzene cation the C5F3+ fragment is most abundant whilst from the benzene cation C4H4+ is far more abundant than C5H3+. These contrasting results are connected, as shown by the PEPICO spectra of other fluorinated benzenes, by a regular change of the dominant fragment from C5F3+ to C4H4+ as the number of fluorine atoms decreases. This regularity is consistent with the appearance energies, at QCISD[T] or CBS-QB3 levels, of C5FmH3−m+ in the acetylene-cyclopropene structure and C4FnH4−n+ in the methylene-cyclopropene and/or the vinyl-acetylene structures. The energy-selected PEPICO data further support the theoretical results.