Carbocations Generated under Stable Conditions by Ionization of Matrix-Isolated Radicals: The Allyl and Benzyl Cations

Carbocations are crucial intermediates
in many chemical reactions;
hence, considerable effort has gone into investigating their structures
and properties, for example, in superacids, in salts, or in the gas
phase. However, studies of the vibrational structure of carbocations
are not abundant, because their infrared spectra are difficult to
obtain in superacids or salts (where furthermore the cations may be
perturbed by counterions), and the generation of gas-phase carbocations
in discharges usually produces several species. We have applied the
technique of ionizing neutral compounds by X-irradiation of cryogenic
Ar matrices to radicals embedded in such matrices, thus producing
closed-shell cations that can be investigated leisurely, and in the
absence of counterions or other perturbing effects, by various forms
of spectroscopy. This Article describes the first set of results that
were obtained by this approach, the IR spectra of the allyl and the
benzyl cation. We use the information obtained in this way, together
with previously obtained data, to assess the changes in chemical bonding
between the allyl and benzyl radicals and cations, respectively.