The elimination of H2 from C2H5 + was investigated at MP2−FC/6-311+G(d,p), QCISD/6-311+G(d,p), and QCISD(T)/6-311++G(3df,2p)//QCISD/6-311+G(d,p) levels. This elimination is an endoergic reaction that proceeds through a late transition state in which two C−H bonds are synchronously stretched with an energy barrier close to its endoergicity, in agreement with experimental results. Back-donation from the HOMO of C2H3 + to the LUMO of H2 plays a crucial role in this process. This back-donation depends on the elongation of the C−C bond so that H2 is eliminated when the synchronous stretching of the two C−H bonds is out of phase with respect to the stretching of the C−C bond.

Rearrangements involving the phenonium ion were investigated by means of a B3LYP/6-311G(d,p) study in which the effect of solvent has been incorporated by using a PCM solvation model. A rationalization of the whole set of experimental facts reported both in the gas phase and in solution was possible thanks to the characterization of protonated benzocyclobutene as a minimum energy structure and, particularly, to the important preferential stabilization in solution of the TS for the isomerization of the phenonium ion to the alpha-methylbenzyl ion, which reduces the Gibbs energy barrier of 26.6 kcal/mol for this process in the gas phase to a more accessible one of 18.7 kcal/mol in solution.

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