A CI study of the classical and nonclassical structures of the vinyl cation and their optimum path for rearrangement

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Abstract
Ab initio calculations including configuration interaction (CI) are reported for the classical (linear) and the nonclassical (bridged) structures of the vinyl cation (C2H3+), and for the minimum energy path for the rearrangement from one to the other. A ’’double‐zeta plus polarization’’ one‐particle basis of contracted Gaussian functions was used to construct an n‐particle space for the CI calculation consisting of the ground state self‐ consistent field (SCF) configuration and all single and double excitations from it. Critical evaluation of the calculations leads to the chemical predictions that (i) the two structures have the same energy, to within ∼1–2 kcal/mole with probably the bridged structure more stable, (ii) that the minimum energy path for rearrangement keeps a planar configuration of the atomic nuclei, and (iii) that the barrier to rearrangement is low, less than ∼1–3 kcal/mole. The calculations include geometry optimization along the rearrangement path as well as the effects of electronic correlation, and we demonstrate that inclusion of both these factors is essential for reliable estimates of the energetics of the rearrangement. A discussion is made of the electronic structure changes during rearrangement, and of the resulting significant differences in the electronic spectra of the linear and bridged forms of the cation.