Structures and Energies of Isomeric C₃H₆²⁺ Dications

The C₃H₆²⁺ potential energy surface was studied by ab initio molecular orbital theory employing the 3-21G and 6-31G* basis sets and Moller-Plesset perturbation theory. The protonated allyl cation 1, which formally has two terminal cation centers, and the propylidene dication 3, which has formally both charges located at its central dimethyl-substituted carbon, have virtually the same energy at MP4/6-31G*. The relative high stability of these two isomers results from effective bishyperconjugation and crosshyperconjugation, respectively. Examples of [1, 3]-H and [1, 2]-CH₄ sigmatropic shifts are discussed for isomeric propylene and vinyl-substituted methonium diions; a transition structure with carbon hexacoordination is involved. Both dications 1 and 3 are predicted to be observable in the gas phase. Energy evaluations are presented for the adiabatic (di)oxidations from neutral and radical cation precursors.