Vinyl cations substituted with beta pi-donors have triplet ground states

Computations at the CASPT2, CBS-QB3, and B3LYP levels of theory demonstrate that beta-substitution of vinyl cations with pi-donors switches the ground state of these ions from the familiar closed-shell singlet state to a carbene-like triplet state similar to the electronic state of triplet phenyl cations. Although the parent vinyl cation is a ground-state singlet species with a very large energy gap to the lowest energy triplet state, substituting the beta hydrogens with just one strong pi-donor (e.g., NH(2), NMe(2), OMe) or two moderate pi-donors (e.g., F, OH, Ar, vinyl) makes the triplet state the computed ground electronic state. In many cases, the singlet states for these beta pi-donor-substituted vinyl cations are prone to rearrangements, although such rearrangements can be inhibited through incorporation of the pi-donors into rings. For example, a vinyl cation based on 1,3-dimethyl-2-methylene imidazolidine (32) is predicted to show a substantial barrier to singlet state rearrangement as well as possess a triplet ground state with a large energy gap. In contrast to the singlet states, the stabilized triplet states appear to be well behaved and more immune to rearrangements. These triplet ions may exhibit substantially different properties and reaction chemistry than those seen for typical closed-shell vinyl cations.