A high-level ab initio and density functional investigation of cyclopropenyl anion and its mono-, di-, and trisubstituted derivatives

High-level ab initio and density functional theory calculations were carried out on cyclopropenyl anion (1) and related species. The parent c- C₃H₃^- ion is predicted to be unstable with respect to electron loss whereas its cyano-substituted derivatives are attractive synthetic targets. MCSCF structures for singlet and triplet cyclopropenyl anion are reported and contrasted to the results obtained from more modest computations. Energetic quantities for 1 were computed including its proton affinity, destabilization energy, singlet-triplet gap, electron binding energy, and ring-opening isomerization energy. The allylic C-H bond strength for cyclopropene also is given. Mono-, di-, and tricyanocyclopropenyl anions were investigated too, and their conjugate acids have lower pK(a)'s than cyclopropene by up to 70.4 kcal/mol or 52 pK(a) units.