Small-ring carbenes carrying a positive charge: the effects of substituting by CN in c-C4H3+

Calculations at B3LYP/6-31+G(d) show c-C4H3+ to have a singlet ground state , with the triplet being 19.5 kcal mol(-1) higher in energy. Singlet c-C4H3 + has a puckered ring structure (C-s symmetry) with an out-oil-plane angle of 51.4 degrees. The barrier to inversion via the planar ring structure (C- 2v symmetry) is 29.1 kcal mol(-1). In the puckered ring the cross-ring dist ance between CH groups is 1.793 Angstrom and the Wiberg bond order is 0.436 , indicating considerable interaction. The stability of the singlet state t hen is attributed to homoaromaticity. Ion c-C4H3+ in its tripIet state is p lanar, but with alternate bonds being long and short, reminiscent of the ge ometries of c-C4H4 and c-C4H4+. Substitution of Il atoms by CN groups stabi lises the tripler state relative to the singlet, and for C-4(CN)(3)(+) the triplet is only 8.4 kcal mol(-1) higher. Substitution also reduces the out- of-plane angle of the ions in the singlet state, going from 51.4 degrees in the unsubstituted ion c-C4H3+ to 43.4 kcal mol(-1) in C-4(CN)(3)(+). A CN group adjacent (cu) to the carbene centre reduces the barrier by similar to 7 kcal mol(-1) whereas substitution at the opposite carbon (beta position) reduces it by similar to 2 kcal mol(-1). These effects are roughly additiv e, with C-4(CN)(3)(+) having a barrier to inversion of only 12.9 kcal mol(- 1). (C) 2000 Elsevier Science B.V.