Calculations at B3LYP/6-31+G(d) show c-C 4 H 3 + to have a singlet ground state, with the triplet being 19.5 kcal mol −1 higher in energy. Singlet c-C 4 H 3 + has a puckered ring structure (C s symmetry) with an out-of-plane angle of 51.4°. 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 distance between CH groups is 1.793 Å and the Wiberg bond order is 0.436, indicating considerable interaction. The stability of the singlet state then is attributed to homoaromaticity. Ion c-C 4 H 3 + in its triplet state is planar, but with alternate bonds being long and short, reminiscent of the geometries of c-C 4 H 4 and c-C 4 H 4 + . Substitution of H atoms by CN groups stabilises the triplet 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° in the unsubstituted ion c-C 4 H 3 + to 43.4 kcal mol −1 in C 4 (CN) 3 + . A CN group adjacent (α) to the carbene centre reduces the barrier by ∼7 kcal mol −1 whereas substitution at the opposite carbon (β position) reduces it by ∼2 kcal mol −1 . These effects are roughly additive, with C 4 (CN) 3 + having a barrier to inversion of only 12.9 kcal mol −1 .