A study of the lowest-lying triplet and singlet states of the cyclopentadienyl cation (c-C5H5+)

High-level abinitio studies [up to the CASSCF+MP2 and RCCSD(T) level] are performed on the cyclopentadienyl cation. The ground state is found to be the 3A2′ state, arising from a ···(a2″)2(e1″)2 electronic configuration, which is of D5h symmetry. The equilibrium bond lengths are determined to be: rC–C=(1.425±0.002) Å, and rC–H=(1.080±0.005) Å. In D5h symmetry, from the ground state electronic configuration, a 1E2′ state and a 1A1′ state also arise, with the former splitting into two states in C2v symmetry (1A1 and 1B2) under the influence of the Jahn–Teller effect. The 1A1′ state (1A1 in C2v symmetry) can then interact with the 1A1 state that arises from the 1E2′ state. It is found that these two electronic states, despite having very different geometries, are almost isoenergetic, and it is not possible to decide conclusively which one is the lower, even at the highest levels of theory used. In addition, the C2v stationary points on the two 1A1 surfaces change their character between minimum and saddle point at different levels of theory. The vibration corresponding to the imaginary frequency suggests an in-plane distortion to a Cs minimum, but this was difficult to converge. The 1B2 state is found to be a saddle point at the lower levels of theory, but to have a C2v minimum at the UMP2/6-311G(2d,p) level; however, one of the harmonic frequencies is anomalously large. The lowest singlet–triplet gap (1A1–3A2′) is calculated to be 6.5 kcal mol-1 (0.28 eV) at the RCCSD(T)/ cc-pVTZ//QCISD/6-31G** level of theory; the 1B2–3A2′ separation is calculated to be 13.3 kcal mol-1 (0.58 eV) at the UQCISD/6-31G** level of theory. c-C5H5+ is often quoted as being a typical "‘4n’' antiaromatic compound, however it will be argued that on geometric grounds, the state could be thought of as aromatic. The geometry of the two 1A1 states indicate antiaromatic behaviour, and that of the 1B2 state indicates close to aromatic behaviour, based on the A parameter of Julg and François.