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

High-level ab initio studies [up to the CASSCF + MP2 and RCCSD(T) level] ar e performed on the cyclopentadienyl cation. The ground state is found to be the (X) over tilde (3)A(2)' state, arising from a ...(a(2)")(2)(e(1)")(2) electronic configuration, which is of D-5h symmetry. The equilibrium bond l engths are determined to be: r(C-C) = (1.425 +/- 0.002) Angstrom, and r(C-H ) = (1.080 +/- 0.005) Angstrom. In D-5h symmetry, from the ground state ele ctronic configuration, a E-1(2)' state and a (1)A(1)' state also arise, wit h the former splitting into two states in C-2v symmetry ((1)A(1) and B-1(2) ) under the influence of the Jahn-Teller effect. The (1)A(1)' state ((1)A(1 ) in C-2v symmetry) can then interact with the (1)A(1) state that arises fr om the E-1(2)' state. It is found that these two electronic states, despite having very different geometries, are almost isoenergetic, and it is not p ossible to decide conclusively which one is the lower, even at the highest levels of theory used. In addition, the C-2v stationary points on the two ( 1)A(1) surfaces change their character between minimum and saddle point at different levels of theory. The vibration corresponding to the imaginary fr equency suggests an in-plane distortion to a C-s minimum, but this was diff icult to converge. The B-1(2) state is found to be a saddle point at the lo wer levels of theory, but to have a C-2v minimum at the UMP2/6-311G(2d,p) l evel; however, one of the harmonic frequencies is anomalously large. The lo west singlet-triplet gap ((1)A(1)-(X) over tilde X (3)A(2)') is calculated to be 6.5 kcal mol(-1) (0.28 eV) at the RCCSD(T)/cc-pVTZ//QCISD/6-31G** lev el of theory; the 1B2-(X) over tilde(3)A(2)' 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 iZ state could be thought of as aromatic. The geometry of the two (1)A(1) states indicate antiaromatic b ehaviour, and that of the B-1(2) state indicates close to aromatic behaviou r, based on the A parameter of Julg and Francois.