Strong pseudo Jahn-Teller effect in the cyclic C3H radical

A computational study of the structure and properties of the ground (X̃ 2B2) and first excited (à 2A1) states of the cyclic C3H radical is reported. Although spectroscopic analyses have been uniformly consistent with a C2v structure for the electronic ground state, previous quantum chemical investigations have predicted that in-plane distortion to a Cs framework symmetry is an energetically favorable process. In contrast, the present results predict a C2v equilibrium geometry for both the ground and excited states. Problems related to symmetry breaking of the zeroth-order wavefunction are avoided in this work by using an equation-of-motion coupled cluster technique. Properties evaluated in calculations for both the X̃ 2B2 and à 2A1 states lend support to a published interpretation of the microwave spectrum.