Second-order jahn-teller effect of cyclobutadiene in low-lying states. An MCSCF study

Abstract

The molecular structures and vibrational frequencies of cyclobutadiene in the ground and low-lying excited states were examined with the ab initio multi-configuration self-consistent-field (MCSCF) method. The harmonic vibrational frequencies at a square structure in both the ground state 1B 1g and the excited state 1A 1s are analyzed in terms of the second-order Jahn-Teller effect. The energy barrier between two rectangular isomers in the ground state is estimated to be 3.2 kcal/mol with the zero-point vibration correction. The lowest excited singlet state 1A 1g is found to have one imaginary frequency toward the rhomboidal distortion from the square structure. The frequency shift about the ring deformation due to the vibronic coupling is compared with the case of the low-lying states of benzene. The geometrical distortion from square to rectangle or rhomboid is also discussed with that of cyclobutadiene radical cation, which shows a typical deformation by the normal Jahn-Teller theorem.