Isomers and excitation energies of C₄

Coupled-cluster (CC) and many-body perturbation theory (MBPT) studies of the rhombic and linear structures of C₄ are reported. For each isomer, the electronic spectra is obtained, and comparisons are made with experimental matrix-isolated ESR and electronic spectra. The closed-shell ¹A_g rhombic ground state is found to be more stable than the ³Σ_g^- state of the linear isomer by 5 kcal/mol at the highest level of calculation performed (CCSDT-1). However, the predicted spectrum for linear C₄ is in reasonable agreement with the observed results. An allowed electronic transition for the rhombus is predicted to lie in the same region, suggesting the possibility that both isomers could coexist in the experiment. Finally, vibrational frequencies for the rhombic isomer are calculated using analytical second-order MBPT second derivatives to aid in the experimental identification of this transient species.