Stability and properties of C₄ isomers

The relative electronic energies of the ¹A_g rhombus and the ³Σ_g^- linear isomers of C ₄ have been computed using a 5s3pld basis at various levels of coupled-cluster and many-body perturbation theory. At the highest level performed, CCSD + T(CCSD), the two isomers are essentially isoenergetic. Themodynamic results indicate, however, that the linear isomer will be significantly more abundant in the plasma in which C₄ is formed. Vibrational frequencies are presented and compared with experimental results; some of the prior assignments are questioned. The isotropic ESR hyperfine splitting parameters for the linear triplet state have been computed and are also compared with experimental results. Finally, the ionization potential and electron affinity for both structures are presented.