(X)over-tilde (3)Sigma(-) and (A)over-tilde (3)Pi electronic states of ketenylidene (CCO): Analysis of the Renner effect in the upper state

Ab initio electronic structure theory has been employed in order to systema tically investigate the (X) over tilde (3)Sigma(-) and (A) over tilde (3)Pi electronic states of ketenylidene (CCO). The total energies and physical p roperties including equilibrium geometries, dipole moments, harmonic vibrat ional frequencies, and associated infrared (IR) intensities of CCO were pre dicted using the SCF, CISD, CCSD, equation of motion coupled cluster with s ingle and double excitations (EOM-CCSD), and CCSD(T) Levels of theory with a wide range of basis sets. The (X) over tilde (3)Sigma(-) state of CCO is linear at equilibrium. The potential energy surface of the (A) over tilde ( 3)Pi State of CCO splits into (3)A'' and (3)A' states on bending and each s urface has its own minimum at the linear configuration, i.e., this state is a type A Renner-Teller state in the nomenclature of Lee, Fox, Schaefer, an d Pitzer.(1) The two harmonic bending frequencies for the two surfaces of t he (A) over tilde (3)Pi state were determined via the EOM-CCSD method. From the harmonic vibrational frequencies of the (A) over tilde (3)Pi state, th e Renner parameter (epsilon), and the average harmonic bending frequency wt can easily be determined. At the EOM-CCSD TZ3P(2f) level of theory, values of -0.153 and 627 cm(-1) were determined for epsilon and omega(2), respect ively, which are in good agreement with experimentally determined values in both sign and magnitude. Theoretically predicted harmonic stretching vibra tional frequencies were in close agreement with experimental fundamental fr equencies, indicating relatively small anharmonicities. At the CCSD(T) leve l of theory with the largest basis set, Dunning's cc-pVQZ, the classical (X ) over tilde-(A) over tilde splitting (T-e value) was predicted to be 33.1 kcal/mol, and the quantum mechanical splitting (Tu value) to be 34.0 kcal/m ol which are in excellent agreement with the experimental To values of 33.3 kcal/mol by Devillers and Ramsay (in 1971), 33.3 kcal/mol by Fujitake, Kir yu, and Ohashi (in 1992), 33.9 kcal/mol by Zengin, Persson, Strong, and Con tinetti (in 1996), and 33.3 kcal/mol by Fulara, Grutter, Wyss, and Maier (i n 1998).