The 2-silaketenylidene (CSiO) radical:: Electronic structure of the (X)over-tilde 3Σ- and (A)over-tilde 3Π states

A theoretical study, using ab initio electronic structure theory, of the ground ((X) over tilde (3)Sigma (-)) and first excited triplet ((A) over tilde (3)Pi) electronic states of the 2-silaketenylidene (CSiO) radical has been reported. The (A) over tilde (3)Pi state is subject to a Renner-Teller interaction and possesses two distinct real vibrational frequencies along the bending coordinate. To avoid variational collapse to a lower-lying state, the (3)A " component of the (A) over tilde (3)Pi state was investigated using the equation of motion (EOM) CCSD technique. With the TZ3P(2f) EOM-CCSD method, the Renner parameter (epsilon) and bending harmonic vibrational frequency (omega (2)) for the (A) over tilde (3)Pi state were determined to be epsilon = -0.253 and omega (2) = 337 cm(-1). At the highest level of theory, cc-pVQZ CCSD(T), the classical (X) over tilde-(A) over tilde splitting of CSiO was predicted to be 29.8 kcal/mol (1.29 eV, 10 400 cm(-1)), and the quantum mechanical energy separation to be 31.1 kcal/mol (1.35 eV, 10 900 cm(-1)). The ground state of CSiO lies 65.4 kcal/mol above the ground state of SiCO. The bond energy for C-SiO was determined to be D-e = 32.6 (D-0 = 31.4) kcal/mol, indicating considerable stability for the ground state of CSiO against the dissociation reaction CSiO ((X) over tilde (3)Sigma (-)) --> C (P-3) + SiO ((X) over tilde (1)Sigma (+)). The most remarkable prediction from the present research is that the C-Si distance in the (A) over tilde (3)Pi excited state is nearly 0.2 Angstrom less than that in the (X) over tilde (3)Sigma (-) ground state. In fact, this distance (1.662 Angstrom) is one of the shortest known Si-C bond distances with a formal bond order of 5/2.