The silaketenylidene (SiCO) molecule: Characterization of the (X)over-tilde(3)Sigma(-) and (A)over-tilde(3)Pi states

The ground ((X) over tilde (3)Sigma(-)) and first excited triplet ((A) over tilde (3)Pi) electronic states of carbonylsilene or silaketenylidene, SiCO , have been investigated systematically using ab initio electronic structur e theory. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associate d infrared (IR) intensities were predicted using self-consistent-field (SCF ), configuration interaction with single and double excitations (CISD), cou pled cluster with single and double excitations (CCSD), equation-of-motion (EOM) CCSD, CCSD with perturbative triple excitations [CCSD(T)] methods wit h a wide range of basis sets. The linear (X) over tilde (3)Sigma(-) ground state of SiCO has a real degenerate bending vibrational frequency, whereas the (A) over tilde (3)Pi state of SiCO is subject to the Renner-Teller effe ct and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequency of the (A) over tilde (3)Pi state was evaluated via the EOM-CCSD technique. At the highest level of the ory with the largest basis set, cc-pVQZ CCSD(T), the adiabatic (X) over til de-(A) over tilde splitting without the zero-point vibrational energy (ZPVE ) correction (T-e value) was determined to be 68.5 kcal/mol (2.97 eV, 23 90 0 cm(-1)) and the adiabatic splitting with the ZPVE energy correction (T-0 value) to be 69.0 kcal/mol (2.99 eV, 24 100 cm(-1)), which are in excellent agreement with the experimental T-0 value of 68.78 kcal/mol (2.983 eV, 24 056 cm(-1)). The theoretical ground state harmonic Si-C stretching frequenc y omega(3) = 564 cm(-1) is much less than the experimental estimate of 800 cm(-1). (C) 2000 American Institute of Physics. [S0021-9606(00)30205-7].