The silaketenylidene (SiCO) molecule: Characterization of the X̃ 3Σ− and à 3Π states

Abstract

The ground (X̃ 3Σ−) and first excited triplet (à 3Π) electronic states of carbonylsilene or silaketenylidene, SiCO, have been investigated systematically using ab initio electronic structure theory. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were predicted using self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), equation-of-motion (EOM) CCSD, CCSD with perturbative triple excitations [CCSD(T)] methods with a wide range of basis sets. The linear X̃ 3Σ− ground state of SiCO has a real degenerate bending vibrational frequency, whereas the à 3Π state of SiCO is subject to the Renner–Teller effect and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequency of the à 3Π state was evaluated via the EOM-CCSD technique. At the highest level of theory with the largest basis set, cc-pVQZ CCSD(T), the adiabatic X̃–à splitting without the zero-point vibrational energy (ZPVE) correction (Te value) was determined to be 68.5 kcal/mol (2.97 eV, 23 900 cm−1) and the adiabatic splitting with the ZPVE energy correction (T0 value) to be 69.0 kcal/mol (2.99 eV, 24 100 cm−1), which are in excellent agreement with the experimental T0 value of 68.78 kcal/mol (2.983 eV, 24 056 cm−1). The theoretical ground state harmonic Si–C stretching frequency ω3=564 cm−1 is much less than the experimental estimate of 800 cm−1.