X̃3Σ − and Ã3Π electronic states of linear disilaketenylidene (SiSiO): analysis of the Renner effect in the Ã3Π state. Comparison with the analogous multiple bonded systems SiCO, CSiO, and CCO

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Ab initio electronic structure theory has been employed to investigate systematically the X̃ 3Σ − and à 3Π electronic states of linear disilaketenylidene (SiSiO). The ground state displays a SiSi double bond while the excited à 3Π state may be assigned a SiSi bond order of 5/2. The ground ( X̃ 3Σ −) state of SiSiO possesses a real degenerate bending vibrational frequency, while the first excited triplet ( à 3Π) state is subject to the Renner–Teller interaction, resulting in two distinct real vibrational frequencies along the bending coordinate. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities of the two lowest-lying triplet states were predicted using the SCF, CISD, CCSD, and CCSD(T) levels of theory with a wide range of basis sets. The à 3Π state was also studied using the equation-of-motion CCSD (EOM-CCSD) technique in order to avoid a possible variational collapse to the lower-lying state. With the cc-pVTZ EOM-CCSD method, the Renner parameter and average harmonic bending vibrational frequency for the à 3Π state were determined to be ε=−0.078 and ω 2=219 cm −1, respectively. With our most reliable method, cc-pVQZ CCSD(T), the X̃– à splitting ( T e value) of SiSiO was predicted to be 55.7 kcal mol −1 (2.41 eV, 19 500 cm −1) and the splitting including zero-point vibrational energy correction ( T 0 value) to be 56.1 kcal mol −1 (2.43 eV, 19 600 cm −1). The bond energy for SiSiO was determined to be D e=26.1 ( D 0=25.2) kcal mol −1 indicating considerable thermodynamic stability for the ground state of SiSiO against the dissociation reaction SiSiO ( X̃ 3Σ −)→Si ( 3 P g)+SiO ( X̃ 1Σ +).