The disilaketenyl radical (HSiSiO) in its ground and first excited electronic states

The disilaketenyl (HSiSiO) radical, an isovalent isomer of the ketenyl (HCC O) radical, has been investigated theoretically using ab initio electronic structure theory. For the two lowest-lying electronic states ((X) over tild e (2)A " and (A) over tilde (2)A') of HSiSiO, total energies and physical p roperties including equilibrium geometries, dipole moments, harmonic vibrat ional frequencies, and associated infrared (IR) intensities were predicted at the self-consistent-field (SCF) and configuration interaction with singl e and double excitations (CISD) levels of theory with a wide range of basis sets. At the CISD optimized geometries coupled cluster with single and dou ble excitations (CCSD) and CCSD with perturbative triple excitations [CCSD( T)] energies were also determined. The ground and first excited electronic states of HSiSiO were predicted to be transplanar bent structures, while th e linear 1 (2)Pi state was found to be a saddle point with two imaginary vi brational frequencies. The (X) over tilde (2)A " and (A) over tilde (2)A' s tates of HSiSiO are more distorted from linearity and more polar than the c orresponding states of HCCO. In particular the HSiSiO ground state is predi cted to have a peculiarly acute HSiSi bond angle of only 75 degrees, almost suggesting an Si-Si bridging hydrogen. At the CCSD(T) level of theory with the largest basis set, Dunning's cc-pVQZ, the first excited state was pred icted to lie 36.3 kcal/mol (1.57 eV, 12 700 cm(-1)) classically above the g round state. With the same method the barriers to linearity were determined to be 45.2 kcal/mol (1.96 eV, 15 800 cm(-1)) for the ground state and 8.9 kcal/mol (0.39 eV, 3100 cm(-1)) for the first excited state, respectively. Due to their large dipole moments and relatively large vibrational infrared (IR) intensities, the two lowest-lying electronic states of HSiSiO may be suitable for IR spectroscopic studies, and the ground state for microwave s pectroscopic investigations. (C) 1999 American Institute of Physics. [S0021 -9606(99)30225-7].