The 1-silaketenyl radical „HSiCO...: Ground and first excited electronic states

The two lowest-lying ~X̃ A9 and à A8! electronic states and lowest linear stationary point (1 P) of the 1-silaketenyl radical ~HSiCO! have been investigated systematically using ab initio electronic structure theory. The lowest linear stationary point possesses two distinct imaginary vibrational frequencies along the HSiC bending coordinates, indicating a strong Renner–Teller interaction. The ground and first excited states of HSiCO are found to have trans-planar bent structures and they are more distorted from linearity but less polar than the corresponding states of HCCO. Specifically, the X̃ A9 structure features a small HSiC bond angle of 84°. With our most reliable method, cc-pVQZ CCSD~T!, the classical X̃2à splitting has been predicted to be 35.7 kcal/mol ~1.55 eV, 12 500 cm!. The barriers to linearity were determined to be 53.5 kcal/mol ~2.32 eV, 18 700 cm! for the X̃ A9 state and 17.8 kcal/mol ~0.77 eV, 6240 cm! for the à A8 state. The ground state of HSiCO was found to be relatively stable thermodynamically against the two dissociation reactions HSiCO(X̃ 2A9)→H(2S)1SiCO(X̃ S) and HSiCO(X̃ 2A9)→SiH(X̃ P)1CO(X̃ S). Due to the large infrared ~IR! intensities of some of the vibrational modes, IR spectroscopic investigation of the HSiCO radical may be feasible. HSiCO is the global minimum for these four atoms, lying energetically below SiCOH ~38.5 kcal/mol!, HCSiO ~40.7 kcal/mol!, and CSiOH ~76.3 kcal/mol! at the TZ2P( f ,d) configuration interaction with single and double excitations ~CISD! level of theory. © 2000 American Institute of Physics. @S0021-9606~00!31104-7#