The 1-silaketenyl radical (HSiCO): Ground and first excited electronic states

The two lowest-lying ((X) over tilde (2)A " and (A) over tilde (2)A') electronic states and lowest linear stationary point (1 (2)Pi) 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 (X) over tilde (2)A " structure features a small HSiC bond angle of 84 degrees. With our most reliable method, cc-pVQZ CCSD(T), the classical (X) over tilde-(A) over tilde splitting has been predicted to be 35.7 kcal/mol (1.55 eV, 12 500 cm(-1)). The barriers to linearity were determined to be 53.5 kcal/mol (2.32 eV, 18 700 cm(-1)) for the (X) over tilde (2)A " state and 17.8 kcal/mol (0.77 eV, 6240 cm(-1)) for the (A) over tilde (2)A' state. The ground state of HSiCO was found to be relatively stable thermodynamically against the two dissociation reactions HSiCO((X) over tilde (2)A ")--> H(S-2)+SiCO((X) over tilde (3)Sigma(-)) and HSiCO((X) over tilde (2)A ")--> SiH((X) over tilde (2)Pi)+CO((X) over tilde (1)Sigma(+)). 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. (C) 2000 American Institute of Physics. [S0021-9606(00)31104-7].