THEORETICAL-STUDIES OF THE NEW RADICALS SINNH AND SICOH

Geometry optimization and harmonic vibrational analysis were carried out for new radicals, SiNNH and SiCOH, as well as their isomers, HSiNN and HSiCO, at several levels of ab initio SCF calculations (ROHF, UHF and MCSCF) with the 6-31G* basis set. To examine the effects of electron correlation on the equilibrium geometry, the bond lengths r(SiN) and r(NN) of the isomer SiNNH were optimized with CI calculations. Each of these four molecules has been found to be slightly asymmetric with a 2A" ground state. The predicted rotational constants suggest that the isomer SiNNH is one of the best candidates for the free radical detected in the envelope of the carbon star IRC + 10216, and none of the other three species, HSiNN, SiCOH and HSiCO, can be that one. From the calculated results, the nature of the chemical bonds in these molecules has been discussed in detail.'