A theoretical study of the excited electronic states of the SiCN system

Ab initio methods have been used to study the lowest-lying electronic states of the SiCN radical, which has two stable linear isomers in its electronic ground state, SiCN and SiNC. Vertical excitation energies and oscillator strengths have been computed for a number of states lying up to 8 eV. The geometries of the lowest-lying doublet and quartet states have been determined. The lowest-lying excited doublet state of SiNC (12Σ+, 4.0 eV) arises from a HOMO-LUMO excitation (3π → 10σ), although the 12Δ state (9σ → 3π) is very close in energy. In the case of the SiCN isomer the lowest excited state is 12Δ, which arises from an excitation from the highest occupied σ orbital into the HOMO (9σ → 3π) and lies 3.6 eV above the ground state. SiCN should present very strong absorptions at 4.9 and 6.1 eV whereas SiNC should have relatively strong absorptions in the region of 5.7-5.9 eV. The smallest adiabatic energy gaps with respect to the ground state of SiNC and SiCN are very close (about 2.8 eV) and the excited state is the same 12A′, which has angular equilibrium geometries for both isomers. We have determined accurate values for enthalpies of formation of the two linear doublet forms ΔfH298o[SiCN]=103.6kcal/mol and ΔfH298o[SiNC]=106.3kcal/mol.