METHYLCARBENE - THE SINGLET TRIPLET ENERGY SEPARATION

Ab initio molecular electronic structure theory is used to examine the energy separation between the ground triplet and lowest lying singlet states of methylcarbene. Zero-point corrections to the singlet-triplet energy separation are considered. Correlation methods, specifically configuration interaction, are employed in this study. The structures of the above-mentioned electronic states are optimized with a Gaussian basis set of double zeta plus polarization quality, DZP. Single-point energies are determined at the DZP optimized structures with a much larger basis set, TZ+2P+f, which takes into account the effects of carbon atom f functions and is otherwise of triple zeta plus double polarization quality. Harmonic vibrational frequencies are determined for both methylcarbene electronic states with the double zeta plus polarization basis set. The methylcarbene singlet-triplet energy difference is predicted to be 5 +/- 1 kcal/mol. Thus the CH3 substituent has the effect of reducing the methylene singlet-triplet splitting (9 kcal/mol) by about 4 kcal/mol.