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 {plus_minus} 1 kcal/mol. Thus the CH{sub 3} substituent has the effect of reducing the methylene singlet-triplet splitting (9 kcal/mol) by about 4 kcal/mol. 23 refs., 2 figs., 2 tabs.