DIMETHYLCARBENE - A SINGLET GROUND-STATE

Ab initio molecular electronic structure theory has been used to deter mine the energy separation between the lowest B-3(1) and (1)A states o f dimethylcarbene. The geometries of both states have been optimized a t the self-consistent field (SCF) and the single and double excitation configuration interaction (CISD) levels of theory using the double-ze ta plus polarization (DZP), the triple-zeta plus double polarization ( TZ2P), and the TZ2P basis set with a set of higher angular momentum fu nctions on the central carbon atom (TZ2P+f) basis sets. For singlet di methylcarbene, structures have also been optimized using the two refer ence CISD method. Single point energies at the coupled cluster with si ngle and double excitations (CCSD) and the CCSD with perturbative trip le excitations [CCSD(T)] levels of theory were determined at the CISD equilibrium geometries with the same basis set. Harmonic vibrational f requencies and infrared (IR) intensities were determined for both stat es at the SCF level of theory using all three basis sets and at the CI SD level of theory using the DZP and TZ2P basis sets. The energy separ ation between the lowest triplet state (B-3(1)) and the lowest singlet state ((1)A) for dimethylcarbene decreases with increasing basis set size and electron correlation. At the highest level of theory employed in this research, TZ2P+f CCSD(T), the singlet state is predicted to b e lower in energy than the triplet state by 0.8 kcal mol(-1). This ene rgy separation becomes 1.4 kcal mol(-1) with the inclusion of zero-poi nt vibrational energy (ZPVE) corrections.