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.