Ab initio calculations have been performed to determine the structure and energies of the ground and first excited electronic states of bromine-and iodine-containing singlet carbenes. Effective core potential basis sets augmented with polarization functions were utilized at the CASSCF, CASPT2, and CISD levels of theory.Validation of the effective core potential basis sets for the ground and excited states of the singlet carbenes was carried out by comparison with previous results from all-electron basis set calculations. As was the case in previous studies of chlorine-and fluorine-containing halocarbenes, the bromine-and iodine-containing singlet carbenes are characterized by small bond angles in their ground states, ranging from 100°to 112°, and dramatically larger bond angles in their first excited states, ranging from 125°to 132°. This increase is due to the promotion of an electron from a carbon lone pair orbital coplanar with the carbon-halogen bonds to a carbon p-type orbital perpendicular to the bonds. Adiabatic transition energies for transitions from the ground to first excited state for the singlet carbenes determined at the CASPT2(18,12) and CISD levels range from 21 277 to 10 870 cm -1 and are in excellent agreement with experimental measurements where comparisons are available.