Singlet-triplet splitting of carbohydroxycarbene

Ab initio quantum mechanical methods have been applied to determine the singlet-triplet splitting of carbohydroxycarbene, which is the prototype of carbalkoxycarbenes. These methods employ basis sets up to triple-zeta plus double polarization plus f functions (TZ2Pf) in calibre and levels of correlation up to coupled cluster including single, double and perturbatively treated connected triple excitations (CCSD(T)). The triplet state is the ground state, which has the planar C-s symmetry structure, while the lowest singlet state has C-1 symmetry. At the highest level of theory employed, the singlet-triplet splitting is predicted to be 4.7 kcal mol(-1), which falls between an early theoretical prediction of 7 kcal mol(-1) by Kim and Schaefer (1980, J. Amer. Chem. Sec., 102, 5389) and recent experimental conclusions of Toscano, Platz, Nikolaev and Popic (1994, J. Amer. Chem. Sec., 116, 8146) from solution kinetics. The harmonic vibrational frequencies and their infrared intensities are also reported.