Singlet−Triplet Gap, and the Electronic and Vibrational Spectra of Chlorophenylcarbene: A Combined Theoretical and Experimental Study

Minimum energy structures of singlet and triplet chlorophenylcarbene, a prototypical carbene, were computed. The singlet−triplet energy separation was predicted to be 7.84 and 7.70 kcal/mol at the UCCSD(T)/6-31+G* and QCISD(T)/6-31+G** levels of theory, respectively, after zero-point correction. This is slightly larger than that predicted by the CAS(6,6) (4.5 kcal/mol), local spin density approximation (5.6 kcal/mol), and the BLYP (7.3 kcal/mol) methods with the 6-31G* basis set reported by Trindle et al. The UV−vis and IR spectra of chlorophenylcarbene were analyzed with the aid of the CASPT2/CASSCF(10,10) and the B3LYP/6-31G* levels of theory, respectively. The UV−vis and IR spectra of chlorophenylcarbene were assigned on the basis of these calculations. The ab initio calculations predicted the existence of strong absorption bands in the UV and a weak band in the visible in good agreement with published spectra. The long (750 nm) wavelength band corresponds to electron promotion from the lone pair σ (HOMO) to the π * (LUMO). On the basis of the calculated harmonic frequencies, we cannot assign the 1244 and 1600 cm -1 IR bands observed in an argon matrix to chlorophenyl carbene. The most intense IR band (1225 cm -1 ) corresponds to the symmetric C−C stretch of the carbene and aromatic carbon. The asymmetric and symmetric C−C−Cl stretches are assigned to the bands observed at 847 and 739 cm -1, respectively.