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 pro totypical 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. Thi s is slightly larger than that predicted by the CAS(6,6) (4.5 kcal/mol), lo cal 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 the se calculations. The ab initio calculations predicted the existence of stro ng absorption bands in the UV and a weak band in the visible in good agreem ent with published spectra. The long (750 nm) wavelength band corresponds t o electron promotion from the lone pair sigma (HOMO) to the pi* (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 carbe ne. 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), r espectively.