No abstract

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.

Laser flash photolysis of alkylphenyldiazomethanes in the presence of pyridine produces easily detected ylides. The data indicate that photolysis of alkylphenyldiazomethanes leads to both carbene formation and direct formation of rearrangement products which do not derive from relaxed carbene intermediates.

Absolute rate constants of reaction of chlorophenylcarbene, bromophenylcarbene, and chloro-p-nitrophenylcarbene with tetramethylethylene (TME; k TME ) are similar in alkane solvent, Freon-113, benzene, tetrahydrofuran, and acetonitrile. Solvation of the carbenes has little influence on k TME . Strong specific solvation which impedes cycloaddition reactions of the arylhalocarbenes with TME is not found with these carbenes, but the existence of weakly bound complexes with spectra and kinetics similar to those of free carbenes cannot be ruled out.