CHEMICAL SHIELDING TENSER OF A CARBENE

The first experimental determination of the chemical shielding tenser of a singlet carbene center, 1,3,4,5-tetramethylimidazol-2-ylidene (1) , is reported. Detailed ab initio theoretical calculations of the chem ical shielding tensors are reported for local density functional theor y (LDFT) and the Hartree-Fock levels (HF) in the LORG and IGLO framewo rks. The chemical shielding anisotropy is quite large, which is charac teristic of the lowest-energy-singlet carbene centers. The anisotropy at the carbene center in 1 is similar to 240 ppm. This is one of the l argest anisotropies observed for carbon in a strictly organic framewor k. The calculations indicate that the orientation of the chemical shie lding tensor of the carbene is such that the most shielded;component i s perpendicular to the molecular plane, with the intermediate componen t oriented approximately along the direction of the lone pair, and the least shielded component perpendicular to the other two. This orienta tion, as well as the relative size of the anisotropy, agrees with calc ulations on the singlet carbenes 1, imidazol-2-ylidene (2), and :CF2. The size of the anisotropy is predicted to be enhanced for the parent carbene :CH2.