APPLICATIONS AND EVALUATION OF IGAIM C-13 AND H-1 CHEMICAL-SHIFT CALCULATIONS FOR UNSATURATED-HYDROCARBONS AND ORGANOLITHIUM COMPOUNDS

Wave functions obtained at the RHF/6-31+G(d) level of theory were used with the new method IGAIM (individual gauges for atoms in molecules) developed by Keith and Bader to calculate the isotropic C-13 and H-1 N MR chemical shifts of a group of neutral molecules (bicyclo[3.2.1]octa -2,6-diene (1), bicyclo[3.2.1]oct-6-ene (2), bicyclo[2.2.1]hepta-2,5-d iene (3), benzene (4)), carbanions (prop-2-en-1-yl (allyl) (5), bicycl o[3.2.1]octa-3,6-dien-2-yl (8)), and lithium complexes (prop-2-en-1-yl lithium (6) and its dimer 7, bicyclo[3.2.1]octa-3,6-dien-2-yllithium ( 9)). The theoretical isotropic C-13 NMR chemical shifts of the neutral molecules, relative to the calculated value for TMS(tetramethylsilane ), are in excellent agreement with the experimental values, with diffe rences between the sets of data ranging from +4.9 to -7.1 ppm. For the same group of compounds the theoretical H-1 shifts are lower than the experimental values by increments ranging between 0.4 and 1.29 ppm. F or allyllithium, which exists as an unsymmetrical fluxional dimer, the theoretical averaged C-13 shifts are larger, 2.6 ppm for the terminal carbons and 16.7 ppm for the central carbon, than the experimental va lues. In the case of 8, originally considered to be a bishomoaromatic species, the theoretical C-13 chemical shifts of its Li+ complex 9 dif fer from the experimental ones for THF-solvated 9 by values that range from +6.2 to -15.0 ppm. Yet, the relative theoretical chemical shifts - of special importance is the fact that the carbons of the vinylene bridge of this compound are unusually shielded relative to the parent diene 1 - correlate with the experimental data. The H-1 chemical shift s calculated for the hydrocarbons 1, 2, 3, 4 and the lithium complexes 7 and 9 range from 0.08 to 1.38 ppm less than the experimental values . To gain information on whether variations in charge density play a s ignificant role in determining the magnitudes of the chemical shifts, we used AIMPAC calculations to obtain the atom electron populations of diene 1, 5, 6, dimer 7, 8, and 9. We find no obvious correlation betw een the charges on the carbon atoms and the C-13 shifts for this set o f compounds.