Albert R. Garber

Semiempirical theory of magnetic resonance parameters. I. Theory of chemical shielding constants, using gauge-invariant atomic orbitals, parameterization, and carbon-13 chemical shifts

Abstract The present calculation can account for the gross structural trends in13C shieldings for hydrocarbons. These trends appear to be governed by the so-called “one-center” contribution to the total shielding constant. It is argued that when one is utilizing a CNDO/INDO semiempirical wavefunction, one is logically obligated to include all of the possible two-center contributions to the shielding constant. However, it is also demonstrated that for certain experimental systems, three-center contributions to the shielding constant are essential for a complete description of substituent effects. That is, two-center and three-center contributions to the shielding constant serve to augment certain types of substituent effects. For those systems that appear to be insensitive to three-center contributions, the present calculation is consistent with experimental experience of the stereochemical dependence of13C chemical shifts.

13C-{1H,11B} triple-resonance experiments. Sign determination of 1J(11B-11B), J(13C-11B), and 2J(1H-11B) in some organoboron compounds

Abstract The first sign determination for a 11 B- 11 B spin-spin coupling constant is reported. Using 13 C-{ 1 H, 11 B} triple-resonance and various double-resonance techniques a positive sign has been determined for 1 J 11 B a - 11 B b (= +18.9 Hz) in 1- 13 C-methylpentaborane-9. Furthermore, a negative sign is obtained for 2 J 1 H- 11 B a (= −6.8 Hz) and a positive sign for 1 J 13 C- 11 B a (= +73.1 Hz) in this molecule. The signs and magnitudes for some of the 13 C- 11 B and 1 H- 11 B coupling constants in the tetramethylborate (1−) and tetraphenylborate (1−) anions are also determined. The results are in accordance with both qualitative and quantitative theories for coupling constants involving boron.