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

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.