New Semi-empirical Approach for the Calculation of 13C Chemical-Shift Tensors.

The semi-empirical bond polarization theory is applied to the calculation of 13C chemical-shift tensors. This method allows prediction of shift tensors with deviations from experiment comparable to the errors of the ab initio methods. In contrast to ab initio calculations, a set of empirical parameters is needed, which can be estimated from experimental chemical-shift tensors solving a set of linear equations. The coefficients of this overdetermined set of equations are bond polarization energies that must be calculated within the framework of this theory. The parameters for C-C, C-H, and C-O bonds of sp3 and sp2 hybridized carbons and C-N bonds of sp3 carbons were obtained from 606 equations formed from experimental data from 20 substances taken from the literature. The substances include sugars, aromatic compounds, amino acids, and organic acids. The mean deviation of calculated from experimental 13C chemical-shift tensor components is 9 ppm.