NEW SEMIEMPIRICAL APPROACH FOR THE CALCULATION OF C-13 CHEMICAL-SHIFTTENSORS

The semi-empirical bond polarization theory is applied to the calculat ion of C-13 chemical-shift tensors. This method allows prediction of s hift 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 experi mental chemical-shift tensors solving a set of linear equations. The c oefficients of this overdetermined set of equations are bond polarizat ion energies that must be calculated within the framework of this theo ry. The parameters for C-C, C-H, and C-O bonds of sp(3) and sp(2) hybr idized carbons and C-N bonds of sp(3) carbons were obtained from 606 e quations formed from experimental data from 20 substances taken from t he literature. The substances include sugars, aromatic compounds, amin o acids, and organic acids. The mean deviation of calculated from expe rimental C-13 chemical-shift tensor components is 9 ppm. (C) 1987 Acad emic Press.