THE INFLUENCE OF STRUCTURE AND COORDINATION ON THE P-31-CHEMICAL SHIFT IN PHOSPHATES

In the first part of the paper a semiempirical formula for the calculation of the P-31-chemical shift is derived from the recently developed bond polarization theory. This formula contains in the case of phosphates only three empirical parameters: (i) the chemical shift of the unpolarized first bond sphere represented by an isolated (PO4)3-, (ii) the chemical shift change with P - O sigma-bond polarization, and (iii) the change with P - O d-p-pi-bond polarization. The last effect is essential for a meaningful description of the phosphorous chemical shift. The three parameters are estimated from a calculation of the bond polarization energies of 18 phosphates. The presented correlation has a mean error of only 2.9 ppm and represents therefore the first quantitative predictive tool for the calculation of chemical shifts of phosphates. The same method can be used for the calculation of atomic charges. Former empirical correlations between structure and chemical shift can be explained using the bond polarization formula.