Theoretical Interpretation of 31P NMR Chemical Shifts. I

Abstract

An ASP—LCAO—MO quantum‐mechanical calculation of ³¹P chemical shifts has been made using s, p, and d orbitals and allowing full latitude in the π bonding. This has been accomplished by use of an artfully chosen coordinate notation which allows the chemical‐shift expression to be stated in a particularly simple functional form, using a minimum number of hybridization parameters. Numerical examples of the calculations are presented graphically, and they have been applied to experimental data for the symmetrically substituted 1 phosphines, 2 phosphoryl, 3 thiophosphoryl, and 4 selenophosphoryl derivatives, as well as to the 5 phosphonium salts. The calculations show that the ³¹P chemical shift is primarily sensitive to asymmetric electronic loading and affords a method for estimating the amount of p_π—d_π bonding to phosphorus.