Theoretical Interpretation of 31P NMR Chemical Shifts. I

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 pπ—dπ bonding to phosphorus.