An ab initio study of angle effects on P-31 nuclear shielding in simple phosphines

Ab initio shieldings for a number of methyl and ethyl phosphines agree well with experimental values. An increase of the intersubstituent angles gener ally causes a deshielding effect that may be understood in terms of a decre ase of the HOMOLUMO energy gap. However, alpha-, beta-, and gamma-substitue nt effects play a dominant role in determining the overall NMR shieldings o bserved. Simple additive effects involving successive replacement of hydrog en by methyl and ethyl groups are not present, and the effect on the shield ing by angle changes is not sufficient to remove this nonadditivity. (C) 19 99 John Wiley & Sons, Inc.