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.