An ab initio study of angle effects on31P 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 generally causes a deshielding effect that may be understood in terms of a decrease of the HOMO-LUMO energy gap. However, α-, β-, and γ-substituent effects play a dominant role in determining the overall NMR shieldings observed. Simple additive effects involving successive replacement of hydrogen by methyl and ethyl groups are not present, and the effect on the shielding by angle changes is not sufficient to remove this nonadditivity.