Steric effects in 31P NMR spectra: ‘Gamma’ shielding in aliphatic phosphorus compounds

An examination of published ³¹P NMR spectral data for aliphatic phosphorus compounds has revealed that chain lengthening and branching effects on the chemical shift can be interpreted in terms similar to those used for ¹³C and ¹⁵N shifts. For six families of phosphorus compounds, a β-carbon substituent was shown to deshield phosphorus, while a γ-carbon caused shielding. The effects are additive, and good agreement was obtained between ³¹P shifts calculated with the appropriate constants and the experimental values. Shielding by γ-carbon is indicative of the operation of a steric influence on ³¹P chemical shifts, not heretofore articulated. The γ-effect is also useful in explaining the unusually large shielding found in six-membered cyclic phosphines.