Chemical Shifts
The ordinary range of chemical shifts ranges from abut δ250 to -δ250, which is much wider than typical for 1H NMR. Unlike 1H NMR spectroscopy, 31P NMR shifts are primarily not determined by the magnitude of the diamagnetic shielding, but are dominated by the so-called paramagnetic shielding tensor (unrelated to paramagnetism). The paramagnetic shielding tensor, σp, includes terms that describe the radial expansion (related to charge), energies of excited states, and bond overlap. Illustrative of the effects lead to big changes in chemical shifts, the chemical shifts of the two phosphate esters (MeO)3PO (δ2.1) and (t-BuO)3PO (δ-13.3). More dramatic are the shifts for phosphine derivatives H3P (δ-240), (CH3)3P (δ-62), (i-Pr)3P (δ20), and (t-Bu)3P (δ61.9).
Read more about this topic: Phosphorus-31 NMR Spectroscopy, Applications in Chemistry
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