8.24: Other Nuclides: 31P, 19F, 15N NMR
Many organic, inorganic, and biological molecules contain spin-half nuclei such as nitrogen-15, fluorine-19, and phosphorus-31. As a result, NMR studies of these nuclei have found extensive applications in chemical and biological research.
While fluorine-19 and phosphorous-31 have high natural abundances (100%) and positive gyromagnetic ratios, nitrogen-15 has a low natural abundance and a negative gyromagnetic ratio. However, nitrogen-15 is still preferred over nitrogen-14 (which has a high natural abundance) because the latter is quadrupolar and produces broad signals. All three nuclei require different standard references and have different chemical shift ranges. For NMR studies of nitrogen-15, nitromethane is used as the standard reference, whereas trichlorofluoromethane and phosphoric acids are used as standard references for fluorine-19 and phosphorus-31 NMR studies, respectively.
In general, nitrogen-15 chemical shifts are lowest for saturated systems and become increasingly positive with electronegative substitution. However, the lone electron pair on nitrogen can undergo coordination and protonation with the solvent, making the chemical shift solvent-dependent.
While fluorine-19 chemical shifts are also subject to solvent effects, they are predominantly influenced by paramagnetic shielding effects. In addition, fluorine-19 chemical shifts are practically unaffected by ring currents and neighboring group effects. Interestingly, phosphorus-31 chemical shifts are more affected by the coordination number of phosphorus and not strongly influenced by electronegative substituents.
