Safety note: Some of the materials mentioned here are very dangerous. Ask a qualified chemist for advice before handling them. Qualified chemists should check the relevant safety literature before handling or giving advice about unfamiliar substances. NMR solvents are toxic and most are flammable. Specifically, potassium salts are non-toxic on account of the potassium under unless ingested or injected in decagram quantities, however, they may be toxic on account of their anion as is the case for KOCD₃ and to a lesser extent KNO₂ mentioned below. ⁴⁰K is slightly radioactive, if isotopically enriched this may pose a radiation hazard.

Potassium NMR

Potassium (K) has three NMR active nuclei, ³⁹K, ⁴⁰K and ⁴¹K (fig. 1), that are all quadrupolar and yields slightly to very broad lines over a small chemical shift range. Usually ³⁹K is preferred as it has the highest sensitivity and yields sharper signals than ⁴¹K. Although ⁴⁰K yields sharper signals than ³⁹K, it has such a low sensitivity that its use is severely restricted. Potassium NMR is mostly used for detecting binding in aqueous solutions by measureing its relaxation rate. However, some chemical shift information of organic potassium compounds is obtainable.

Fig. 1. Comparison of the potassium isotopes under comparable conditions for KNO₂ (~15 M) in D₂O

All the isotopes have the same chemical shifts (fig. 2) although there is little information available about these shifts.

Fig. 2. Chemical shift ranges for potassium NMR

³⁹Potassium NMR

(³⁹K) ³⁹Potassium is a spin 3/2 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment. Hence, in the fig. 3, KCl is narrower than KOMe. ³⁹K NMR is mostly used for detecting binding in aqueous solutions by measureing its relaxation rate. However, some chemical shift information of organic potassium compounds is obtainable.

Fig. 3. ³⁹K-NMR spectra showing increasing linewidth with increasing environmental asymmetry

²⁹K has a low intrinsic sensitivity although it has a high natural abundance. Although it yields broader signals than ⁴⁰K, it is the nucleus of choice because it is the much more sensitive and for most applications, resolution is not the overriding issue.

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Properties of ³⁹K

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⁴⁰Potassium NMR

(⁴⁰K) ⁴⁰Potassium is a spin 4 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment. ⁴⁰K has a medium intrinsic sensitivity but has a very low natural abundance. It is also slightly radiocative so, although this is not a safety issue at natural abundance, enrichment could lead to safety issues in addition to the high expense of enrichment. Although ⁴⁰K yields sharper signals (fig. 4) than ³⁹K, it has never been reported as the nucleus of choice because it is the much less sensitive and for most applications, resolution is not the overriding issue.

Fig. 4. ⁴⁰K-NMR spectrum of KNO₂ (~15 M) in D₂O

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Properties of ⁴⁰K

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⁴¹Potassium NMR

(⁴¹K) ⁴¹Potassium is a spin 3/2 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment. ⁴¹K NMR is not the preferred potassium nucleus as it yields broader signals (fig. 5) than both ³⁹K and ⁴⁹K and is less sensitive than ³⁹K. The most likely application for this nucleus is for measuring isotopic enrichment of potassium.

Fig. 5. ⁴¹K-NMR spectrum of KNO₂ (~15 M) in D₂O

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Properties of ⁴¹K

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