(⁴³Ca) Calcium NMR

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⁴³Calcium (⁴³Ca) is a moderately sensitivity nucleus but with a very low natural abundance that yields moderately sharp lines (fig. 1) in symmetric environments over a small chemical shift range. ⁴³Ca is a spin 7/2 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment. ⁴³Ca NMR is mostly used in relaxation studies to determine binding to DNA, proteins, etc. Because of its extremely low sensitivity at natural abundance, it is usually isotopically enriched for these studies despite the cost. There is little information available about chemical shifts (fig. 2).

Fig. 1. ⁴³Ca-NMR spectrum of CaCl₂ (1M) in D₂O

Fig. 2. Chemical shift ranges for calcium NMR

Properties of ⁴³Ca

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Property	Value
Spin	7/2
Natural abundance	0.135%
Chemical shift range	70 ppm, from -35 to 35
Frequency ratio (Ξ)	6.730029%
Reference compound	0.1 M CaCl2 in D2O
Linewidth of reference	0.5 Hz
T1 of reference	1 s
Receptivity rel. to 1H at natural abundance	8.68 × 10-6
Receptivity rel. to 1H when enriched	6.42 × 10-3
Receptivity rel. to 13C at natural abundance	0.051
Receptivity rel. to 13C when enriched	37.8
Linewidth parameter	2.3 fm4

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, calcium salts are non-toxic on account of their calcium under normal circumstances. However, their anions may be toxic.