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(Cl) Chlorine has two useful NMR active nuclei ³⁵Cl and ³⁷Cl. Both yield relatively broad signals but have a broad chemical shift range. ³⁵Cl is more sensitive but ³⁷Cl yields slightly higher resolution (fig. 1). In most cases, the extra sensitivity of ³⁵Cl is more important than the extra resolution of ³⁷Cl. Either nucleus can be used to detect and quantify the presence of ionic chlorides. Organic chlorides yield very broad signals of the order of tens of kHz so Cl NMR is of limited to the smallest for organic chlorides.

Fig. 1. Comparison of ³⁵Cl and ³⁷Cl NMR of NaCl solution showing that ³⁷Cl yields slightly sharper signals at the expense of a significant reduction in sensitivity

The main use of chlorine NMR is for very small organic and inorganic molecules and the study of chloride binding. Each type of compound has its characterictic chemical shift range (fig. 2).

Fig. 2. Chemical shift ranges for chlorine NMR

³⁵Chlorine NMR

³⁵Cl NMR is significantly more sensitive than ³⁷Cl NMR and although it yields slightly broader signals it is usually preferred over ³⁷Cl. ³⁵Cl is a spin 3/2 quadrupolar nucleus and its signals become broader with increasing molecular size and asymmetry (fig. 3). ³⁵Cl can be used to study ionic and inorganic chlorides. Organic chlorides yield very broad signals of the order of tens of kHz so ³⁵Cl NMR is of limited to the smallest for organic chlorides.

Fig. 3. ³⁵Cl-NMR spectra showing increasing linewidth with increasing environmental asymmetry

Properties of ³⁵Cl

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Property	Value
Spin	3/2
Natural abundance	75.78%
Chemical shift range	1100 ppm, from -50 to 1050
Frequency ratio (Ξ)	9.797909%
Reference compound	0.1 M NaCl in D2O
Linewidth of reference	11.4 Hz
T1 of reference	0.04 s
Receptivity rel. to 1H at natural abundance	3.58 × 10-3
Receptivity rel. to 1H when enriched	4.72 × 10-3
Receptivity rel. to 13C at natural abundance	21.0
Receptivity rel. to 13C when enriched	27.7
Linewidth parameter	89 fm4

³⁷Chlorine NMR

³⁷Cl NMR yields slightly narrower signals than ³⁵Cl NMR. However, because it is significantly less sensitive ³⁵Cl is usually preferred over it. ³⁷Cl is a spin 3/2 quadrupolar nucleus and its signals become broader with increasing molecular size and asymmetry (fig. 4). ³⁷Cl can be used to study ionic and inorganic chlorides. Organic chlorides yield very broad signals of the order of tens of kHz so ³⁷Cl NMR is of limited to the smallest for organic chlorides.

Fig. 4. ³⁷Cl-NMR spectra showing increasing linewidth with increasing environmental asymmetry

Properties of ³⁷Cl

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Property	Value
Spin	3/2
Natural abundance	24.22%
Chemical shift range	1100 ppm, from -50 to 1050
Frequency ratio (Ξ)	8.155725%
Reference compound	0.1 M NaCl in D2O
Linewidth of reference	7.5 Hz
T1 of reference	0.06 s
Receptivity rel. to 1H at natural abundance	6.59 × 10-4
Receptivity rel. to 1H when enriched	2.72 × 10-3
Receptivity rel. to 13C at natural abundance	3.87
Receptivity rel. to 13C when enriched	16.0
Linewidth parameter	55 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, chloride is non-toxic under normal conditions, however, other oxidation states of chlorine, and organic chlorides are toxic and may be carcinogens, hydrochloric acid is corrosive and toxic: wear protective gloves and, if volatile, work in a hood.