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Rubidium (Rb) has two NMR active nuclei, ⁸⁵Rb and ⁸⁷Rb, that are both quadrupolar and yield broad to very broad lines over a wide chemical shift range. ⁸⁷Rb is preferred as it has the highest sensitivity and yields less broad signals than ⁸⁵Rb (fig. 1). Rubidium NMR is mostly used for detecting binding in aqueous solutions as a substitute for the less NMR sensitive potassium cation by measuring its relaxation rate.

Fig. 1. Comparison of the rubidium isotopes under comparable conditions for RbCl (1M) in D₂O

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

Fig. 2. Chemical shift ranges for rubidium NMR

⁸⁵Rubidium NMR

(⁸⁵Rb) ⁸⁵Rubidium (fig. 3) is a spin 5/2 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment. It is less sensitive and yields broader signals than ⁸⁷Rb so ⁸⁵Rb is not the nucleus of choice for rubidium NMR unless studying isotopic enrichment.

Fig. 3. ⁸⁵Rb-NMR spectrum of RbCl (1M) in D₂O

Properties of ⁸⁵Rb

(Click here for explanation)

Property	Value
Spin	5/2
Natural abundance	72.17%
Chemical shift range	110 ppm, from -80 to 30
Frequency ratio (Ξ)	9.654943%
Reference compound	0.01 M RbCl in D2O
Linewidth of reference	152 Hz
T1 of reference	0.00185 s
Receptivity rel. to 1H at natural abundance	7.67 × 10-3
Receptivity rel. to 1H when enriched	0.0106
Receptivity rel. to 13C at natural abundance	45.0
Receptivity rel. to 13C when enriched	62.4
Linewidth parameter	240 fm4

⁸⁷Rubidium NMR

(⁸⁷Rb) ⁸⁷Rubidium is a spin 3/2 nucleus and is therefore quadrupolar. As a result, the signal width increases with asymmetry of the environment. ⁸⁷Rb is more sensitive and yields narrower lines than ⁸⁵Rb so is the rubidium nucleus of choice. In fig. 4, the RbCl signal is much narrower than that of rubidium diphenylanthracene.

Fig. 4. ⁸⁷Rb-NMR spectra of RbCl (1M) in D₂O and rubidium diphenylanthracene

Properties of ⁸⁷Rb

(Click here for explanation)

Property	Value
Spin	3/2
Natural abundance	27.83%
Chemical shift range	110 ppm, from -80 to 30
Frequency ratio (Ξ)	32.720454%
Reference compound	0.01 M RbCl in D2O
Linewidth of reference	149 Hz
T1 of reference	0.005 s
Receptivity rel. to 1H at natural abundance	0.0493
Receptivity rel. to 1H when enriched	0.177
Receptivity rel. to 13C at natural abundance	290
Receptivity rel. to 13C when enriched	1042
Linewidth parameter	83 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, rubidium salts are toxic in very large doses. This is besides any toxicity that may arise from the anion. ⁸⁷Rb is very slightly radioactive, but is unlikely to pose a radiation hazard.

References

• B. Lindman and I. Lindqvist, "Nuclear quadrupole relaxation of rubidium-85 in an aqueous solution of a humic acid", Acta Chem. Scand., 23, 2215-2216 (1969).
• Z. H. Endre, J. L. Allis, P. J. Ratcliffe and G. K. Radd, "87-rubidium NMR : a novel method of measuring cation flux in intact kidney" Kidney international, 35, 1249-1256 (1989).
• O. Jilkina, B. Xiang, B. Kuzio, J. Rendell and V. V. Kupriyanov, "Potassium transport in langendorff-perfused mouse hearts assessed by ⁸⁷Rb NMR spectroscopy" Magn. Reson. Med., 53, 1172-1176 (2005).