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.

Rubidium NMR

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

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Properties of ⁸⁵Rb

(Click here for explanation)

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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

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Properties of ⁸⁷Rb

(Click here for explanation)

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References

1. 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).
2. 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).
3. 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).

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