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, xenon displaces oxygen in the air and is therefore an asphyxiant. XeOF₄ is unstable and reacts with water to yield HF that is highly corrosive, dissolves glass, very toxic, causes serious burns and other nasty biological damage including death, requires special equipment (chemical hood, gloves, eye protection, 2.5% calcium gluconate gel to treat burns by application and/or injection, etc.) and special skills to handle.

Xenon NMR

Xenon has two medium to low sensitivity nuclei that have a very wide chemical shift range. ¹²⁹Xe is a medium sensitivity spin ½ nucleus that yields sharp signals. ¹³¹Xe is a low sensitivity spin 3/2 nucleus that yields narrow lines in highly symmetrical environments and broader signals in less symmetrical environments. ¹²⁹Xe is therefore the preferred nucleus although the quadrupolar relaxation of ¹³¹Xe is sometimes used as a probe for environmental asymmetry. Xenon NMR is used extensively as an inert physical probe of the solution and gas state in addition to studies of its chemistry and weak molecular bonding.

Having such a wide, chemical shift range (fig. 1), xenon's chemical shift is very susceptible to physical conditions. For example the chemical shift of xenon gas varies with pressure by 0.5 ppm/atm. The dissolved gas has a chemical shift range of approximately 200 ppm depending on the properties of the liquid. It is therefore an excellent indicator of the physical properties of the solution. Each type of xenon compound has its characteristic chemical shift range.

Fig. 1. Chemical shift ranges for xenon NMR

¹²⁹Xenon NMR

¹²⁹Xe is a medium sensitivity spin ½ nucleus that yields sharp signals (fig. 2). ¹²⁹Xe is the xenon nucleus of choice because it is more sensitive and yields sharper signals than ¹³¹Xe.

Fig. 2. ¹²⁹Xe spectrum of xenon gas

Spectrum courtesy of W. Makulski and K. Jackowski

¹²⁹Xe in molecules often couplings to other nuclei. Two and three bond couplings to fluorine are between 20 and 200 Hz. Couplings have also been reported to ¹H, ¹³C, ¹⁴N, ¹⁵N, ¹⁷O and ¹²⁵Te.

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Properties of ¹²⁹Xe

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¹³¹Xenon NMR

¹³¹Xe is a low sensitivity quadrupolar nucleus, less sensitive than ¹²⁹Xe. Although it yields sharp signals in symmetrical environments (fig. 3), its signals get much broader in asymmetric environments. ¹³¹Xe is therefore not the xenon nucleus of choice. However, the quadrupolar broadening of ¹³¹Xe is sometimes used as a measure of asymmetry in the molecular environment.

Fig. 3. ¹³¹Xe spectrum of xenon gas

Spectrum courtesy of W. Makulski and K. Jackowski

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Properties of ¹³¹Xe

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References

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