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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 (fig. 1). 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.

Fig. 1. Comparison of ¹²⁹Xe and ¹³¹Xe spectra of xenon gas (23 atm) in CD₃OD

Having such a wide, chemical shift range (fig. 2), xenon's chemical shift is very susceptible to physical conditions. For example the chemical shift of xenon gas varies with pressure by 0.9 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. The chemical shift scale is referenced by IUPAC to XeOF₄ but is more usually referenced to low pressure xenon gas, a difference of 5386 ppm.

Fig. 2. Chemical shift ranges for xenon NMR

¹²⁹Xenon NMR

¹²⁹Xe is a medium sensitivity spin ½ nucleus that yields sharp signals (fig. 3). ¹²⁹Xe is the xenon nucleus of choice because it is more sensitive and yields sharper signals than ¹³¹Xe.

Fig. 3. ¹²⁹Xe spectrum of xenon gas (23 atm) in CD₃OD

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.

Properties of ¹²⁹Xe

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Property	Value
Spin	1/2
Natural abundance	26.44%
Chemical shift range IUPAC	5800 ppm, from -5700 to 100
Chemical shift range rel. to gas	5800 ppm, from -400 to 5400
Frequency ratio IUPAC (Ξ)	27.810186%
Frequency ratio rel. to gas (Ξ)	27.6603909%
Reference compound	XeOF4 (neat) or Xe gas at low pressure
Linewidth of reference	0.9 Hz*
T1 of reference	>200 s
Receptivity rel. to 1H at natural abundance	5.72 × 10-3
Receptivity rel. to 1H when enriched	0.0216
Receptivity rel. to 13C at natural abundance	33.6
Receptivity rel. to 13C when enriched	127

*Theoretically the line should be much narrower but thermal inhomogeneity broadens the line.

¹³¹Xenon NMR

¹³¹Xe is a low sensitivity quadrupolar nucleus, less sensitive than ¹²⁹Xe. Although it yields sharp signals in the gas phase it is broad even as dissovled gas (fig. 4) and 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. 4. ¹³¹Xe spectrum of xenon gas (23 atm) in CD₃OD

Properties of ¹³¹Xe

(Click here for explanation)

Property	Value
Spin	3/2
Natural abundance	21.18%
Chemical shift range IUPAC	5800 ppm, from -5700 to 100
Chemical shift range rel. to gas	5800 ppm, from -400 to 5400
Frequency ratio IUPAC (Ξ)	8.243921%
Frequency ratio rel. to gas (Ξ)	8.1995134%
Reference compound	XeOF4 (neat) or Xe gas at low pressure
Linewidth of reference	0.7 Hz
T1 of reference	1.4 s
Receptivity rel. to 1H at natural abundance	5.96 × 10-4
Receptivity rel. to 1H when enriched	2.81 × 10-3
Receptivity rel. to 13C at natural abundance	3.5
Receptivity rel. to 13C when enriched	16.5
Linewidth parameter	170 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, 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.

References

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