Argon does not have any NMR active nuclei that occur naturally in significant quantities. However, 39Ar is NMR active but is radioactive and occurs naturally in tiny quantities. 39Ar is synthesized by a nuclear reaction of 39K with fast neutrons. Because of the difficultly and limited utility of performing a 39Ar experiment, it has never been done. Nonetheless, it is possible to predict many of the properties of nucleus theoretically (fig. 1). 39Ar has a spin of 7/2 and is therefore quadrupolar although it is always found in highly symmetrical environments so its signals are not very broad. The chemistry of argon is limited to that of endoherdral fullerenes. We have no experience of 39Ar in our laboratory and we do not have the equipment to synthesize or handle it.
Fig. 1. 39Ar chemical shift ranges as theoretically predicted
|Chemical shift range||110 ppm, from -110 to 0 (theoretical prediction)|
|Frequency ratio (Ξ)||~8.1222%|
|Reference compound||39Ar gas|
|Linewidth of reference||4 Hz (theoretical prediction)|
|T1 of reference||0.1 s (theoretical prediction)|
|Receptivity rel. to 1H at natural abundance||~1 × 10-16|
|Receptivity rel. to 1H when enriched||0.0113|
|Receptivity rel. to 13C at natural abundance||~6 × 10-13|
|Receptivity rel. to 13C when enriched||66.3|
|Linewidth parameter||91 fm4|
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, argon displaces oxygen in the air and is therefore an asphyxiant and 39Ar is radioactive.