Hazardous references

Robin K. Harris
University of Durham, England

Chemical & Engineering News (14 Jul 1997) Vol. 75, No. 28, pp. 7.

I strongly endorse David Live's position. As Live points out, and as was first suggested decades ago, referencing for NMR spectra of any nucleus can be achieved via a proton resonance standard (usually TMS for nonaqueous solutions) without the direct use of a standard containing the nucleus of interest. The procedure is readily viable and is, indeed, recommended. The International Union of Pure & Applied Chemistry is currently in the process of giving approval to two documents relevant to this discussion, one jointly with the International Union of Pure & Applied Biophysics.

The appropriate ratios of frequencies have been determined for most nuclei so that compounds such as dimethylmercury may be regarded as secondary standards that do not need to be directly handled. A list of relevant frequencies (given with respect to the 1H resonance of TMS at exactly 100 MHz) has been produced by Pierre Granger of the University of Strasbourg, in France. The value for 199Hg in dimethylmercury is listed by him as 17.910323 MHz.

Of course, some NMR practitioners may still wish to make direct use of secondary references, but clearly there is no need to handle dimethylmercury for this purpose, even if results are quoted relative to its signal, since other systems in use will have known chemical shifts. My own research involves mainly solid-state NMR, and a useful secondary standard for 199Hg in such situations is hexakis(dimethyl sulfoxide)mercury(II) trifluoromethanesulfonate, [Hg(DMSO)6][ CF3SO3]2, which has a chemical shift relative to dimethylmercury of -2,313 ppm[ Magn. Reson. Chem., 33, 77 (1995)]. Being a solid, this material is far less hazardous than dimethylmercury, though anyone working with mercury compounds should certainly be aware of toxicity and take every available precaution.

The chemical shift of this material has not, as far as I am aware, been measured using the technique mentioned by Live (but maybe it should be), so the value quoted should perhaps be regarded as approximate. In any case, if the ratio method is employed for any compound being studied, direct measurement of secondary standard samples is unnecessary.

The basic conclusion is that, apart from any attempt to remeasure the fundamental frequency ratio for dimethylmercury (for which there seems to be no call), there is absolutely no need for practitioners of 199Hg to involve themselves with handling this dangerous material. This fact should be widely known. Similar situations may exist for other metals with toxic compounds.

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