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, chromium compounds, including K₂CrO₄ are toxic carcinogens: wear gloves. Oxidation states VI and VII including K₂CrO₄ are corrosive.

⁵³Chromium NMR

⁵³Chromium (⁵³Cr) is the only NMR active nucleus of chromium. It is a low sensitivity nucleus that yields moderately broad lines in symmetric environments (fig. 1) over a very wide chemical shift range. For larger complexes and molecules, the signals become too broad to observe with a high-resolution NMR spectrometer. Even number oxidation states, such as (0) and (VI) of chromium are diamagnetic while odd number oxidation states are paramagnetic yielding signals much too broad to observe with a high-resolution NMR spectrometer. In most cases, a specific oxidation state of chromium is contaminated with other paramagnetic oxidation states so the NMR of ⁵³Cr is restricted to chromates, carbonyls and carbenes. The Cr(II) oxidation state usually contains too much paramagnetic Cr(III) to yield signals in the high-resolution NMR spectrum. Only in the case of (CO)₃Cr carbenes have signals been observed on a high-resolution NMR spectrometer. There is little information available about chemical shifts (fig. 2).

Fig. 1. ⁵³Cr-NMR spectrum of Na₂CrO₄ (1M) in D₂O

Fig. 2. Chemical shift ranges for chromium NMR

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Properties of ⁵³Cr

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References

1. E. Haid, D. Koessler, O. Lutz and W. Schich, J. Magn. Reson., 55, 145 (1983).
2. A. Hafnerm, L. S. Hegedus and K. H. Doetz, Adv. Metal Carbene Chem., 247-349 (1989).

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