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Paramagnetic nuclear magnetic resonance spectroscopy
Paramagnetic NMR spectroscopy refers to NMR spectroscopy of paramagnetic compounds.〔Köhler, F. H., "Paramagnetic Complexes in Solution: The NMR Approach," in eMagRes, 2007, John Wiley. 〕〔R. S. Drago "Physical Methods in Inorganic Chemistry" 1977, W. B. Saunders, Philadelphia. ISBN 0-7216-3184-3〕 Although most NMR measurements are conducted on diamagnetic compounds, paramagnetic samples are also amenable to analysis and give rise to special effects indicated by a wide chemical shift range and broadened signals. Paramagnetism diminishes the resolution of an NMR spectrum to the extent that coupling is rarely resolved. Nonetheless spectra of paramagnetic provide insights into the bonding and structure of the sample. For example, the broadening of signals is compensated in part by the wide chemical shift range (often 200 ppm). Since paramagnetism leads to shorter relaxation times (1/T1), the rate of spectral acquisition can be high.
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It should be noted that chemical shifts in diamagnetic compounds are described using the Ramsey equation, which describes so-called diamagnetic and paramagnetic contributions. In this equation, paramagnetic refers to excited state contributions, not to contributions from truly paramagnetic species.〔
==Isotropic shift==
The difference between the chemical shift of a given nucleus in a diamagnetic vs paramagnetic environments is called the isotropic shift. Thus, the isotropic chemical shift for nickelocene is -255 ppm, which is the difference between the observed shift (ca. -260 ppm) and the shift observed for a diamagnetic analogue ferrocene (ca. 5 ppm). The isotropic shift contains contributions from the pseudocontact (called dipolar) and contact (also called scalar) terms.〔Hrobárik, P., Reviakine, R., Arbuznikov, A. V., Malkina, O. L., Malkin, V. G., Köhler, F. H., Kaupp, M., "Density functional calculations of NMR shielding tensors for paramagnetic systems with arbitrary spin multiplicity: Validation on 3d metallocenes", J. Chem. Phys. 2007, volume 126, p. 024107. 〕〔Kruck, M., Sauer, D. C., Enders, M., Wadepohl, H., Gade, L. H., "Bis(2-pyridylimino)isoindolato iron(II) and cobalt(II) complexes: Structural chemistry and paramagnetic NMR spectroscopy", Dalton Trans. 2011, volume 40, p. 10406. 〕
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