Paramagnetic NMR spectroscopy
Paramagnetic NMR spectroscopy refers to NMR spectroscopy of paramagnetic compounds. 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. In many cases, the effects are considered to be deleterious, but they can provide insights into the bonding and structure of the specimen.
Contact vs pseudocontact shifts
Unpaired electron spins influence the NMR responsive nuclei (usually 1H) in two ways, causing contact shifts and pseudocontact shifts. Both effects operate simultaneously but one or the other term can be dominant. Contact shifts result from spin polarization conveyed through the molecular orbitals of the molecule. Pseudocontact shifts result from the magnetic field emanating from the paramagnetic center. Pseudocontact shifts follow an 1/r3 dependence and tend to be smaller, often within the normal 1-10 ppm range for 1H NMR. NMR shift reagents such as EuFOD exploit this effect.
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 and contact terms.
- Friebolin, H., "Basic One- and Two- Dimensional NMR Spectroscopy," VCH: Weinheim, 2010. ISBN 978-3-527-32782-9.