Magnetic susceptibility measurement by NMR

Magnetic susceptibility is used to detect and measure the presence of unpaired electrons in paramagnetic materials. In diamagnetic samples susceptibility is an indicator of degree of bond ionization. Usually a dedicated susceptometer is needed to measure diamagnetism accurately. An NMR spectrometer is more readily available in most chemistry departments but till now has failed to measure magnetic susceptibility accurately. An improved NMR method is introduced to measure the magnetic susceptibility, or diamagnetism, with similar absolute accuracy as other methods. This is achieved by careful modelling of the NMR sample shape and the response profile of the probe.

Additionally, magnetic susceptibility affects the resonance frequency of NMR signals and is an important factor when comparing signals in separate samples. Susceptibility is especially important when comparing different solvents, changes in environmental conditions and studies of nanostructured liquids such as microemulsions and liquid crystals.

The new method is validated by comparing the measured diamagnetism of water against the literature standard to within 0.05%.

As a first example of its application, the diamagnetism of CDCl₃ was measured over a range of temperatures and used to reanalyze earlier measurements of the variation of the chemical shift of TMS in CDCl₃ against ³He gas. This improved on the accuracy and reliability of the result and allows for the first time accurate studies of the absolute effect of temperature on chemical shift.