Friday, June 6, 2008
NMR to Determine Paramagnetic Susceptibilities
The chemical shift of an inert test compound (such as TMS) will depend on the paramagnetic susceptibility of the medium in which it is dissolved. The chemical shift difference between the test compound in the presence and absence of the paramagnetic medium can be used to determine the paramagnetic susceptibility of the medium. Furthermore, if the concentration of the paramagnetic material in the medium and the mass susceptibility of the solvent are known, the mass susceptibility of the paramagnetic material can be determined. The measurement is made by preparing two solutions: one containing a dilute test compound in an appropriate solvent and an identical solution with a known mass of a paramagnetic material added. One of the solutions is sealed in a capillary and placed in an NMR tube containing the other solution. A spectrum is then collected and the chemical shift difference between the resonances of the test compound in the presence and absence of the paramagnetic material is measured and used to calculate the paramagnetic susceptibility. (D.F. Evans, J. Chem. Soc. 2003, (1959)) The figure below illustrates this for the proton spectrum of TMS in a CDCl3 solution of Cr(acac)3 and in a similar solution without the Cr(acac)3. For another example and additional references see the very highly recommended book: S. Berger and S. Braun, 200 and More NMR Experiments p. 305 (2004).