Measurement of magnetic susceptibility and magnetic moment of paramagnetic molecules in solution by high-field fourier transform NMR spectroscopy

Abstract

Since the original mention of the ‘H NMR method of susceptibility measurement by Evans in 1959 ( I ) , the magnetic susceptibilities of numerous paramagnetic metal complexes and free radicals in solution have been successfully determined by this technique (2-6). The method and its variation were later described in detail by subsequent workers (2-5, 7-10). The NMR method is simple, quite reliable, and widely used by many as an alternative to Gouy or other standard susceptibility measurement techniques. In this NMR method, two dilute solutions are prepared by using a reference compound in pure solvent. The solution of the paramagnetic species whose susceptibility is to be determined is incorporated in an inner narrow-bore tube while the solvent containing the reference compound is placed in the NMR sample tube or vice versa. The paramagnetic solution also contains the reference compound in an identical amount. The ‘H NMR spectra ofthe reference compound in the two coaxial tubes exhibit chemical-shift differences (paramagnetic shift, Av) which are measured and are then used to calculate the mass susceptibility (xr) of the dissolved paramagnetic molecule from the expression ( I )