Determination of Magnetic Susceptibility by NMR Method

Determination of Magnetic Susceptibility by NMR Method | Evans method

Determination of Magnetic Susceptibility by NMR Method

NMR method is based on the principle that the position of proton resonance lines of a compound is dependent on the bulk susceptibility of the medium in which the compound is placed. In this method, a concentric cell of length > diameter is used. The inner tube of the cell contains the aqueous solution of the paramagnetic substance whose magnetic susceptibility is to be measured and ~3% tert-butanol as internal standard. An identical solvent (H2O) without the paramagnetic substance is placed in the annular section of the cell.

Determination of Magnetic Susceptibility by NMR Method


The paramagnetic substance shifts the proton resonance lines of the standard (that is tert-butanol) and consequently two resonance lines are observed for the methyl protons of the tert-butanol. The shift of the proton resonance lines is given by the equation-
ΔH = (2π/3) ΔV.H
where H is the applied magnetic field and ΔV = Vsoln. – Vsolv., the Vsoln. and Vsolv. being the volume susceptibility of the solution and the solvent, respectively.

The gram magnetic susceptibility of the paramagnetic substance can be calculated from the equation-
χg = χsolv + [χsolv(dsolv − dsoln)/m] . (3/2πm).(ΔH/H)


Here χsolv is the gram susceptibility of the solvent (H2O) (−0.72 × 10–6 cgs units for 3% aqueous solution of tert-butanol), m is the mass of the paramagnetic substance contained in 1 mL of the solution, and dsolv and dsoln are the density of the solvent and the solution, respectively. It is notable here that 1% teramethylsilane or 1% benzene can also be used as standard in this method.


Advantages of NMR Method

✍︎ It requires small amount of solution (~0.2 mL).
✍︎ The measurement is simple and speedy.
✍︎ Temperature can be controlled and varied.

Disadvantages of NMR Method

✍︎ Only sample in the form of solution can be handled. ✍︎ Costly NMR spectrometer is required to record proton shifts.


Source: R.C.Maurya Inorganic Chemistry  

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