Adjust and control the probe-head temperature at 298 K. Load standard ¹H NMR acquisition parameters, adjust the homogeneity, and record a spectrum of the solution in the NMR tube without the capillary. Introduce the capillary and record the spectrum again. The sample should be spun so as to center the capillary in the NMR tube. Measure the frequency difference between the t-butanol signals in the two compartments.

5. Processing

Use standard 1D processing as described in Experiment 3.1.

6. Result

Tert-butanol proton signals used to determine paramagnetic susceptibility

The figure shows the result obtained on an AM-400 spectrometer. The frequency difference between the t-butanol signals was 459 Hz using 7.2 mg FeSO₄·7H₂O for the solution in the capillary.

Note that the equations and spectra shown in the early literature apply to iron magnets where the magnetic field is perpendicular to the axis of the NMR tube. For measurements using superconducting magnets the factor 2π/3 must be replaced by −4π/3; thus the effect is larger and has the opposite sign. The difference between the frequencies of the signals in the outer compartment (ν_o) and in the capillary (ν_i) is related to the volume susceptibilities X by Equation (1).

(ν_o − ν_i)/ν_o = −(4π/3)(X_i − X_o)(1)

The volume susceptibilities X are usually replaced by the mass susceptibilities χ, where χ = X/ρ, and the density ρ is equal to the mass m of the paramagnetic compound in 1 ml of solution. Thus (1) rearranges to give Equation (2). Here χ_o is approximately equal to the mass susceptibility of water, which is −0.72·10⁻⁶ cm³g⁻¹.