The Evans Method

1. Preparation of Capillary Insert

1. Using a lighter or other gas flame, melt the tip of a long Pasteur pipette. Gently rotate the pipette tip in the flame until a small bulb forms. Allow the glass to cool.
2. In a scintillation vial, prepare a 50:1 (volume) solution of deuterated:proteo chloroform. Pipette 2 mL of deuterated solvent, and to this add 40 µL of proteo solvent. Cap the vial.
3. Carefully add a few drops of the solvent mixture to the sealed glass pipette. Gently flick the tip of the sealed pipette so that the liquid enters the capillary. Repeat until the solution has a depth of ~ 2 inches from the bottom of the capillary. Make sure that there are no bubbles of air.
4. Cap the pipette with a 14/20 rubber septum. Using a 3-mL syringe capped with a needle, insert the needle into the pipette and pull out 3 mL of air. This creates a partial vacuum, facilitating the next step.
5. Seal the top of the capillary. Horizontally clamp the pipette to a ring stand. Use a lighter to soften the glass above the solution in the bottom of the pipette. Once the glass softens, begin to rotate the tip of the pipette and pull the tip of the pipette away from the clamped base. Let the sealed capillary cool.

2. Preparation of Paramagnetic Solution

1. Using an analytical balance, mass a scintillation vial and lid. Note the mass.
2. Mass out 5-10 mg of the Fe(acac)₃ in the scintillation vial, and note the mass. Fe(acac)₃ has a very high solution magnetic moment. Therefore 5-10 mg will generate a large change of the chemical shift. Typically, 10 - 15 mg is a more appropriate mass to use for Evans method samples.
3. Pipette ~600 µL of the prepared solvent mixture into the vial containing the paramagnetic species. Cap the vial, and note the mass. Make sure that the solid completely dissolves.

3. Preparation of NMR Sample

1. In a standard NMR tube, carefully drop the capillary insert at an angle, to ensure it does not break it.
2. Pipette in the solution containing the paramagnetic species.
3. Cap the NMR tube. For air-sensitive samples, wrap Parafilm around the cap.

4. Data Collection

1. Acquire and save a standard ¹H NMR spectrum.
2. Note the temperature of the probe.
3. Note the radiofrequency.

5. Data Analysis and Results

1. Using the mass and density of the solvent, calculate the volume of the solvent used to prepare the paramagnetic solution.
2. Calculate the concentration (M) of the paramagnetic solution.
3. Calculate the peak separation of the solvent resonance between that of pure solvent (in the capillary) and that shifted by the paramagnet (outside of the capillary) (Δ_ppm). If this is done in ppm, convert it to Hz by Equation 5:
      (5)
   F = spectrometer radiofrequency in Hz
4. Calculate the magnetic susceptibility using Equation 4.
5. Calculate the magnetic moment using Equation 1.
6. Compare the magnetic moment obtained with that predicted for n unpaired electrons from Equation 3. The magnetic susceptibility will be slightly greater than the anticipated spin-only value given in the table, but should be less than that which corresponds to n+1 unpaired electrons.
7. Give the number of unpaired electrons for the paramagnetic species.

6. Troubleshooting

1. If two well-resolved solvent peaks are not observed, try the following:
   1. Use a spectrometer with a greater field strength to increase the chemical shift difference (in ppm) of the two peaks.
   2. Make the sample more concentrated, so that the shift is larger.
2. Sometimes the value does not make sense. If a value that is too low is obtained, try the following:
   1. Repeat, taking greater care in massing out the solvent and paramagnetic species.
   2. Make sure that the paramagnetic species being used is pure. Even solvent impurities in crystals will affect the mass and hence concentration.
   3. For large molecules, the diamagnetism may be so significant that a diamagnetic correction must be made. This term is subtracted to Equation 4:
      
3. Sometimes the value does not make sense. If a value that is too high is obtained, try the following:
   1. Follow the same steps as 6.2.1-6.2.3.
   2. For heavier metals, inclusion of orbital contributions may be necessary.

7. Air-sensitive Samples

1. Air-sensitive samples can readily be analyzed using this technique. Steps 1.2-1.4, step 2, and step 3 are simply performed inside a glove-box.