6: 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.

The Evans method is a technique for calculating the number of unpaired electrons in solution-state metal complexes.

Many transition metal complexes have unpaired electrons, making them attracted to magnetic fields. These complexes are called paramagnetic. Complexes with all paired electrons are called diamagnetic.

Knowing the number of unpaired electrons is important for predicting the reactivity of a compound. The Evans method uses NMR spectroscopy to measure the parameters needed to calculate the number of unpaired electrons.

This video will illustrate the procedure for performing the Evans method, demonstrate the analysis of Fe(acac)3, and introduce a few applications of counting unpaired electrons in chemistry.

The number of unpaired electrons in a complex can be determined from the magnetic moment of the given molecule. The magnetic moments of 1st?row transition metal complexes can be approximated from the contributions of unpaired electrons, called the spin-only magnetic moment. For the 2nd and 3rd row transition metal complexes, both the spin and orbital contributions must be considered.

The magnetic moment is related to the magnetic susceptibility, which provides the degree of magnetization of a complex in an applied magnetic field.

The chemical shift of a species in an NMR spectrum is affected by the overall magnetic susceptibility of the sample solution. Thus, the chemical shift of a solvent changes if the solute is paramagnetic. The Evans method uses this relationship to obtain the magnetic susceptibility, and thus the magnetic moment, of that paramagnetic solute.

An Evans method sample uses a capillary insert containing a mixture of a deuterated solvent and the matching proteated solvent. The compound of interest is dissolved in the same solvent mixture and placed in an NMR tube with the capillary.

The acquired NMR spectrum shows two solvent peaks: one corresponding to the proteated solvent in solution with the compound, and the other corresponding to the proteated solvent in the capillary.

The magnetic susceptibility is calculated from the frequency difference and the concentration of the paramagnetic compound in the sample.

The magnetic moment is calculated from the magnetic susceptibility in a special unit called the Bohr magneton. The magnetic moment can then be compared to theoretical spin-only values to estimate the number of unpaired electrons in the sample.

Now that you understand the principles of the Evans method, let's go through a procedure for finding the number of unpaired electrons in Fe(acac)3 with the Evans method.

To prepare the capillary insert, melt the tip of a long Pasteur pipette with a flame until the tip melts into a glass bulb. Allow the glass to cool.

Next, combine in a clean scintillation vial 2 mL of a deuterated solvent and 40 ?L of a proteated solvent. Cap the vial and swirl gently.

Carefully add a few drops of the solvent mixture to the cooled pipette. Gently flick or tap the pipette tip until the solvent has gathered at the bottom of the tip.

Continue adding the solvent mixture in this way until the solution fills the sealed pipette tip to a depth of about 2 inches, with no air bubbles.

Cap the pipette with a 14/20 rubber septum. Equip a 3-mL syringe with a needle. Insert the needle through the septum and carefully withdraw 3 mL of air.

Remove the syringe and clamp the pipette to a ring stand horizontally. Use a lighter to soften the glass above the solution in the pipette tip.

Once the glass begins softening, slowly rotate the solution-filled pipette tip to seal in the solution. Continue rotating the newly-formed capillary until it easily separates from the pipette body.

Let the capillary insert cool, and then store it in a labeled container.

To prepare a sample for the Evans method, first record the mass of a scintillation vial and cap. Then, place 5 mg of the paramagnetic compound of interest into the scintillation vial and record the mass.

Pipette about 600 ?L of the mixture of deuterated and proteated solvents into the scintillation vial. Swirl the vial until the solid compound completely dissolves.

Record the mass of the capped vial of sample solution. Then, obtain a standard NMR tube and cap.

Carefully slide the capillary insert into the NMR tube at an angle. Transfer the solution of the paramagnetic compound to the NMR tube and cap the tube. Ensure that the insert is sitting at the bottom of the tube.

Acquire and save a standard 1H NMR spectrum.

First, calculate the concentration of the sample solution in moles per cubic centimeter using the recorded masses and the density of the solvent. Then, convert the difference between the solvent peak chemical shifts from ppm to Hz. Calculate the molar magnetic susceptibility of the sample.

Next, calculate the magnetic moment from the probe temperature and the molar magnetic susceptibility. Compare the calculated value with a table of known values to determine the number of unpaired electrons in the compound.

The number of unpaired electrons is important for modeling chemical and biological complexes. Let's look at a few applications.

Transition metal complexes can be modeled with molecular orbital theory. In this model, electrons are assigned to molecular orbitals shared between atoms. Information about the number of unpaired electrons helps to confirm that an appropriate model is being used. Further, the number of singly-occupied and unoccupied orbitals predicts how the complex will react with other molecules.

Molecules can be classified by the symmetry operations that they can perform, such as being mirrored across an axis. Molecular symmetry can predict many properties, such as the vibrational modes of a compound. As the number of unpaired electrons can provide information about molecular geometry, it is important to accurately determine the number of unpaired electrons when characterizing compounds.

You've just watched JoVE's introduction to the Evans method. You should now understand the underlying principles of the Evans method, the procedure for calculating the number of unpaired electrons, and how unpaired electrons are relevant to understanding chemical reactivity. Thanks for watching!