Magnetic Susceptibility by the Evans Method

Author: J. M. McCormick

Last Update: March 29, 2009

Introduction

The Evans method uses difference in the NMR chemical shift in a solvent caused by the presence of a paramagnetic species. It can be used to determine the magnetic moment of the paramagnetic and the number of unpaired electrons present.¹ And although the temperature dependence of the chemical shift difference can be used to determine the temperature-dependent magnetic moment, the Evans method is usually only performed at room temperature. Note in reference 1 the background on NMR describes the antiquated continuous wave (CW) method of obtaining an NMR spectrum, not the modern Fourier transform (FT) method.

In this exercise you will determine whether this method is sufficiently sensitive to distinguish differences in the magnetic moment of a number of transition metal coordination complexes and explain your results in terms of the metals’ electronic configurations. You will also determine whether the points raised by Piguet² and Ostfeld and Cohen³ are relevant to the compounds that you studied.

Experimental

The method described in the literature¹ will be followed, with the modification suggested by Schubert.⁴ You may either use a series of metal acetylacetonates (we have Cr, Fe, Ni and Co) in CDCl₃ or d⁶-DMSO or the series of simple salts given in reference 1 in D₂O. Please note that some of the salts that they list as anhydrous are only commercially available as hydrates. You will need to prepare the samples with a precisely known concentration with a volume of no more than 1 or 2 mL. You will be using a special NMR tube which is made up to concentric tubes, the inner one has diameter near the bottom is much less than at the top. A sample of pure solvent will go in the inner tube, while the sample containing the paramagnetic species will go in the outer tube. Care must be taken not to add too much of the paramagnetic sample to the outer tube such that the sample is expelled when the inner tube is inserted.

Results and Analysis

Perform the calculations described in reference 1 to determine the magnetic moment of your compounds. Discuss any trends or anomalies. Give quantitative evidence for or against your hypothesis of whether Piguet’s and Ostfeld and Cohen’s comments are relevant to your systems.

References

1. Garland, C. W.; Nibler, J. W. and Shoemaker, D. P. Experiments in Physical Chemistry, 8^th Ed.; McGraw-Hill; New York: 2003, p. 371-379.

2. Piguet, C. J. Chem. Educ. 1997, 74, 815-816. Click here to download this article as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).

3. Ostfeld, D. and Cohen, I. A. J. Chem. Educ. 1972, 49, 829. Click here to download this article as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).

4. Schubert, E. M. J. Chem. Educ. 1992, 69, 62. Click here to download this article as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).