1. The Evans method Measuring magnetic susceptibility by NMR—history Created by Adam R. Johnson, Harvey Mudd College (adam_johnson@hmc.edu) and posted on VIPEr on June 9, 2016. Copyright Adam R. Johnson, 2016. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike License. To view a copy of this license visit http://creativecommons.org/licenses/by-nc-sa/4.0/

2. Equation for magnetic susceptibility as a function of mass concentration (g/mL) History ∆f = peak separation (Hz) f = NMR frequency (Hz) m = mass per cm 3  m = mass susceptibility

3. Historical perspective When this technique was first being developed, NMR sensitivity was low. Reference in capillary had to be neat TMS or other material However, diamagnetic shift of solvent reference was observed; the  for TMS is NOT equal to the  for 1% TMS in CCl 3 for example

4. Two competing factors The reference peak in the sample will be shifted due to the paramagnetic sample and the solvent shift (  s ) due to different environments This means 2 unknowns to solve for,  s and 

5. Double reference set-up A double reference was used Spherical reference has no paramagnetic shift (  = 4  /3), only diamagnetic shift Cylindrical reference has both diamagnetic and paramagnetic shift

6. Double reference Evans method Reference tubes contain pure substance (water) Sample contains pure substance (CH 3 CN)

7. Superconducting magnets Modern spectrometers are much more sensitive, and can get good data with only a cylindrical reference tube The reference need not be neat solvent, but can be 1% TMS in CDCl 3

8. Modern Evans method Using 1 H NMR to determine magnetic susceptibility –NMR tube Sample solution Capillary with pure solvent –NMR spectrum collected NMR solvent in capillary (shifted peak) NMR solvent in tube (reference peak) –Use peak shift to calculate unpaired electrons

9. Magnet orientation Modern instrumentation: coaxial superconducting magnet Applied field parallel to sample ∆f = peak separation (Hz) f = NMR frequency (Hz) m = mass per cm 3  m = mass susceptibility