8 Jun 2016

Historical overview of Evans method

Five Slides About

Submitted by Adam R. Johnson, Harvey Mudd College
Categories
Description: 

This LO grew out of my interest in understanding (deeply) the machinery behind the Evans method calculations. I did these calculations as a grad student to characterize my compounds, and I teach it in both my lecture and lab. Currently I use the metal acac synthesis lab to motivate the problem.

As I crawled back in time, I found a number of helpful references in unusual journals (at least for an organometallic chemist). I hope that my historical presentation is of some interest to faculty, and maybe even for students. However, since I did spend all this time working it up, I am planning to devote a day to the history of the field next spring when I teach again. I already do a brief history…

You can expect a similar (though less in depth) LO on SQUID magnetometry later this summer!

The main points I would want students to get out of this presentation are as follows:

  • In the olden days: spectrometers were so weak that the reference capillary had to be neat TMS, water or other reference compound.
  • The chemical shift of TMS in TMS is NOT the same as TMS at 1% in CDCl3. Thus, there are two competing factors: paramagnetic shift, and diamagnetic shift due to solvent.
  • The shape factor for a spherical cell is -2pi/3, there is no net paramagnetic shift for a spherical cell, only diamagnetic.
  • A complicated NMR tube with both a cylindrical and spherical reference could be constructed to solve for both the paramagnetic and diamagnetic shifts in one experiment.
  • Only one experiment could be done because of instrument drift, and the fact that this was all done on chart recorders with rheostats.
  • Superconducting magnets made it possible to see the TMS peaks in a 1%v/v solution in CDCl3, making the terms related to diamagnetic effects go away since now the reference and sample solutions had the same diamagnetic shift.

 

 

Learning Goals: 

Students will see the path through history of measuring magnetic susceptibility by NMR, including instrument advances.

Students will understand how much NMR has advanced as a tool since the early 1950s

Implementation Notes: 

I have not yet used this in class so I would appreciate any feedback!

Time Required: 
20-30 minutes
Evaluation
Evaluation Methods: 

I would probably not evaluate students on the material presented here; it is of historical interest.

Evaluation Results: 

n/a

Creative Commons License: 
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