## Historical overview of Evans method

Submitted by Adam R. Johnson, Harvey Mudd CollegeThis 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 CDCl
_{3}. 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 CDCl
_{3}, making the terms related to diamagnetic effects go away since now the reference and sample solutions had the same diamagnetic shift.

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

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