# Historical overview of Evans method

## Five Slides About

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.

Attachment | Size |
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powerpoint slides about the history of magnetic measurements by NMR | 733.5 KB |

a list of useful references on the field | 150.62 KB |

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!