Set 3 – Electron Shielding

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Page ID: 79417

Thomas Wenzel
Bates College

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Students are then asked the following question.

What “things” in a molecule generate magnetic fields that will influence B_o for a particular hydrogen nucleus?

Most students have had some introduction to NMR from a prior course and have some recollection of shielding and coupling so know that electrons and other nuclei have an effect. Also, there’s a realization from prior parts of this module that hydrogen nuclei product a magnetic field that should influence neighbors.

I then describe how electron shielding occurs in a molecule using a drawing similar to Figure 9 and give them the following question.

Does electron density affect the magnitude of B_e? If so, what is the relationship?

Many students already know this from their prior experience with NMR from organic chemistry, but even if they have not taken this course, it makes intuitive sense that more electron density will lead to larger shielding.

Does a more highly shielded nucleus absorb higher frequency (higher energy) or lower frequency (lower energy) radiation?

Groups can rationalize out the relationship between shielding and relationship to excitation frequency using equations previously developed in the unit. That allows me to draw Figure 10 in the text for them.

I then point out how NMR spectrometers are designated by the frequency at which hydrogen nuclei are excited and not the applied magnetic field strength. I also describe why it is essential to use a zero reference in NMR spectroscopy, that the ppm scale has been devised as a way to report the location of resonances in the spectrum, and give them the equation for determining the ppm of a resonance. Then I give them the following question.

Suppose the resonant frequency of the TMS singlet on a 400 MHZ NMR spectrometer is exactly 400 MHz. What is the chemical shift in ppm for a signal that has a resonant frequency of 400,000,400 Hz?

Groups are able to plug the values into Equation 10 and determine that the answer is 1 ppm. I then draw a spectrum from 1 to 10 ppm and indicate which end of the spectrum corresponds to higher energy and higher frequency. I also indicate which end is deshielded and the distinction between the upfield and downfield part of the spectrum. I then ask them the following question.

What is different as B_APPLis varied?

It is helpful to prompt the students to consider a peak at 1 ppm on a 100 or 600 MHz instrument. When they think about the values in the equation, they realize that the Hz/ppm varies with field strength.

At this point, I like to describe some examples of unusual shielding effects produced by aryl rings and double bonds as shown in Figure 12 in the text.