Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR is pretty complex and not a high-yield topic for the MCAT¨. We will talk briefly about it, but refer to a prof, a text or your class notes for a more in-depth discussion of NMR.

How does NMR identify organic molecules?

NMR recognizes the spin of protons ($^{1}$H or $^{13}$C) in their normal states (a lower ``$\alpha$'' energy state) and in a state excited by a magnetic field (a higher ``$\beta$'' state).

Applying a magnetic field results in resonance of $^{1}$H or $^{13}$C nuclei, and each give a distinct pattern, depending on their locations in a molecule and in relation to other atoms.

This pattern has been analyzed and allows us to identify molecules.

How do NMR ÒspikesÓ relate to each other?

The area under a curve is directly related to the number of nuclei which it represents.

Therefore, for a given plot, one can compare one spike to another. If one has an area of, let's say X, and another has an area three times X, then the latter exists three-fold more than the former (see Figure 58.1 below).

Can two nuclei interact with each other?

Yes, nuclei within three bonds of each other can influence each other.

When nearby nuclei are in different energy states they can ``shift'' each other's resonance, causing a normal spike to split; this is also called spin-spin splitting.

Figure 58.1: NMR $^{1}$H of ethanol.