Chemistry: Graduate Lecture Series: (CT6) Understanding NMR Spectroscopy (11L)

CHARACTERISATION TECHNIQUES (CT)

(CT6) Understanding NMR Spectroscopy (11L)

By now you will be familiar with the use of NMR as a qualitative tool for structure determination, but little has been said so far about what NMR spectroscopy actually is and how it works. One of the beauties of NMR is that you can use it every day to help in identifying chemical structures without ever having to worry about what is actually going on in the experiment. However, there comes a time when either our curiosity, or our need to understand more deeply what we are doing, brings us to the point where we really want to know what NMR is. This is where this course fits in.

The course starts out by considering the basic NMR experiment which, it turns out, is performed in rather a different way to virtually all other kinds of spectroscopy. Rather than looking for the absorption of radiation by the spins, we excite the spins with a short burst of radiation and then detect the ringing signal which is induced. The Fourier transform of this ringing signal is the familiar spectrum. In order to understand this most basic experiment we will have to develop the vector model, which is a precise semi-classical way of understanding the behaviour of the spins. Once we have the vector model we can begin to explore other experiments which involving pulses, including the famous spin echo experiment, which is the basis for many further developments.

Useful though the vector model is, it is not able to describe the behaviour of coupled spins, and in particular the important phenomena of coherence transfer and multiple quantum coherence. To deal with these effects we need the quantum mechanical approach offered by the product operator method. We will not concern ourselves too much with where this theory comes from, but will find that it can be used in a simple and intuitive way to explain all of the important phenomena in modern NMR. In particular, we will be able to understand how two-dimensional experiments, with such delightful names as COSY, DQF-COSY and HMQC work. It is these experiment which have so revolutionized the application of NMR over the past twenty years.

Time permitting, we will also look at relaxation in NMR spectroscopy. In contrast to most other kinds of spectroscopy, the excited states generated in pulsed NMR are very long-lived, and this means that it is relatively easy to study the way in which these states return to equilibrium – which is what relaxation is. The rate of relaxation gives important insight into molecular motion, and relaxation is also responsible for the nuclear Overhauser effect (NOE) which is an exceptionally important tool in structure determination by NMR.

Recommended books:

James Keeler Understanding NMR Spectroscopy , Wiley 2005 (The course will largely be based on this text) Hore, P. J., Nuclear Magnetic Resonance , OUP 1995.

Postgraduates in the Department of Chemistry

Number of sessions: 11

Lecture

1 hour