Understanding Nmr Spectroscopy

Understanding NMR Spectroscopy
James Keeler
University of Cambridge, Department of Chemistry

c James Keeler, 2002

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What this course is about

This course is aimed at those who are already familiar with using NMR on a day-to-day basis, but who wish to deepen their understanding of how NMR experiments work and the theory behind them. It will be assumed that you are familiar withthe concepts of chemical shifts and couplings, and are used to interpreting proton and 13 C spectra. It will also be assumed that you have at least come across simple two-dimensional spectra such as COSY and HMQC and perhaps may have used such spectra in the course of your work. Similarly, some familiarity with the nuclear Overhauser effect (NOE) will be assumed. That NMR is a useful for chemistswill be taken as self evident. This course will always use the same approach. We will first start with something familiar – such as multiplets we commonly see in proton NMR spectra – and then go deeper into the explanation behind this, introducing along the way new ideas and new concepts. In this way the new things that we are learning are always rooted in the familiar, and we should always be ableto see why we are doing something. In NMR there is no escape from the plain fact that to understand all but the simplest experiments we need to use quantum mechanics. Luckily for us, the quantum mechanics we need for NMR is really rather simple, and if we are prepared to take it on trust, we will find that we can make quantum mechanical calculations simply by applying a set of rules. Also, thequantum mechanical tools we will use are quite intuitive and many of the calculations can be imagined in a very physical way. So, although we will be using quantum mechanical ideas, we will not be using any heavy-duty theory. It is not necessary to have studied quantum mechanics at anything more than the most elementary level. Inevitably, we will have to use some mathematics in our description of NMR.However, the level of mathematics we need is quite low and should not present any problems for a science graduate. Occasionally we will use a few ideas from calculus, but even then it is not essential to understand this in great detail. Course structure The course is accompanied by a detailed set of handouts, which for convenience is divided up into “chapters”. You will notice an inconsistency inthe style of these chapters; this comes about because they have been prepared (or at least the early versions of them) over a number of years for a variety of purposes. The notes are sufficiently complete that you should not need to take many extra notes during the lectures. Each chapter has associated with it some exercises which are intended to illustrate the course material; unless you do theexercises you will not understand the material. In addition, there will be some practical exercises which

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What this course is about involve mainly data processing on a PC. These exercises will give you a feel for what you can do with NMR data and how what you see relates to the theory you have studied. Quite a lot of the exercises will involve processing experimental data. Chapter 2considers how we can understand the form of the NMR spectrum in terms of the underlying nuclear spin energy levels. Although this approach is more complex than the familiar “successive splitting” method for constructing multiplets it does help us understand how to think about multiplets in terms of “active” and “passive” spins. This approach also makes it possible to understand the form of multiplequantum spectra, which will be useful to us later on in the course. The chapter closes with a discussion of strongly coupled spectra and how they can be analysed. Chapter 3 introduces the vector model of NMR. This model has its limitations, but it is very useful for understanding how pulses excite NMR signals. We can also use the vector model to understand the basic, but very important, NMR...