Understanding NMR Spectroscopy doc

[...]... vector model For most kinds of spectroscopy it is sufficient to think about energy levels and selection rules; this is not true for NMR For example, using this energy level approach we cannot even describe how the most basic pulsed NMR experiment works, let alone the large number of subtle two-dimensional experiments which have been developed To make any progress in understanding NMR experiments we need some... Spectrum As you may know from other kinds of spectroscopy you have met, only certain transitions are allowed i.e only certain ones actually take place There are usually rules – called selection rules – about which transitions can take place; these rules normally relate to the quantum numbers which are characteristic of each state or energy level In the case of NMR, the selection rule refers to the quantum... understanding NMR experiments we need some more tools, and the first of these we are going to explore is the vector model This model has been around as long as NMR itself, and not surprisingly the language and ideas which flow from the model have become the language of NMR to a large extent In fact, in the strictest sense, the vector model can only be applied to a surprisingly small number of situations However,... which gives rise the energy levels and ultimately an NMR spectrum In many ways, it is permissible to think of the nucleus as behaving like a small bar magnet or, to be more precise, a magnetic moment We will not go into the details here, but note that the quantum mechanics tells us that the magnetic moment can be aligned in any direction1 In an NMR experiment, we do not observe just one nucleus but... what we actually detect in an NMR experiment All we have to do is to mount a small coil of wire round the sample, with the axis of the coil aligned in the x y-plane; this is illustrated in Fig 3.3 As the magnetization vector “cuts” the coil a current is induced which we can amplify and then record – this is the so-called free induction signal which is detected in a pulse NMR experiment The whole process... only true if we set the receiver reference frequency to be equal to the transmitter frequency; this is almost always the case More details will be given in Chapter 5 z M0 cos β Hard pulses In practical NMR spectroscopy we usually have several resonances in the spectrum, each of which has a different Larmor frequency; we cannot therefore be on resonance with all of the lines in the spectrum However, if... spectrum more and more strongly coupled The spectrum at the bottom is almost weakly coupled; the peaks are just about all the same intensity and where we expect them to be 1 See, for example, Chapter 10 of NMR: The Toolkit, by P J Hore, J A Jones and S Wimperis (Oxford University Press, 2000) 2.6 Strong coupling 2–15 ~ ν0,2 = –10 –ν0,2 J12 ν0,2 = –20 –ν0,2 12 34 J12 ν0,2 = –50 –ν0,2 13 24 0 12 34 20 –ν0,1... doublet In fact it is easy Fig 2.15 The intensity distributions in multiplets from strongly-coupled spectra are such that the multiplets “tilt” towards one another; this is called the “roof” effect 2–16 NMR and energy levels enough to work out the Larmor frequencies using the following method; the idea is illustrated in Fig 2.16 If we denote the frequency of transition 1–2 as ν12 and so on, it is clear... we could think about the spectrum of three coupled spins in terms of sub-spectra in which the Larmor frequencies were replaced by effective Larmor frequencies This kind of approach is very useful for understanding the AB part of the ABX spectrum 2.6 Strong coupling 2–17 full spectrum –ν0,A –ν0,B β sub-spectrum –ν0,A+1/2JAX –ν0,B+1/2JBX α sub-spectrum 0 10 –ν0,A–1/2JAX 20 30 –ν0,B–1/2JBX 40 50 frequency... general it contains 6 lines, rather than the four which would be expected in the weak coupling limit The two extra lines are combination lines which become observable when strong coupling is present 2–18 NMR and energy levels 2.7 Exercises E 2–1 In a proton spectrum the peak from TMS is found to be at 400.135705 MHz What is the shift, in ppm, of a peak which has a frequency of 400.136305 MHz? Recalculate . NMR. This model has its limita- tions, 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 impor- tant, NMR. 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.