Understanding NMR Spectroscopy by James Keeler

By James Keeler

Keeler (chemistry, Cambridge U.) compiles and rationalizes lecture notes he has used over a few years at a variety of summer season colleges and graduate classes, right into a reference for those who are already accustomed to using regimen nuclear magnetic resonance for opting for constitution, yet are looking to deepen their realizing of the way precisely the know-how works, might be for you to do whatever particularly unconventional with it. It deals no functional recommendation on the right way to decide on and run a selected test or interpret effects; nor is it excited about concept. quite the emphasis is at the physics of what's really occurring.

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In (a) the line which is on resonance (at zero frequency) is in pure absorption, but as the offset increases the phase error increases. Such an frequency dependent phase error would result from the use of a pulse whose RF field strength was not much larger than the range of offsets. The spectrum can be returned to the absorption mode, (c), by applying a phase correction which varies with the offset in a linear manner, as shown in (b). Of course, to obtain a correctly phased spectrum we have to choose the correct slope of the graph of phase against offset.

Here we plot the phase, φ, as a function of time. During the first τ delay the phase increases linearly with time. The effect of the 180◦ pulse is to change the phase from ( τ ) to (π − τ ); this is the jump on the diagram at time τ . Further evolution for time τ causes the phase to increase by ( τ ) leading to a final phase at the end of the second τ delay of π . This conclusion is independent of the value of the offset ; the diagram illustrates this by the dashed line which represents the evolution of vector with a smaller offset.

Selective excitation Sometimes we want to excite just a portion of the spectrum, for example just the lines of a single multiplet. We can achieve this by putting the transmitter in the centre of the region we wish to excite and then reducing the RF field 3–18 The vector model strong pulse excitation selective pulse offset transmitter Fig. 27 Visualization of the use of selective excitation to excite just one line in the spectrum. At the top is shown the spectrum that would be excited using a hard pulse.

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