MULTIPLE PULSE NMR

Shriver, J., Product Operators and Coherence Transfer in Multiple-Pulse NMR Experiments, Concepts in Magnetic Resonance An Educational Journal, 1992,4 (No 1)... 

Multiple-Pulse NMR of Solid Polymers Dynamics of Polytetrafluoroethylene... 

Shown herein are the first 2D multiple-pulse NMR images following a chemical reaction for which the image contrast is Ti rather than T2. A multiple pulse line narrowing sequence is critical to efficient image acquisition where broad lines are expected. The gas-solid reaction between ammonia and a crystal of 4-bromobenzoic acid was monitored optically and by NMR imaging. Some anisotropy in the reaction... 

Notice how we diagram a multiple-pulse NMR experiment the horizontal axis represents time and the vertical axis represents RF amplitude for pulses. The times and amplitudes are not drawn to scale—they are just cartoon representations. 90° pulses are shown as half the width of 180° pulses, and recording of the FID is shown as a decaying signal. Each RF channel is labeled according to the nucleus being irradiated (pulses) and/or observed (FID). 

Shriver J. Product operators and coherence transfer in multiple-pulse NMR experiments. Concepts Magn. Reson. 1992 4 1-33. 

Chapter 9 does not form a part of the course, but is an optional advanced topic. The chapter is concerned with the two methods used in multiple pulse NMR to select a particular outcome in an NMR experiment phase cycling and field gradient pulses. An understanding of how these work is helpful in getting to grips with the details of how experiments are actually run. 

In NMR, the Hamiltonian is seen as having a more subtle effect than simply determining the energy levels. This comes about because the Hamiltonian also affects how the spin system evolves in time. By altering the Hamiltonian the time evolution of the spins can be manipulated and it is precisely this that lies at the heart of multiple-pulse NMR. 

The overall result is that anti-phase magnetization of spin 1 has been transferred into anti-phase magnetization of spin 2. Such a process is called coherence transfer and is exceptionally important in multiple-pulse NMR. 

In heteronuclear systems it is possible to choose whether or not to allow the offset and the coupling to evolve this gives great freedom in generating and manipulating anti-phase states which play a key role in multiple pulse NMR experiments. 

In designing a multiple-pulse NMR experiment the intention is to have specific orders of coherence present at various points in the sequence. One way of indicating this is to use a coherence transfer pathway (CTP) diagram along with the timing diagram for the pulse sequence. An example of shown below, which gives the pulse sequence and CTP for the DQF COSY experiment. 

Refocusing and inversion pulses play an important role in multiple-pulse NMR experiments and so the interaction between such pulses and field gradient pulses will be explored in some detail. As has been noted above in section 9.5.3, a perfect refocusing pulse simply changes the sign of the order of any coherences present, p — -p. If the pulse is imperfect, there will be transfer to coherence orders other than -p. 

We also described elementary ideas in pulse and multiple pulse NMR. Many of the pulse trains manipulate the Hamiltonian, that is, the nuclear interactions, rather than just having the operator passively observe its time evolution. This manipulation of the Hamiltonian is used to select information In... 

The basic principle behind the multiple-pulse NMR techniques to achieve line narrowing (i.e., eliminate the H- H dipolar interaction) is to manipulate the H spin system with r.f. pulses rather than by motion of the whole system, as is done with MAS. This manipulation is performed by using a series of well-timed r.f. pulses such that the average Hamiltonian over the entire period of the pulse sequence does not include the homonuclear dipolar interaction, but still maintains a scaled-down chemical shift e ct. Because of the strict requirements on r.f. pulse widths, shapes, phasing and timing, the multiple-pulse techniques represent some of the most difficult solid-state NMR techniques to implement on a routine basis. The most popular multiple-pulse techniques are currently the eight-pulse MREV-8 and the 24-pulse BR-24 sequence. ... 

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