Line narrowing pulse sequences

The original homonuclear line-narrowing pulse sequence (WAHUHA) [23] was a four-pulse sequence later elaborations involve more pulses in the total cycle and offer compensation for pulse imperfections and/or higher order averaging of the homonuclear dipolar interaction [24]. Currently, the most popular multiple pulse sequences are the eight-pulse MREV-8 sequence [24a] and the 24-pulse BR-24 sequence [24b]. 

The combination of cross polarization (basically a pulse sequence) and MAS is sufficient to drastically reduce the linewidths of spin-Vi nuclei. Liquid-state proton NMR spectra, as we have seen, are characterized by extremely narrow lines and complex multiplets due to spin-spin coupling in addition, the normal chemical shift range is only around 10 ppm. 

Low frequency spectra of liquids are notably deficient of any structure, and it has long been hoped that a technique would be discovered that provides the same type of line narrowing enjoyed in echo-based electronic and NMR spectroscopy. Tanimura and Mukamel observed that such a technique was possible, and proposed a two-time interval, fifth-order Raman pulse sequence capable of distinguishing, for example, inhomogeneous and homogeneous contributions to the lineshape.[4] The pulse sequence, shown in Fig. 1, is simply an extension of conventional time-domain third-order Raman-based methods. At the... 

A number of different multiple pulse sequences (8-, 24- and 52-pulse sequences) have also been introduced in order to obtain better resolution or line narrowing, i.e. to affect the first- and second-order terms in the average Hamiltonian. Since pulse imperfections are the major source of resolution limitations, these composite pulse sequences are designed with corresponding symmetry properties which allows the canceling of specific rf pulse imperfections. 

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... 

Fig. 15. The pulse sequence for the 13C 2H correlation experiment.38 This two-dimensional experiment, conducted under magic-angle spinning, separates 2H line-shapes according to the 13C isotropic chemical shift of nearby l3C spins, i.e. the bonded l3C in practice. The narrow black pulses are 90° pulses wide ones are 180° pulses. The 2H pulses are placed symmetrically within the rotor period.

The electronic absorption spectra of complex molecules at elevated temperatures in condensed matter are generally very broad and virtually featureless. In contrast, vibrational spectra of complex molecules, even in room-temperature liquids, can display sharp, well-defined peaks, many of which can be assigned to specific vibrational modes. The inverse of the line width sets a time scale for the dynamics associated with a transition. The relatively narrow line widths associated with many vibrational transitions make it possible to use pulse durations with correspondingly narrow bandwidths to extract information. For a vibration with sufficiently large anharmonicity or a sufficiently narrow absorption line, the system behaves as a two-level transition coupled to its environment. In this respect, time domain vibrational spectroscopy of internal molecular modes is more akin to NMR than to electronic spectroscopy. The potential has already been demonstrated, as described in some of the chapters in this book, to perform pulse sequences that are, in many respects, analogous to those used in NMR. Commercial equipment is available that can produce the necessary infrared (IR) pulses for such experiments, and the equipment is rapidly becoming less expensive, more compact, and more reliable. It is possible, even likely, that coherent IR pulse-sequence vibrational spectrometers will... 

In previous chapters we have referred to sequences of pulses that can be used to make particular measurements, such as 7j and T2 (Section 2.9), and more complex sequences that narrow lines in solids (Section 7.8). In this chapter we explore the use of pulse sequences in more detail and investigate the behavior of magnetizations when subjected to arrays of pulses. We follow these spin gymnastics about as far as possible with the classical picture of magnetization vectors and set the stage for invoking the more powerful formalisms described in Chapter 11. 

Fig. 1. The basic line-narrowing m.p. sequence and definition of the pulse spacing t, the pulsewidth t, and the cycle time The pulses embraced by t constitute a cycle because they impose a zero net rotation on the nuclear magnetization. The cycle is repeated over and over and the NMR signal is sampled at integer multiples of t. ...

Our procedure also offers the opportunity to study the effects of various kinds of pulse errors. The general result of our simulations is that known line-narrowing m.p. sequences like the MREV-8 (Mansfield, 1971 Rhim et al., 1973a, b) and BR-24 (Burum et ah, 1979b) sequences perform so well and are so robust against pulse errors that the spectral resolution in actual experiments will rarely be limited by the m.p. sequence as such provided that T can be made as short as 1.5 (MREV-8) or 3 /is (BR-24). lliese are numbers that are well within the reach of our spectrometer and other specialized m.p. spectrometers. 

It is known that for the WAHUHA sequence all purely dipolar terms in the Magnus expansion of the effective Hamiltonian F vanish for a two-spin system in the S-pulse limit (Bowman, 1969). The lines in a WAHUHA m.p. spectrum (S-pulse limit) of a two-spin system should, therefore, be infinitely narrow, irrespective of the pulse spacing t. A two-spin system is, hence, obviously too small for out purpose. Likewise, two-spin systems are too small to meaningfully test any line-narrowing... 

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