Line narrowing multiple pulse sequences

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. 

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. ...

The combination of MAS and multiple-pulse sequences for line-narrowing in solids with homogeneous spin interactions has been analyzed using Floquet... 

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]. 

Figure 34.8 shows the difference in H line-narrowing capabilities betweai CRAMPS and MAS-only (no multiple-pulse sequence) with a modestly high MAS speed... 

Instead of averaging away the anisotropic nuclear spin interactions by MAS or multiple pulse sequences, it is possible to take advantage of the anisotropy of these interactions, provided that macro-scopically oriented samples are available. This kind of static solid state NMR approach is entirely different from the experiments discussed above. It can lead to highly resolved H and spectra with narrow lines, whose position carries information about... 

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. 2. A 2D SLF pulse sequence (A) with S-spin magnetization evolution (B) subject only to heteronuclear dipolar couplings in the q period and detection of chemical shift spectrum in the 2 period. Various multiple pulse (MP) sequences can be used to suppress dipolar coupling among I spins in the laboratory frame during the h period, which enables line-narrowing in the I-S dipolar coupling dimension (i.e., the a>i frequency dimension of the 2D spectrum). This experiment under MAS can be used for separating I-S dipolar sideband patterns by isotropic chemical shifts the re-pulse and the start of the acquisition need to be synchronized with rotational echoes. Other aspects of this pulse sequence are similar to the SLF sequence in Fig. 1.

Phase considerations intrude even in the simplest experiments of observing an FID or an echo. Accurately adjusting the phases of rf pulses can be very important, particularly in experiments involving trains of pulses such as the Carr-Purcell Meiboom-Gill train or the multiple pulse line narrowing sequences. In other sections we have considered how phase shifts originate and how to cope with them. 

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. ... 

MQMAS (Multiple Quantum MAS) is used to resolve resonances of noninteger quadruplar nuclei complete narrowing of the second-order quadrapolar line shape caimot be achieved by MAS alone, but this is feasible when MAS is combined with multiple quantum pulse sequences.This was used extensively in studies of zeolites. 

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