Radiofrequency pulse sequences

The NMR characterisation of polymeric systems requires first the search for the existence of networks. The observation of a time reversal effect, specific to residual spin-spin interactions, gives evidence for the presence of polymeric networks [3]. This property is reflected by so-called pseudo-solid spin-echoes formed by applying a suitable radiofrequency pulse sequence that results in a rotation of the spin operators (Figure 8.3). 

Y. K. Lee. N. D. Kurur, M. Helmle, O. G. Johannessen, N. C. Nielsen and M. H. Levitt, Efficient dipolar recoupling in the NMR of rotating solids. A sevenfold symmetric radiofrequency pulse sequence. Chem. Phys. Lett.. 1995. 242, 304-309. 

M. Carravetta, 1. Eden, X. Zhao, A. Brinkmann and 1. H. Levitt, Symmetry principles for the design of radiofrequency pulse sequences in the nuclear magnetic resonance of rotating solids. Chem. Phys. Lett., 2000, 321, 205-215. 

R. Freeman, S. P. Kempsell, and M. H. Levitt, "Radiofrequency pulse sequences which compensate their own imperfections,"... 

Cross-Polarization, CP In NMR, a fundamental radiofrequency pulse sequence used in most solid-state experiments. 

In the previous chapter we saw how the matrix elements of a NMR density matrix relate to spectral lines. In the context of NMR QIP, an algorithm is nothing but a radiofrequency pulse sequence which encodes quantum logic gates. Each radiofrequency pulse implements an unitary transformation, which is used to prepare the initial state, and process the information and the computation. Under a sequence of unitary operators U (ti), U(t2), U(t ), the initial equilibrium density matrix transforms according to ... 

Fig. 2.9.2 Radiofrequency, field gradient and current distributions requires a three-dimen-ionic current pulse sequences for two-dimen- sional imaging sequence [see Figure 2.9.1(a)] sional current density mapping. TE is the Hahn and multiple experiments with the orientation spin-echo time, Tc is the total application time of the sample relative to the magnetic field of ionic currents through the sample. The 180°- incremented until a full 360°-revolution is pulse combined with the z gradient is slice reached. The polarity of the current pulses...

The CPMG pulse sequence is composed of the initial 90° radiofrequency (rf) excitation pulse followed by a series of 180° rf pulses spaced to allow a train of... 

Radiofrequency spectroscopy (NMR) was introduced in 1946 [158,159]. The development of the NMR method over the last 30 years has been characterised by evolution in magnet design and cryotechnology, the introduction of computer-based operating systems and pulsed Fourier transform methods, which permit the performance of new types of experiment that control production, acquisition and processing of the experimental data. New pulse sequences, double-resonance techniques and gradient spectroscopy allow different experiments and have opened up the area of multidimensional NMR and NMRI. 

As explained in depth in the next sections, most of the advanced high-resolution experiments on quadrupolar nuclei are based on the selection of specific coherences and on the transfer of these coherences along the selected pathways, which always terminate at the observable SQ coherence p = — 1. Most of the time, the selection is done using nested phase-cycling of the radiofrequency (rf) pulses included in the pulse sequence [34]. Recently, new methods have been proposed to optimize the nested phase-cycling procedure, including the schemes referred to as cogwheel [36-39] and multiplex [40—421. 

Fig. 5. Pulse sequence for MR detection of vibration using a radiofrequency field gradient. A binomial 1331 radiofrequency pulse (pulse length D, interpulse delay r) is applied in-phase with the mechanical wave. Thus the vibration period 7V is equal to 4(D + r). The number of cycles can be increased to ensure a better frequency selectivity. The constant RF field gradient generated by a dedicated RF coil allows space encoding without using conventional static field gradients (from Ref. 16 with permission from Elsevier).

Two types of low resolution solid state NMR techniques can be distinguished a) broad line NMR in which the absorption signal is obtained by sweeping the magnetic induction B in the vicinity of the resonance value B0 (Eq. (1)), and b) pulsed techniques which are based on the possibility of rotating the magnetization under the influence of particular radiofrequency pulses, or pulse sequences. 

Introducing parallelism into NMR data acquisition requires simultaneous acquisition of signals from more than one sample. The simplest way to achieve this is to use multiple samples within a single radiofrequency (RF) coil, and to design either pulse sequences or post-processing routines to separate the signals... 

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