Pulse sequence rotational

The pulses in a pulse sequence rotate the magnetization about a specific axis by a specific angle. However, this ideal behavior is, in practice, not... 

To understand cross polarization and its many variants, we need several new concepts. These concepts include spinlocking (one element of this double-resonance pulse sequence), rotating-frame relaxation times, spin temperature and Curie s law, and finally, establishing contact, or mixing, between spin systems by means of applied rf. 

A variety of experiments based upon particular pulse sequences can be applied to a sample to produce a specific form of NMR signals. For example, the 90-FID pulse sequence rotates the net magnetization down into the xy-plane, through which the relaxation times of the nuclei can be determined. Amongst the more... 

Figure Bl.15.11. Fomiation of electron spin echoes. (A) Magnetization of spin packets i,j, /rand / during a two-pulse experiment (rotating frame representation). (B) The pulse sequence used to produce a stimulated echo. In addition to this echo, which appears at r after the third pulse, all possible pairs of the tluee pulses produce primary echoes. These occur at times 2x, 2(x+T) and (x+2T).

Precession A characteristic rotation of the nuclear magnetic moments about the axis of the applied magnetic field Bo at the Larmor frequencies. Preparation period The first segment of the pulse sequence, consisting of an equilibration delay. It is followed by one or more pulses applied at the beginning of the subsequent evolution period. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

Another practical tool is dynamical decoupling, a technique that uses sequences of fast qubit rotations to mitigate the effects of decoherence. The pulse sequences are designed such that the interactions of each qubit with its environment tend to average out [34, 35]. While still a major concern, decoherence may thus not be the strong impediment it originally seemed to represent for the advent of QC. 

Fig. 2 (a) DRAMA pulse sequence (using % = t/2 = rr/4 in the text) and a representative calculated dipolar recoupled frequency domain spectrum (reproduced from [23] with permission), (b) RFDR pulse sequence inserted as mixing block in a 2D 13C-13C chemical shift correlation experiment, along with an experimental spectrum of 13C-labeled alanine (reproduced from [24] with permission), (c) Rotational resonance inversion sequence along with an n = 3 rotational resonance differential dephasing curve for 13C-labeled alanine (reproduced from [21] with permission), (d) Double-quantum HORROR experiment along with a 2D HORROR nutation spectrum of 13C2-2,3-L-alanine (reproduced from [26] with permission)... 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

The evolution of the density operator can be calculated in the Eigenframe first and the result is then transformed to the rotating frame. For simplicity, a pulse sequence with two PIPs (refer to Fig. 20) is taken into account first. The calculation is then extended to a general case with any number of PIPs. 

Fig. 11.7 a Pulse sequence for rotational-resonance recoupling of homonuclear spin pairs, b The spinning frequency is matched to the isotropic chemical-shift difference, and one of the resonances is selectively inverted and the polarization exchange measured as a function of the mixing time, c The difference polarization as a function of the mixing can be evaluated to give the dipolar coupling constant. 

T2 measurements usually employ either Carr-Purcell-Meiboom-Gill (CPMG) [7, 8] spin-echo pulse sequences or experiments that measure spin relaxation (Tlp) in the rotating frame. The time delay between successive 180° pulses in the CPMG pulse sequence is typically set to 1 ms or shorter to minimize the effects of evolution under the heteronuc-lear scalar coupling between 1H and 15N spins [3]. 

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