Refocusing chemical shifts

The different information obtainable from a chemical-shift weight and a relaxation-parameter image is illusiraled in Fig. 8.3,2 Pra3] In covulcanization of different rubber sheets, for example, sheets from SBR and NR, an interface may arise depending on the materials and the conditions of vulcanization (Fig. 8.3.2(a)). A sufficiently long acquisition delay t without chemical-shift refocusing introduces the chemical-shift... 

Fig. 15. Basic pulse sequence for the acquisition of MQ spectra with chemical shift refocusing.

Fig. 16. 6Q spectrum of 4-chlorotoluene (3) aligned in nematic liquid crystalline solution (a) with chemical shift refocusing and (b) with no chemical shift refocusing. Signals marked ( ) arise from the use of imperfect pulses. (Adapted with permission from ref. 83.)... 

Because only one resonance can be on-resonance, a slight modification to the pulse sequence is in order if we are to determine whether or not each is protonated. This modification allows the experiment to work for resonances that are off-resonance, and is accomplished with what is called a chemical shift refocusing pulse. If, after the first x, we simultaneously apply both a 180° and 180° pulse, we will (1) change the spin state of the H s boimd to the C s, and (2) flip the vectors so that after the x period they will refocus along either the +y- or —y-axis of the rotating frame. 

Figure 4. 300 MHz spectra of the protons in naph-thaquinone dissolved in the nematic solvent ZLI1132. (a) Five-quantum spectrum obtained from a pulse sequence without chemical-shift refocusation, (b) five-quantum spectrum with chemical-shift refocusation, (c) one-quantum spectrum. Real lines resulting from five-quantum transitions are marked with to distinguish them from spurious peaks arising from pulse imperfections. Reprinted with permission from [81].

The spin-echo experiment therefore leads to the refocusing not only of the individual nuclear resonances but also of the field inhomogeneity components lying in front or behind those resonances, a maximum negative amplitude being observed at time 2t after the initial 90° pulse. The frequency of rotation of each signal in the rotating frame will depend on its chemical shift and after the vector has been flipped by the 180° pulse, it... 

Nuclei resonating at different chemical shifts will also experience similar refocusing effects. This is illustrated by the accompanying diagram of a two-vector system (acetone and water), the nuclei of which have different chemical shifts but are refocused together by the spin-echo pulse (M, = magnetization vector of acetone methyl protons, M(v = magnetization vector of water protons). 

Figure 5.18 (A) Pulse sequence for homonuclear 2D y-resolved spectroscopy. (B) Effect of 90° H and 180° H pulses on an H doublet. (C) In the absence of coupling, the vectors are refocused by the 180° H pulse after t. This serves to remove any field inhomogeneities or chemical shift differences.

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

Spin-echo The refocusing of vectors in the xy-plane caused by a (t-180°-t) pulse sequence produces a spin-echo signal. It is used to remove field inhomogeneity effects or chemical shift precession effects. 

In strongly coupled systems, it is not possible to eliminate chemical shifts by refocusing nor is it possible to describe the evolution in terms of an effective Hamiltonian.44 A 90° or a 180° pulse leads to coherence transfer between various transitions, and a multitude of new effective precession frequencies may appear in the F1 dimension. A detailed analysis shows artefacts resulting of strong coupling induced by the 180° pulse applied on the H channel can be efficiently removed by applying a LPJF before acquisition.42 Likewise, artefacts present in HMBC with a terminal LPJF are suppressed by an LPJF in the beginning of the sequence as in conventional HMBC. 

The REDOR experiment (Fig. la), introduced by Gullion and Schaefer in 1989 [22], invokes a strong n pulse for one of the spin species (e.g., the I spin in an I-S spin system) in the middle of each rotor period xr - in addition to one n pulse at each rotor echo on the 5-spin channel to refocus isotropic chemical shift effects and... 

In 2006 Wimperis et al. proposed a method called satellite transitions acquired in real time by MAS (STARTMAS) [142, 202], which allows for the real-time acquisition of high-resolution NMR spectra of spin-3/2 nuclei under MAS. This method combines a train of pulses, similar to CPMG [109, 110], with sample rotation at the magic angle to refocus the quadrupolar broadening in a series of echoes, while allowing the isotropic quadrupolar shift and chemical shift to evolve. 

Fig. 7 A chemical shift imaging pulse sequence. The MR signal is spatially encoded prior to acquiring the spectral signal in the absence of any applied magnetic field gradients. The shaded gradient pulses applied along z either side of the n refocusing pulse are homospoil gradients.

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

The first part, before the t period, of the experiment is identical to the HN(CO)CA-TROSY scheme. This step chooses solely the sequential pathway in an HN(CO)CA-TROSY manner. The chemical shift of the 13C nucleus is recorded during the t evolution period. The back-transfer route is, however, quite different. We transfer the desired coherence from 13C directly back to the 15N nucleus and remove the second 13C -> 13C INEPT step found in HN(CO)CA-TROSY and replace it with the HNCA like back-transfer step. The antiphase 2./N<> coupling then refocuses simultaneously with VNc during the 13C 15N back-INEPT step. Thus, the HN(CO)CANH-TROSY... 

---