Pulse sequence gated spin echo

Figure 2.4 (A) Pulse sequence for the gated spin-echo (GASPE) or attached proton test (APT) experiment. (B) Effect of the pulse sequence on the C magnetization vectors of a CH group.

F ure 2.4 (A) Pulse sequence for the gated spin-echo (GASPE) or attached proton... 

Figure 5.7. (A) The gated spin-echo (GASPE, or Attached Proton Test APT) pulse sequence. (B) Effects of a gated spin-echo sequence on a CH doublet, (a) At equilibrium (b) after the 90° pulse (c) after first period t, angle a = (Q — coo)t radians (d) after the 180° pulse and broad band decoupling (e) after final period t.

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Figure 4.13. J-modulated spin-echo sequences, (a) The decoupler-gated variant and (b) the pulsed variant.

The spin-echo experiment is particularly simple to set up as it does not require proton pulses or their calibration, a desirable property when the experiment was first introduced but of little consequence nowadays. The same results can, in fact, be obtained by the use of proton 180° pulses rather than by gating of the decoupler [23] (Fig. 4.15b). In this case the A period is broken in two periods of 1/27 separated by the simultaneous application of proton and carbon 180° pulses. These serve to refocus carbon chemical shifts but at the same time allow couplings to continue to evolve during the second A/2 period (Section 2.2). Hence, the total evolution period in which coupling is active is 1/7, as in the decoupler-gating experiment above, and identical modulation patterns are produced. It is this shorter pulsed form of the heteronuclear spin-echo that is widely used in numerous pulse sequences to refocus shift evolution yet leave couplings to evolve. 

Fig. 8.6 Pulse sequence for the amplitude modulated 2DJ-resolved NMR experiment. The experiment is based on a J-modulated spin echo. The first 90° pulse tips magnetization into the xy-plane where it evolves during the first half of the evolution period, t]/2. The 180° pulse is applied and the decoupler is simultaneously gated off for the second half of evolution.

Figure 7 Pulsed-gradient spin-echo (PGSE) electrophoretic NMR sequence. This ENMR pulse sequence is used to detect electrophoretic mobility during the gated electric field E, which is applied fora time duration 3 equal to the spacing between a pair of magnetic field gradient pulses. Reprinted with permission from Ref. [37], Copyright [2014], AlP Publishing LLC.

There are several types of heteronuclear 2D J-resolved experiments (i) the gated decoupler method, (ii) the spin-flip method, (iii) the selective spin-flip method, (iv) the semi-selective spin-flip method, and (v) the use of polarization transfer, e.g., INEPT. In all these experiments, the pulse sequence results in the generation of spin echoes which are modulated during the evolution period by coupling frequencies. 

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