Half pulse sequence

In the case of the t domain, since it is only the number N of data points that determines the resolution, and not the time involved in the pulse sequence with various delays, it is advisable to acquire only half the theoretical number of FIDs and to obtain the required digital resolution by zero-filling. Thus the resolution in the Fi domain will be given by R = 2SWi/A i that in the F2 domain is given by / = 1/AQ = 2SW2/A2. 

Figure 5.7 (A) Pulse sequence for gated decoupled /-resolved spectroscopy. It involves decoupling only during the first half of the evolution period

Many variations of this experiment are known. Some of the pulse sequences used for recording heteronuclear 2D/resolved spectra are shown in Fig. 5.8. In a modified gated decoupler sequence (Fig. 5.8b), the decoupler is off during the first half of the evolution period and is svdtched on during the second half. Any C resonances that are folded over in the F, domain may be removed by employing the fold-over corrected gated decoupler sequence (FOCSY) (Fig. 5.8c) or the refocused fold-over corrected decoupler sequence (RE-FOCSY) (Fig. 5.8d). 

The pulse sequence used in homonuclear 2D y-resolved spectroscopy is shown in Fig. 5.18. Let us consider a proton, A, coupled to another proton, X. The 90° pulse bends the magnetization of proton A to the y -axis. During the first half of the evolution period, the two vectors (faster... 

Figure 7.5 Pulses sequences for ID NOESY and ID relayed NOESY experiments, (a) A ID NOESY sequence with full Gaussian pulses is inferior to the ID NOESY sequence (b) with half-Gaussian excitation, (c) A 90° Gaussian pulse in the pulse sequence of 1D relayed NOESYis appropriate for excitation, since antiphase magnetization is required for the first mixing step. (Reprinted from Mag. Reson. Chem. 29, H. Kessler el al, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

The pulse sequence for the ID TOCSY experiment is shown in Fig. 7.6. The original experiment used a Gaussian pulse, but a half-Gaussian... 

The pulse sequence involving excitation by a half-Gaussian pulse is shown in Fig. 7.15 (Kessler et al., 1989a). Its use was demonstrated by semiselective excitation of the NH spectral region of a hexapeptide. 

Figure 9 Timing diagram of the BIRD-HMBC pulse sequence for the detection of nJch correlations, including an additional two-step low-pass J filter. Thin and thick bars represent 90° and 180° pulses, respectively. 13C180° pulses are replaced by 90°y — 180°x — 90°y composite pulses. <5 is set to 0.5/(Vch) and A is set to 0.5/("JCH). Phases are cycled as follows fa = y, y, —y, —y 4>j = x, —x fa — 8(x), 8(—x) fa = 4(x), 4(— x) ( rec = 2 (x, — x), 4(—x, x), 2(x, —x). Phases not shown are along the x-axis. Gradient pulses are represented by filled half-ellipses denoted by Gi-G3. They should be applied in the ratio 50 30 40.1.

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. 10.13. 2D J-resolved NMR spectrum of santonin (4). The data were acquired using the pulse sequence shown in Fig. 10.12. Chemical shifts are sorted along the F2 axis with heteronuclear coupling constant information displayed orthogonally in F . Coupling constants are scaled as J/2, since they evolve only during the second half of the evolution period, t /2. 13C signals are amplitude modulated during the evolution period as opposed to being phase modulated as in other 13C-detected heteronuclear shift correlation experiments.

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.

Fig. 4. The CPMG pulse sequence. An echo is formed halfway between two consecutive K pulses. The echo amplitude (or the Fourier transform of the half-echo) provides an evaluation of T2 less affected by translational diffusion than in the simple Hahn sequence. The phase change of k pulses with respect to the initial Jt/2 pulse cancels the effect of (re) pulse imperfections.

Fig. 1. Pulse sequences of ID selective experiments. The shaped 90° pulses were half-Gaussian. The 180° selective pulses were produced by a DANTE-Z pulse train [44]. T denotes the trim pulse of phase ip. tnoe stands for the NOE-mixing time.

The ID TOCSY-TOCSY experiment is illustrated by the identification of two partial spin systems of a capsular polysaccharide 1, starting with two overlapping protons H-la and H-lb. A 20 ms half-Gaussian pulse was used in an exploratory ID TOCSY experiment (pulse sequence of fig. 1(b)). It has been found (fig. 11(b)) that although the corresponding H-2 protons overlapped partially, H-3a and H-3b were separated completely. In the following ID TOCSY-TOCSY experiments, using the pulse sequence of fig. 10(a), the second TOCSY transfer was initiated from protons H-3a and H-3b, respectively. The two spectra (fig. 11(c), (d)) clearly separate both partial spin system of a and b residues. 

ID NOESY spectra (b) and (c) were acquired using the pulse sequence of fig. 1(a). A 59.2 ms half-Gaussian pulse was applied to proton H-ld. Mixing time was 25 ms in (b) and 200 ms in (c). Water presaturation was applied during the relaxation delay and the NOE mixing... 

Figure 4 shows a TOCSY spectrum with C(wi)-half-filter recorded with the small globular protein bovine pancreatic trypsin inhibitor (BPTI) using the pulse sequence of fig. 3. Although proton multiplets are usually difficult... 

R. W. Schurko, 1. Hung and C. M. Widdifield, Signal enhancement in NMR spectra of half-integer quadrupolar nuclei via DFS-QCPMG and RAPT-QCPMG pulse sequences. Chem. Phys. Lett., 2003, 379,1-10. 

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