Decoupling during evolution period

Fig. 2. The HSQC or Overbodenhausen experiment of Bodenhausen and Ruben (1980) is shown. An INEPT-type magnetization transfer to the heteronucleus occurs with the first 90° C(X) pulse the heteronuclear component of magnetization is then allowed to evolve. Proton chemical shift evolution can be removed by either applying a 180° pulse midway through the evolution period or by decoupling during evolution. Magnetization is then transferred back to proton, refocused, and detected with heteronucleus decoupling as shown...

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). 

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. 10.14. 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 180° 13C pulse at the middle of the evolution period interchanges the precession frequencies of the a and /3 spins (see Fig. 9.2, bottom) and effectively decouples the spins during tu and a broadband decoupling sequence, such as WALTZ or GARP, is applied during f2.Thus, the 13C spectrum in the F2 dimension is decoupled, and the H spectrum in the Fx dimension retains homonuclear couplings but is also decoupled from 13C, as illustrated in Fig. 10.106. 

It is not necessary that the evolving 13C coherences be detected immediately. As shown in Section 9.6, they can be allowed to precess until they are in phase, then detected while protons are decoupled to provide a single enhanced signal. Alternatively, the entire INEPT sequence can be treated as the preparation period of a 2D experiment. The coherences then evolve during a period t, and can be manipulated in various ways by further pulses. One of the most commonly used methods is to apply a second INEPT sequence, without the initial 90v pulse, after the evolution period to convert the 13C coherences back into H coherences, which can be observed. As we mentioned in Chapter 10, this method, heteronuclear single quantum coherence (HSQC), is widely employed to obtain... 

Attached proton test (APT) can be carried out in several ways, one of which is depicted in Fig. 12.1. It can be readily understood with only the vector picture. The 180° pulse refocuses chemical shifts, but spin coupling evolution occurs only during the period of 1 /J after the first 180° pulse, when the decoupler is turned off. At the end of that period the total signals S are given by... 

Inadvertent homonuclear Hartmann-Hahn transfer during the application of heteronuclear decoupling sequences in a detection period can give rise to undesirable linewidth anomalies (Barker et al., 1985 Shaka and Keeler, 1986). However, no application of Hartmann-Hahn transfer during the detection period of an NMR experiment is known to the authors from the literature. Potential applications include the direct (single shot) acquisition of Hartmann-Hahn coherence-transfer functions in the detection period rather than in an evolution period (Luy et al., 1996). 

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