Decoupling, broadband heteronuclear

In practice, any available HMBC pulse sequence could be used to record fast-HMBC spectra.88 As HMBC are recorded without broadband heteronuclear decoupling, the duty cycle is no longer an issue with FAST-HMBC schemes, as it is using the standard SOFAST-HMQC or FAST-HMQC pulse sequences. 

An alternative method for excitation of nuclei over a range of chemical shifts is by irradiation with a weak, noise-modulated radio-frequency, instead of with strong r.f. pulses. In one realization of this method, protons were irradiated with repetitive sequences of noise that was truly random,162 and, in another,163 fluorine nuclei were excited by pseudo-random noise generated by amplitude modulation of the r.f. with maximum-length sequences of pulses from a computer or shift register (a series of flip-flop devices connected by feedback loops). With the carrier wave suppressed, the latter process is equivalent to phase modulation of the r.f. by+7r/2 radians when the pulse is turned on, and by —ir/2 radians when it is turned off. This method is identical with that used in most broadband, heteronuclear, noise decouplers, except that greater power is required for decoupling. 

Even better homonuclear Hartmann-Hahn transfer can be achieved (Bax and Davis, 1985b) when broadband heteronuclear decoupling schemes such as MLEV-16 (Levitt et a/., 1982) and WALTZ-16 (Shaka et al 1983a, b) are employed, which are based on composite 180° pulses (Levitt et al., 1983 Barker et al., 1985 Shaka and Keeler, 1987). 

Phase cycling has improved procedures for broadband heteronuclear decoupling. As described in Section 5-3, modem methods use repeated 180° pulses rather than continuous irradiation. Imperfections in the 180° pulse, however, would accumulate and render the method unworkable. Consequently, phase-cycling procedures have been developed to cancel out the imperfections. The most successful to date is the WALTZ method of Freeman, which uses the sequence 90°, 180°, 270° in place of the 180° pulse (90 — 180 + 270 — 180), with significant cancelation of imperfections. The expanded WALTZ-16 sequence cycles through various orders of the simple pulses and achieves an effective decoupling result. 

In NMR-SIM broadband heteronuclear decoupling can be "switched on" by assigning a rf channel to the decoupled nucleus. As shown in Fig. 2.9 decoupling can be switched on during acquisition and/or prior to the detection pulse. In the following section the most common decoupling methods will be briefly discussed. 

Fig. 6. Pulse sequence for the refocused- or D-HMBC experiment. The idea of refocusing anti-phase proton single quantum coherence prior to acquisition to allow broadband heteronuclear decoupling during acquisition was first reported by Bermel et al in 1989. Evidently unaware of the initial report, Furihata and Seto again de.scribed this experiment in a 199.5 communication, giving it the acronym D-HMBC. There have been a number of applications of this experiment for the acquisition of both H- Cand long-range...

To keep the pulse sequence symmetric to allow broadband heteronuclear decoupling during acquisition, a second variable delay, vd, follows the recreation of antiphase single quantum proton magnetization. A reverse low-pass J follows to make ACCORD-HMBC sequence completely symmetric, followed by acquisition with broadband heteronuclear decoupling. 

Fig. 7. HMQC-TOCSY pulse sequence described by Lerner and Bax (1986). Proton magnetization is manipulated and labeled with the chemical shift of the directly attached in a fashion identical to the HMQC experiment (see Fig. 1). After a refocusing period, A, proton magnetization is propagated from the directly attached proton to its vicinal neighbors using an MLEV-17-based isotropic mixing period. In the original work, the receiver was enabled and broadband heteronuclear decoupling initiated after a fixed delay, A = 1/2( Jch) to suppress the direct responses. Alternative considerations regarding direct responses in an HMQC-TOCSY spectrum are discussed in the text...

Fig. 11. A Pulse sequence to invert direct (one-bond) responses based on HSQC-TOCSY (Domke 1991). B Pulse sequence to invert direct responses based on HMQC-TOCSY (Martin et al. 1992). When the adjustable pulse, P, is set to 180°, the direct responses are inverted as in the original work of Domke (1991). In contrast, when p = 90°, direct responses will be canceled in a manner analogous to the procedure used to calibrate decoupler pulses (Thomas et al. 1981 Bax 1983b). In experiments when the direct response is to be canceled, there is no need of broadband heteronuclear decoupling during acquisition, allowing higher levels of digital resolution in F2 than would otherwise be possible...

A. Bax, Broadband homonuclear decoupling in heteronuclear shift correlation NMR spectroscopy, J. Magn. Resonance 53, 517 (1983). 

In C NMR spectroscopy, three kinds of heteronuclear spin decoupling are used In proton broadband decoupling of C NMR spectra, decoupling is carried out unselectively across a frequency range which encompasses the whole range of the proton shifts. The speetrum then displays up to n singlet signals for the n non-equivalent C atoms of the moleeule. 

Heteronuclear two-dimensional /-resolved spectra contain the chemical shift information of one nuclear species (e.g., C) along one axis, and its coupling information with another type of nucleus (say, H) along the other axis. 2D /-resolved spectra are therefore often referred to as /,8-spectra. The heteronuclear 2D /-resolved spectrum of stricticine, a new alkaloid isolated by one of the authors from Rhazya stricta, is shown in Fig. 5.1. On the extreme left is the broadband H-decoupled C-NMR spectrum, in the center is the 2D /-resolved spectrum recorded as a stacked plot, and on the right is the con tour plot, the most common way to present such spectra. The multiplicity of each carbon can be seen clearly in the contour plot. 

Figure 5.5 A heteronuclear 2D /-resolved spectrum of camphor, along with a broadband decoupled spectrum.

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