Gradient heteronuclear multiple quantum

Fig. 8.19 Schematic representation of the gradient heteronuclear multiple quantum coherence or GHMQC pulse sequence. The gradient version of this experiment now in use [114] is derived from the earlier non-gradient experiment described by Bax and Subramanian [113]. Coherence pathway selection is obtained by the application of gradients in a ratio of 2 2 1 as shown. Other ratios are also possible, as considered in the reports of Ruiz-Cabello et al. [115] and Parella [116]. The experiment creates heteronuclear multiple quantum coherence with the 90° C pulse that precedes evolution. Both zero and double quantum coherences are created and begin to evolve through the first half...

COR, H- F COSY, and H- F gradient heteronuclear multiple quantum coherence (gHMQC). Since the abundance of F and H nuclei are both 100%, het-... 

The first of the proton-detected experiments is the Heteronuclear Multiple Quantum Correlation HMQC experiment of Bax, Griffey and Hawkins reported in 1983, which was first demonstrated using 1H-15N heteronuclear shift correlation [42]. The version that has come into wide-spread usage, particularly among the natural products community, is that of Bax and Subramanian reported in 1986 [43]. A more contemporary gradient-enhanced version of the experiment is shown in Fig. 10.14 [44],... 

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.

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

In the heteronuclear experiment category, the experiments of interest are the heteronuclear multiple quantum correlation (HMQC) experiment, the heteronuclear single quantum correlation (HSQC) experiment, and the heteronuclear multiple bond correlation (HMBC, including the gradient-selected version gHMBC) experiment. Both the HMQC and HSQC produce similar results, but each has its own unique advantages and disadvantages. 

Figure 8.19 shows the gradient version of the HMQC experiment since in most cases users will want to opt for the improved performance of the gradient experiment. Following a preparation period, heteronuclear multiple quantum coherence (zero and double) is created by the 90° X-nucleus pulse applied at the initiation of the evolution period, ti. Evolution occurs and the 180° pulse serves to refocus... 

Eye. -detected gradient-enhanced heteronuclear multiple-quantum coherence spectroscopy has been used to determine the distribution of [l- C]glucose and [l- C]sorbitol in the bovine lens. Following incubation of lenses in 50 mM [l- C]glucose for 1, 2 and 4 days, [l- C]glucose and [l- C]sorbitol were mainly detected in the periphery lens. ... 

Tolman, J. R., Chung, J. and Prestegard, J. H. Pure-phase heteronuclear multiple-quantum spectroscopy using field gradient selection. J. Magn. Reson. 98 462—467, 1992. 

Hurd, Ralph E. and John, B. K. Gradient-enhanced proton-detected heteronuclear multiple-quantum coherence spectroscopy. J. Magn. Reson. 91 648, 1991. 

G = Gy= G. Heteronuclear multiple-quantum Spin echoes and gradient pulses... 

Fig. 10.15. Pulse sequence for the multiplicity-edited gradient HSQC experiment. Heteronuclear single quantum coherence is created by the first INEPT step within the pulse sequence, followed by the evolution period, t . Following evolution, the heteronuclear single quantum coherence is reconverted to observable proton magnetization by the reverse INEPT step. The simultaneous 180° XH and 13C pulses flanked by the delays, A = l/2( 1 edits magnetization inverting signals for methylene resonances, while leaving methine and methyl signals with positive phase (Fig. 16A). Eliminating this pulse sequence element affords a heteronuclear shift correlation experiment in which all resonances have the same phase (Fig. 16B).

More recently, a gradient-enhanced heteronuclear single quantum multiple bond correlation (HSQMBC) experiment and its variants34,47,60-62 have been proposed for the measurement of heteronuclear coupling... 

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