2D HMBC

Likewise, the attenuation of the long-range signals "Jch is demonstrated by selected F2 sections taken through the 2D HMBC spectra. Figure 8 shows the responses at the coz frequencies of C-2 at 68.73 ppm and C-3 at 78.46 ppm. 

The principle of multiple selective excitation has been incorporated into a few ID and 2D experiments, the schemes of which are shown below (fig. 1). Depending on the experiment, either a DANTE pulse train (ID TOCSY [2]), frequency selective 180° pulses (ID NOE [3], ID INADEQUATE [4], ID C/H COSY [5] and 2D TOCSY-COSY [6]) or frequency selective 90° pulses (2D HMBC [11]) are applied to selectively perturb and uniquely label selected spins. Besides the DANTE pulse , composed itself of a series of non-selective rectangular pulses, Gaussian-shaped 180° and... 

Fig. 1. Pulse sequences modified for multiple selective excitation. I ID TOCSY, II het-eronuclear ID NOE, III ID INADEQUATE, IVa heteronuclear ID COSY (optimized to detect Jch), IVb heteronuclear ID COSY (optimized to detect "Jch), V 2D TOCSY-COSY, Via 2D HMBC (designed to detect heteronuclear long-range couplings "Jch only), VIh 2D HMBC (extended pulse sequence to detect both heteronuclear long-range "Jch and...

TOPHAT-shaped 90° pulses are used in other cases as the best compromise with respect to the excitation profile, the phase homogeneity and length. Depending on the type of the detected spin-spin interaction - being either scalar or dipolar coupling - each selected spin is initially perturbed only once (ID TOCSY, ID INADEQUATE, ID C/H COSY, 2D TOCSY-COSY and 2D HMBC), or for several times (ID NOE). With each of the selected spins initially perturbed only once the inherently smaller transient NOEs would be detected in the latter case, whereas with the multiple excitation of a selected spin within the NOE build-up period the stronger steady-state NOEs are more or less approximated. 

They could well be replaced today by a single multiselective 90° or 180° pulse somewhere in the pulse sequence leading to probably superior variants as demonstrated with the modified 2D HMBC experiment. Former hardware and software limitations, however, forced us to use the variant with initially applied trains of selective 180° pulses. 

A series of 2D HMBC experiments have been performed to demonstrate the benefits of LP with 2D data. Whereas the number of time domain data points in t2 (TD2) remains the same for each experiment, the number of time domain data points in 11 (TD1) and the number of scan.s (NS) has been varied according to Table 5.3. Thi.s allows a comparison of the results obtained with/without the application of LP on the basis of the same measuring times but different number of scans per increment (005001,006001, 007001)... 

JOC6700>. Their structures were determined primarily by analysis of high field 2D (HMBC, HMQC, COSY, and NOESY) NMR data. 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

Bazzo and co-workers have proposed an optimised 2D HMBC method of higher sensitivity for the measurement of the /hc couplings. Parella and Belloc have presented simple ID HMQC and HSQC pulse schemes which allow the Juc sign and magnitude measurement for samples at natural carbon abundance. 

FIGURE 12.12 2D HMBC spectrum of 6-hydroxy buspirone (carbonyl region). 

The increasing analytic ability of modern NMR spectroscopy will make use of long-range coupling constants "/( Si,X) (n> 3) for assignment purposes, in particular for X = H, when gradient-enhanced 2D HMBC experiments can be set up to detect Si resonances as a consequence of very small scalar Si- H spin-spin coupling. 

The 2D HMBC experiment can provide further information needed to make many of the remaining assignments. This experiment is essential to... 

Figure 21. 2D HMBC spectrum in D2O of polyglucuronic acid-4-amino TEMPO. All the...

---