Examples of HMBC Spectra

Some of the CH2 crosspeaks are degenerate, meaning that the two protons have the same ll chemical shift (d and r). This can be a coincidence, but it is more likely to happen in a flexible chain, so we would suspect carbons 22-24 in cholesterol, although 22 is less likely because it is next to a chiral center. 

The HMBC also incorporates a low-pass filter that tries to reject the one-bond correlations seen in HSQC/HMQC. Low pass means that only the low values of 7ch (0-10 Hz) are allowed to pass through and produce crosspeaks in the 2D spectrum. Because there is no 13 C decoupling, the one-bond correlations appear as wide doublets (7 150 Hz) centered on the XH peak position in F2 (Fig. 11.10—squares). They obscure the weak HMBC crosspeaks and can easily be misinterpreted as long-range correlations, especially if one of the two components of the doublet happens to fall at the position of another peak in the XH spectrum. The low-pass filter is set to reject a particular 7 value, typically 135 Hz for molecules dominated by saturated hydrocarbon (e.g., 3-heptanone, menthol, cholesterol), 142 Hz for sugars, and 170 Hz for molecules dominated by aromatic carbons. The same 

A table of predicted and observed HMBC correlations confirms that many crosspeaks are missed, even in a fairly concentrated sample  

In this example, we had already assigned all of the carbon peaks in the 13C spectrum, so there was no mystery. In the most difficult case of a complete unknown, the peaks in the 13C spectrum are numbered arbitrarily from upfield to downfield, and the protons are numbered correspondingly according to the carbon they correlate to in the HSQC/HMQC spectrum. Then the HMBC correlations are tabulated (e.g., C-5-H-16a) and the puzzle solving begins. 

INVERSE HETERONUCLEAR 2D EXPERIMENTS HSQC, HMQC, AND HMBC 

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Similar spectra can be obtained more rapidly and with less sample if the data are acquired through the proton signals, which are much more intense. Basically, the H NMR data are acquired and the H- C coupling constant used as the delay in a pulse sequence, which enables us to obtain the carbon spectrum. This method of obtaining the data is called inverse-mode , since the carbon atoms are detected through their attached hydrogen atoms rather than by direct detection, with obvious benefits in the sensitivity and the time taken to obtain a spectrum. HMQC and HMBC are both examples of inverse-mode spectra and this method is so much quicker than CH COSY that an entire HMQC spectrum can be obtained in much less time than it takes to obtain the proton-decoupled C... 

The elucidation of the primary structure of small organic molecules by tracing their carbon skeletons was, traditionally, the main focus of INADEQUATE experiments. It is not the first NMR experiment to be considered for such a task typically a standard set of NMR spectra, that is COSY, TOCSY, NOESY, HSQC and HMBC, are performed and analysed first. If ambiguities remain after inspection of standard spectra, the tracing of carbon-carbon connectivities is embarked on. Nevertheless, an example is presented below where carbon-carbon connectivities are included at an earlier stage in order to reduce the number of computer-generated structures compatible with the experimental data. 

Other strategies that show great promise in reducing NMR acquisition time utilise methods to obtain multiple sets of data from one experiment through a concept known as time-shared evolution. An example of this process that should find utility in natural products elucidation was demonstrated by a pulse sequence called CN-HMBC.93 Traditionally, a separate 13C-HMBC and 15N-HMBC were acquired independently. However, the CN-HMBC allows both 13C- and 15N-HMBC spectra to be obtained simultaneously. By acquiring both data sets simultaneously, an effective 50% time reduction can be achieved.93 This approach has also been demonstrated for a sensitivity-enhanced 2D HSQC-TOCSY (heteronuclear multiple bond correlation total correlation spectroscopy) and HSQMBC (heteronuclear single quantum... 

As an illustrative example, selected regions of TOCSY (a), HMQC (b), and ROESY (c) spectra of a sulfated trisaccharide which is the most frequently found in the saponins isolated fi om B. rigidum, are depicted in Fig. (3). Although the selected region is one of the more heavily crowded of the spectra, the neat separation of proton and carbon signals achieved with the use of such techniques permit assignment of all of them to each unit. Connections among the different residues can also be deduced from the ROESY Fig. (3c) and HMBC spectra (not shown). 

It is also important that LP not be abused. A sufficient number of increments must be taken from which the FID s can confidently be extended. A total of 64 increments has, for example, been found to be insufficient, while LP s have successfully been carried out with 96 increments. A good practice is to acquire at least 128 increments for accurate prediction. A second concern is that LP not be extended too far (e.g., 128 points predicted to 4,096). W. F. Reynolds (2002) has found that, as a general rule, data presented in the phase-sensitive mode can be predicted fourfold (e.g., 256 data points can be predicted to 1,024), while absolute-value data can be extended twofold, 256 points to 512. A significant exception to the fourfold rule for phase-sensitive experiments concerns the H-detected, heteronuclear chemical-shift correlation experiments. In marked contrast to COSY and HMBC spectra, for which the interferograms are frequently composed of many signals, those of HMQC and HSQC spectra constitute only one (due to the directly attached C). LP s up to sixteen-fold can be performed in these experiments (Sections 7-8a and 7-8b). 

Any NMR spectroscopic analysis of an organic sample will normally begin with the examination of a simple H-NMR spectrum, which serves to assess purity, concentration of minor components (if any), and overall complexity of the structures in the sample. Furthermore, the JH spectrum provides an opportunity to examine line shape characteristics of the sample s components, and, if necessary, reevaluate solvent choice, sample concentration, or acquisition temperature. If large quantities of pure compound are available, ID 13C-NMR spectra may also be useful. However, in most cases acquisition of a pair of (1H,13C)-HSQC and H,1 )-HMBC spectra will be a better use of spectrometer time, unless structural features are suspected that preclude full characterization by HSQC and HMBC. For example, compounds that feature quaternary carbon atoms that... 

In a mixture containing a number of complexes at the same time, for example, a catalyst along the reaction pathway, HMBC spectra offer an easy way for signal assignment in ID spectra of sufficiently sensitive nuclei. However, as conventional HMBC experiments take up to 2h, fast reactions are not observable by this technique. Hadamard NMR spectroscopy was described to allow such experiments in the minute scale [15], when chemical shifts in the indirect dimension (e.g., Pt) are identified from preliminary experiments. 

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