The HMBC sequence

Long-range correlations based on the HSQC sequence are generally less effective. Significant evolution of H- H couplings during the A period leads to unwanted COSY- 

Without any doubt, the greatest problem associated with the HMBC sequence lies in the suppression of the parent signals which may 

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The next step in the HMBC sequence is the t evolution period, during which the magnetization of interest is labelled with the 13C chemical shift information. 

In the context of standard proton detected H- Sn correlation experiments one usually aims at addressing V(" Sn, H) coupling correlations simultaneously because they are widely spread in value. This makes the distinction between HMQC and HMBC nomenclature meaningless. Since for H- Sn correlation experiments, a low-pass filter is therefore no longer needed, we believe the nomenclature HMQC to be more appropriate as, historically, the HMBC sequence was the one in which the low-pass filter was introduced. This is understandable if it is remembered that the latter was designed specifically... 

Figure 6.26. The HMBC sequence (a) without and (b) with incoiporation of pulsed field gradients. The sequence is closely relate to the HMQC experiment and follows a similar coherence transfer pathway.

Figure 6.27. The low-pass filter for removing spurious one-bond correlation peaks from HMBC spectra. This comprises an additional A1 period and 90° X pulse at the beginning of the HMBC sequence. Ai is set to 1/ Jch and A2 to l/"JcH- Inverting the phase of the first X pulse on alternate scans without inverting the receiver phase cancels unwanted one-bond contributions.

Figure 7.14 Pulse sequence for the HMBCS (heteronuclear multiple-bond correlation, selective) experiment, which uses advantageously a 270° Gaussian pulse for exciting the carbonyl resonances. It is also called the semisoft inverse COLOC. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Recent results have been summarized in a number of articles, especially in this journal.17-19 This review will focus on newly developed HMBC pulse sequences and application on small to medium-sized organic molecules. After a short introduction into basic theory, a selection of pulse sequences and a look at the large variety of applications will complete the overview of the HMBC experiment. Finally, a brief summary and outlook to future perspectives of HMBC will be given. 

The HMBC spectrum recorded on a sample of 50 mmol cyclosporine A (Figure 3) dissolved in CgD6 using the pulse sequence depicted in Figure 1 is shown in Figure 2. 

Figure 15 HMBC and broadband HMBC spectra of cyclosporine in C6D6 recorded with the pulse sequence shown in Figure 14. (A) HMBC spectrum recorded with A = 65.0 ms and 32 scans. (B) HMBC spectrum where two subspectra of 16 scans each recorded with A = 65.0 ms and 120 ms, and co-added in absolute-value mode. (C) broadband HMBC spectrum where four subspectra of eight scans each were recorded with A = 96.7, 84.4, 81.8, and 80.8 ms, respectively, and co-added in absolute-value mode.

Figure 16 u/tro-HMBC spectrum of cyclosporine in C6D6 recorded with the pulse sequence shown in Figure 14 where four subspectra of eight scans each were recorded with A = 181.1,160.0,115.0, and 99.3 ms, respectively, and co-added in absolute-value mode. 

While the 2/,3/-HMBC experiment, also known as STAR-HMBC, has undeniably its merits, it also suffers from a severe sensitivity penalty which results from the extended pulse sequence and additional delays, pulses and gradients as compared to the standard HMBC sequence. For instance, the 2/,3/-HMBC spectrum shown in Figure 21 has been recorded using 64 scans, for a total experimental time of 184 min, while for obtaining approximately the same signal-to-noise ratio, the corresponding HMBC spectrum could be recorded with only 8 scans and 23 min. 

Figure 22 Pulse sequence of the HMBC-RELAY experiment. Filled and open bars represent 90° and 180° pulses, respectively. All other phases are set as x, excepted otherwise stated. A two-phase cycle x, —x is used for the pulse phases (j>, and

B) up-down HMBC pulse sequence inverting BCH and 13CH3 peaks relative to the standard sequence. (C) up-down + HMBC pulse sequence inverting 13CH and 13CH3 peaks relative to the standard sequence and in the opposite sense to the up-down sequence. The data of the different pulse sequences are recorded in an interleaved manner. After formation of the required two linear combinations in the time domain, the data are processed in the same way as other HMBC-type data. 

Figure 27 Edited broadband HMBC spectrum of cyclosporine using the pulse sequences shown in Figure 26 in an interleaved manner. The two subspectra, CH + CH3 (left) and C + CH2 (right), exemplify the editing properties. The spectrum in the bottom displays the two subspectra, CH + CH3 (black) and C + CH2 (grey) in the same frame. The number of scans was 32 for each of the 128fi increments, the relaxation delay was 1 s, and the range for the third-order low-pass. /-filter was 115 Hz < Vch < 165 Hz. The spectra were processed to maintain the absorptive profiles in F, while a magnitude mode was done in F2.

For those purposes, the authors used constant-time version of the sensitivity-enhanced HMBC sequence,79 combined with a two-step low-pass J filter. Constant-time experiments have no coupling structures in the carbon dimension making it easy to identify the centre of signals in... 

Recently, a robust, sensitive, and versatile HMBC experiment for rapid structure elucidation has been proposed. The suggested IMPACT-HMBC experiment eliminates the weaknesses of the basic HMBC experiment and the overall performance of the pulse sequence is improved significantly. In addition, it can be recorded with short recovery times, which is useful in routine analysis by NMR when the experimental time is limited. 

Figure 34 Excerpts of two-dimensional HMBC spectra of cholesteryl acetate recorded on a Bruker Avancell 400 MHz spectrometer (A) with the standard HMBC pulse sequence (Figure 1), and (B) with the IMPACT-HMBC experiment depicted in Figure 30. The same contour levels are used for all spectra. In (A), F, ridges are still visible (indicated by a vertical arrow), while they are very efficiently suppressed in (B). The proposed sequence results in signals with no coupling structure, as a result of the incorporation of a constant-time period. The improved peak dispersion is shown for the correlation between C-3 and H-2 (expanded in the small boxes). Asterix and the dashed box indicate residual Vch signals. The measurement duration was 22 min for both experiments.

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. 

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