D-HMBC

Decoupled HMBC (D-HMBC), an Improved Version of HMBC... 

For D-HMBC experiments, we usually omit the low pass J-filter (the first 90° pulse for C nucleus in the HMBC pulse sequence) aiming to suppress the cross peaks due to the direct Jc-h correlation, but it can be implemented if desirable. In the D-HMBC spectra, the cross peaks between directly bonded C and H do not, in most cases, hinder the easy analysis of the spectra, because these cross peaks appear as singlets. On the contrary, these peaks even contribute to easy NMR spectral analysis when HMQC spectral data are not in hand. 

Figure 2 compares the HMBC and D-HMBC spectra of an antibiotic monazomycin [10]. It is seen that several cross peaks, which cannot be ob-... 

Fig. 3. Comparison of F and Fj slices between HMBC (a) and D-HMBC (b) spectra of monazomycin. These spectra were prepared at the same...

This improvement is clearly seen in the Fi and F2 slices of the same D-HMBC spectrum shown in fig. 3. The intensities of both the H-and C-signals in the D-HMBC spectrum are approximately twice as strong as those in the conventional HMBC spectrum. In addition, the resolution is improved in the D-HMBC spectrum. Note that the splitting of the proton signals in the Fz axis is caused by H- H spin coupling and not by H- C coupling. 

Fig. 4. Long range couplings observed in the D-HMBC spectra of promothiocin B. A 120 ms for dotted arrows and A = 500 ms for arrows.

The D-HMBC pulse sequence can also be used in combination with the pulse field gradient (PFG) technique [12]. Figure 5(c) shows the successful observation of cross peaks between the methyl group at C-5 of an oxazole unit and adjacent carbons in promothiocin. These cross peaks are hidden by the strong t noise of the solvent peak in the HMBC and D-HMBC spectra. The above results clearly indicate that D-HMBC is a quite useful technique for structural studies of complicated natural products. 

Fig. 5. Comparison of HMBC, D-HMBC and Field Gradient D-HMBC spectra of promoth-iocin B showing long range C- H couplings between methyl protons and relevant carbons of two oxazole units. Delay time was set to 500 ms. The sample (20 mg) was dissolved in...

It may be necessary to mention the relationship between PFG and D-HMBC. PFG is an especially suitable method to observe HMBC spectra with suppressed ti noise, and enables to detect very weak cross peaks which could... 

It may be useful to make some explanations on data processing to prepare good D-HMBC spectral data. Sine-bell window is usually employed for processing of HMBC data to give power-mode spectra as shown in fig. 6(a), because they consist of absorption-mode cosine) and dispersion mode sine) signals for both the t and t2 axes. This procedure causes a considerable loss of signal to noise ratio when cross peaks appear as broad ones and when digital resolution is poor as used for ordinary HMBC measurement. 

This half-absorption mode is also employed for data processing of the D-HMBC spectra with a modification that t2 and t axes data are treated in... 

Figure 7 demonstrates the effects of window functions on the signal to noise ratio of the 5r,r spectrum (for D-HMBC) and 5r,i spectrum. 5r,r spectra shown in fig. 7(c) revealed marked decrease of -noise in the range between 0.8 to 1.1 ppm. Processing of S r r spectral data, however, requires somewhat tedious phase adjustment of the Fi spectrum. In order to achieve this easily, it is recommended to take a ID-NMR spectrum at first under... 

The H- C 2-D HMBC spectra of aromatic and conjugated compounds often allow us to distinguish between C signals that are cis versus trans to a H three bonds away Because the entire network of atoms in an aromatic or conjugated system is often planar (at least locally, barring steric constraints), there are only two vicinal bond angles from which to choose (0° and 180°). Thus, there are only two values likely to be encountered between a H on an sp -hybridized carbon and a C three bonds away the cis- I is smaller (5-6 Hz) than the trans- ] (8-10 Hz). Consequendy, the cis- J generates a weaker HMBC cross peak than the trans- ] does. 

The simplest method for distinguishing between the nonprotonated C resonances in 6-ethyl-3-formylchromone is to examine the structure (Figure 9.6) and identify the carbon sites that are expected to show trans icii s. C3 is not expected to show any trans Ys to H s. C6 is trans to H8. C9 is trans to H2, H5, and H7. CIO is trans to H8. On the basis of these geometries, we therefore expect C9 to show three cross peaks in the 2-D HMBC spectrum to H s on sp -... 

I FIGURE 9.8 The 2-D HMBC NMRspectrum of6-ethyl-3-formylchromone in CDCIj,... 

I FIGURE U.l.hmbc The 2-D HMBC NMR spectrum of a-terpinene in CDCI3. 

I FIGURE MA.hmbc The 2-D HMBC NMRspectrum of longifolene in CDCI3. 

I FIGURE H.W.ghmbc The 2-D HMBC NMRspectmm of unknown 14.10 in CDCI3. 

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