HMBC spectra

Heteronuclear Multiple-Bond Connectivity (HMBC) Spectra... 

Use of the HSQC and HMBC spectra to assign the compound and establish key connectivities in this way, unambiguously establishes the substitution positions of both groups but is a lot more labour-intensive than the NOE-based approach. 

The second drawback of the basic HMBC experiment is due to the actual range of /CH spin coupling constants, which vary from 1 to 50 Hz. Usually HMBC spectra are recorded with a compromise by setting the corresponding delay in the sequence to about 70 ms to allow an optimum transfer for correlation signals where the spin coupling constant is about 7 Hz. Under these conditions, several important connectivities may yield... 

Obviously, the one-step low-pass J filter produces the narrowest profile and is therefore the less efficient in removing direct correlations in HMBC spectra, while the five-step tuned low-pass J filter (f lps) offers the most broadband profile. For this filter, the intensity of the residual /ch signals remains below 0.11% for the range 125 < /ch < 225 Hz, which is remarkable. Interestingly, as long as the /ch couplings range of the molecule remains moderate (—60 Hz), the four- and five-step filters... 

The efficiency of the various low-pass J filters may be appreciated by considering the HMBC spectra of 1,3-butadiynyl (ferf-butyl) diphenylsi-lane (Figure 6), which provides a stringent test because of the extensive range of 1Jch coupling constants present. 

Partial plots extracted from HMBC spectra recorded with the five different low-pass J filters are shown in Figure 7. Clearly, ambiguities may arise, particularly if accidental degeneracy in chemical shifts causes true long-range correlations and residual 1Jch correlations to overlap. 

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. 

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.

HMBC spectra provide a wealth of correlations over multiple bonds, and while this on one hand is highly attractive, it poses the problem of distinguishing particularly between two- and three-bond correlations. Several methods have been proposed to this end, and will be briefly exposed in the following. 

As shown by the authors, the almost exclusivity of two-bond correlations in H2BC makes it possible to extract INADEQUA TE-type connectivity information by overlaying H2BC and HSQC spectra. Also, the complementarity of HSQC, H2BC, and HMBC spectra could be used for a semi-automated analysis of this package of spectra based on the H2BC-HMBC rule of thumb mentioned above and the unique one-bond correlations in HSQC. 

Figure 30 shows the potential of the 10-ppm approach. The full-width HMBC spectra (left) do not allow a clear discrimination of all cross-peaks. For instance, the correlation between the proton resonance of CH3fi2 MeLeu 9 at 0.82 ppm and the close pair of carbons at 22.1 and 22.2 ppm remains ambiguous and it is not possible to attribute the resonance of the CF c) MeLeu 9. In the 10-ppm spectrum, the achieved resolution clearly shows that this correlation can be unambiguously attributed, and that the exact resonance of the CHyh MeLeu 9 is 22.2 ppm. 

The simplicity of the logic for analyzing 10-ppm HMBC spectra is noticeable, but going back and forth between the high- and low-resolution spectra may be quite tedious whether they are printed on paper or... 

In practice, any available HMBC pulse sequence could be used to record fast-HMBC spectra.88 As HMBC are recorded without broadband heteronuclear decoupling, the duty cycle is no longer an issue with FAST-HMBC schemes, as it is using the standard SOFAST-HMQC or FAST-HMQC pulse sequences. 

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.

Figure 35 Two-dimensional HMBC spectra of isopropylidene glycerol recorded at 400 MHz with (A) the standard HMBC experiment, and (B) with the IMPACT-HMBC experiment. The spectra have been recorded using 128 increments, 2 transients, and a recovery delay of 0.2 s. The flip angle a was set to 130°. Measurement duration for both experiments approximately 2 min.

The CN-HMBC experiment concerted recording of 1H-13C and 1H-I5N HMBC spectra... 

Since about one decade, the development of fast experiments containing low number of RF pulses and delays to improve sensitivity and reduce the experimental time has been an active research area. It is likely that this area will involve many NMR spectroscopists for the next years. Current "fast methods" such as non-linear sampling, Hadamard and SMART techniques take a few seconds to produce 2D spectra. While these methods are currently rather mysterious for most of the scientists, there is absolutely no doubt that they will be provided by the NMR manufacturers in the next years. It is therefore likely that most of the HMBC spectra will be obtained in only a few minutes in a near future, which opens new perspectives. 

The 13C NMR spectra of primaquine diphosphate were obtained using a Bruker instrument operating at 75, 100, or 125 MHz. The sample was dissolved in DMSO-d6 and tetramethylsilane (TMS) was added to function as the internal standard. The 13C NMR spectra are shown in Fig. 9. The HSQC and HMBC spectra are shown in Figs. 10 and 11, respectively. The DEPT 90 and DEPT 135 are shown in Figs. 12 and 13, respectively. The assignments for the observed resonance bands associated with the various carbons are provided in Table 4. 

GC analyses of the pupal secretion of E. borealis have indicated the presence of vitamin E acetate and other tocopherol derivatives [49,50]. However, in tests with ants, these compounds proved to be essentially inactive, whereas the secretion itself was potently deterrent. To find and identify the active components in the pupal Epilachna borealis secretion, NMR spectroscopic studies on the fresh secretion were carried out. One and two-dimensional NMR experiments revealed that the tocopheryl acetates account for only a relatively small percentage of the beetles5 total secretion (20%), whereas the major components represented a group of previously undetected compounds. By analysis of the COSY, HSQC and HMBC spectra of the mixture, these components were shown to be esters and amides derived from three (co-l)-(2-hydroxyethylamino)alka-noic acids 44-46. HPLC analyses coupled to a mass spectrometric detector revealed that the secretion contain a highly diverse mixture of macrocyclic polyamines, the polyazamacrolides (PAMLs) 47-52 (Fig. 8). 

For the exo-regioisomers of 6e and f resulting from the Domino-Heck reaction were also identified from the HMBC spectra. In the spectrum of 6f, the interaction of the acetylene carbon (at 103.5 ppm) with the H proton (at 4.24 ppm) was obvious in the isomer 6e, an interaction between the phei rl gronp qnatemaiy carbon (at 139.0 ppm) with the H proton (at 4.40 ppm) was apparent, bnt inboth cases similar effects were not seen in the other isomers. 

This programme turns DQF-COSY and HMBC spectra into bond constraints. Then it turns C DEPT spectra and the molecular formula into building blocks such as -CH3 and -CH2-. These are then assembled into as many complete structures as are compatible with the bond constraints. CISOC-SES is designed to be as compatible with real-world spectra with their attendant ambiguities as possible. CISOC-SES is a result of collaborative work with Bodenhausen et al. who had previously tackled the problem indepen-dently. CISOC-SES has since been commercialized as NMR-SAMS by Spectrum Research, EEC. 

Figure 5.6 Comparative segments from (a) conventional and (b) phase-sensitive 8-Hz optimized HMBC spectra of DP-2 recorded using a 3-mm sample positioned coaxially in a 5-mm gradient inverse triple resonance Varian Cold-probe . ...

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