H HMBC Experiment

HMBC [5.202], J-HMBC [5.203], TANGO-SL-HMBC [5.204], CT-HMBC [5.171], GSQMBC [5.201], ACCORD-HMBC [5.172], IMPEACH-HMBC and CIGAR-HMBC [5.173, 5.174, 5.175] 

With respect to the pulse sequence layout, the HMBC experiment is essentially a HMQC experiment incorporating a low-pass filter to suppress the one-bond correlation peaks. The low-pass filter, consists of a delay d2 = 1/(2 U(C, H)) and a 90° pulse, which transfers the U(C, H) coherence into a multiple quantum state. In a second period coherences which are generated by JCC, H) evolution are also transferred to a multiple quantum state by a 90° l C pulse but with a different phase in relation to the first 90° pulse of the low-pass filter. A combination of appropriate receiver phase cycling and pulse phase cycling enables the exclusive detection of J(C, H) correlation peaks in the 2D experiment. 

Open the configuration file ch5731.cfg. Create the HMBC pulse sequence as shown in the scheme below saving the new sequence files with the name myhmbc2.seq. Check the new pulse sequence using the test spin system chlongrg.ham (calculation time approximately 3 minutes). 

Gradient selection can be implemented in the HMBC experiment in an analogous manner to the HMQC experiment. 

Load the configuration file ch5732.cfg and run a simulation. Inspect the spectrum for residual 1 J(C, H) correlation peaks and compare their intensity with the same peaks of the phase cycled HMBC experiment of Check it 

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H. HMBC experiment performed on the derivatized saponin (256 experiments of 2K, reverse mode without decoupling in the carbon dimension experiment is not phased). Part of the map corresponding to the low field resonances (sugars) is shown below. The observed correlations allow sequencing of the chains of sugars. 

Figure 2.17. HC HMBC experiment of a-pinene [ CDCb, 5 % v/v, 25 °C, 125 MHz for C, 500 MHz for H, 16 scans, 256 experiments, contour plot]. This experiment gives the same information as Fig. 2.16 within 24 min instead of 8 hrs required for the CH-COLOC in Fig. 2.16 due to higher sensitivity because of proton detection and stronger magnetic field. Deviations of proton shifts from those in Fig. 2.16 arise from the change of the solvent. The methylene protons collapsing in Fig. 2.16 at Sh =2.19 (200 MHz) display in this experiment an AB system with Sa = 2.17 and Sb = 2.21 (500 MHz)...

Proton resonances for aU residues were assigned using a combination of COSY, TOCSY, HSQC, and HMBC experiments. The large values of y(NH/H-C(/9)) and the small values of J(H-C(a)/H-C(y9) were indicative of antiperiplanar and synclinal arrangements respectively, around those bonds. In addition, medium-range NOE connectivities H-C(/ )i/NH +i, H-C(a)i/NH, + i, NHi/NH +i were consistent... 

The HMBC spectrum of vasicinone along with the H-NMR assignments are shown. Determine the H/ C long-range heteronuclear shift correlations based on the HMBC experiment, and explain how HMBC correlations are useful in chemical shift assignments of nonprotonated quaternary carbons. 

The H/ C long-range coupling information obtained from the inverse HMBC experiment (Fig. 8.9) allowed the various fragments to be connected together. The proton at 5 3.65 (C-31aH) showed /ch interaction with C-4 (8 45.1) and ch interactions vdth C-3 (8 56.1) and C-5 (8 53.6). The C-31/3 proton (8 3.70) showed /ch interaction with... 

From a sensitivity standpoint, the h,1-ADEQUATE experiment, at least in the author s experience, ranges from two to four times less sensitive than the l,n-ADEQUATE experiment. This difference in relative sensitivity reflects the relative difference in sensitivity between an HSQC and HMBC experiments with which 1, n- and n,l-ADEQUATE begin, respectively. 

Figure 12 Comparison of Vch artefacts intensity illustrated with ID rows taken from a BIRD-HMBC (A), (D) and (G) a G-BIRD-HMBC (B), (E) and (H) and a double tuned G-BIRD-HMBC (C), (F) and (I) experiments showing the Vch artefacts and nJCH responses of C-6 at 135.6 ppm (A), (B) and (C), C-l at 67.2 ppm (D), (E) and (F) and C-10 at 27 ppm (G), (H) and (I) of the 1,3-butadiynyl (tert-butyl) diphenylsilane molecule dissolved in CDCl3. For the BIRD-HMBC and G-BIRD-HMBC experiments, the delays S were adjusted to aV-value of 190 Hz, as an average value for the extreme range of coupling constants for this molecule (125-260 Hz). For the double tuned G-BIRD-HMBC, the /ch nnax and /ch nnin values were set to 240 and 145 Hz, respectively. The corresponding values for the S and S delays were 3.13 and 2.17 ms, adjusted toj values of 160 and 230 Hz, respectively. For both G-BIRD-HMBC experiments, 192 is BIP 720-100-10 pulses have been used for 13C inversion. The same vertical scale is used for all spectra. Residual /ch signals are denoted with arrows.

Figure 13 Timing diagram for the clean HMBC experiment with an initial second-order and terminal adiabatic low-pass 7-filter.42,43 The recommended delays for the filters are the same than for a third-order low-pass J filter. <5 and 8 are gradient delays, where 8 — <5 + accounts for the delay of the first point in the 13C dimension. The integral over each gradient pulse G, is H/2yc times the integral over gradient G2 in order to achieve coherence selection. The recommended phase cycle is c/)n = x, x, x, x 3 — 4(x), 4(y), 4( x), 4(—y) with the receiver phase c/)REC = x, x.

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.

To this end, Ilin et al. have developed the T-HMBC experiment,120 which is itself based on an experiment developed by Vincent and Zwahlen for measuring dipole-dipole cross-correlation in polysaccharides.121 These experiments allow determining the conformation around glycosidic bonds based on 3Jch couplings and C-H-dipolar cross-correlated relaxation. 

FIGURE 27. Partial structures of viridenomycin. (a) Data from H- H COSY experiment (b), (c) The solid line arrows indicate H-13C long-range coupling detected by HMBC... 

The position of the functional groups along the eremophilane core was confirmed by an HMBC experiment. In particular, the correlations C-lO/OH-1, H-6, H-9 C-8/H-9, H-6, H-13 C-2/OH-1, H-4 and C-12/H-13 were consistent with the disposition of the ketone, hydroxyl and lactone functionalities. 

S = 0.0 ppm)) of oximes 44-51 measured for solutions in DMSO-d based on -gradient selected H, N HMBC experiments"... 

The UV spectrum [/Imax 209, 230 (sh), 242 (sh), 275 (sh), 306, 409, and 482 nm] of malasseziazole B (391) was similar to that of malasseziazole A (390), indicating the presence of a similar indolo[3,2-f>]carbazole framework. The H-NMR spectrum of malasseziazole B (391) was also very similar to that of malasseziazole A (390), except for the absence of the C-12 singlet proton at 8.40, along with the presence of an aldehyde proton at 5 11.55. This aldehyde proton showed correlations with the C-1 la, C-12, and C-12a carbons in the HMBC experiment. Based on these spectral analyses and the structural similarity to malasseziazole A (390), the structure 391 was assigned to malasseziazole B (358) (Scheme 2.103). 

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. 

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