The 37-HMBC Experiment

One of the problems with HMBC involves the suppression of the signals of the protons that are directly bonded to C. This interfering H- C magnetization can be dealt with in the following way. A C 90 pulse (known as a y-filter) is inserted into the pulse sequence at a time equal to (2 /ch) tfter the initial H 90° pulse (i.e., during the longer range delay 

Choosing between HMBC and gHMBC may be approached as follows. If a sample is fairly concentrated and displays strong signals in its H NMR spectrum, then ti noise ridges 

Since the time A of HMBC is so much larger than that of either HMQC or HSQC ( 60 ms vs. 3.6 ms), the refocusing delays with accompanying C decoupling of the latter experiments are not used, for reasons of sensitivity. Too much signal would be lost through relaxation processes in the approximately 60 ms that it would take the antiphase H vectors to refocus prior to detection (Section 6-2). 

Because of the uncertainty concerning both the location and intensity of correlations in HMBC contour plots, cross sections should be taken through individual chemical shifts on both the H and C axes. 

steady-state scans = 32 (with gradients) or 8 (for nongradient versions) 

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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. 

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 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). 

To improve the spectral quality the unwanted H signals of the C isotopomers may be further reduced by using field gradients which will allow the proper selection of the coherence pathways in conjunction with the HMBC experiment... 

Table 5.3 Data sets obtained with the HMBC experiments with...

Figure 15.1. (A) COSY, (B) TOCSY, (C) 1H-1T HSQC or HMQC, (D) dl- Y HMBC, for 4-oxopentanal. For clarity, only key assignments have been given as an example. Note that the double-ended arrows indicate how to interpret the spectra. In the case of COSY and TOCSY the information is represented as cross-peaks that are symmetrically oriented with respect to the central diagonal. In the single-bond correlation (HSQC/HMQC) a cross-peak represents in one dimension the carbon chemical shift and in the other dimension the proton chemical shift. Note there is no diagonal in heteronuclear NMR experiments. In the HMBC, lines are drawn vertically to connect the cross-peaks. In HMBC 2-4 bonds, H-13C correlations are often observed. Note that the 4-bond correlation is less common in NMR but has been included here as an example, and 1-bond correlation is commonly filtered from the HMBC experiment to improve detection limits for the weaker 2-4 bond correlations.

There is a carbon detected analogue of the HMBC experiment called COLOC (Correlated spectroscopy for Long range Couplings) that predated the experiment treated here. The COLOC is not used much any more and we will not give any examples. 

The analysis of the HMBC in the region of the carbonyl carbons is hampered by the lack of digital resolution along the FI axis. Recall that the HMBC experiment is proton detected giving good resolution in the proton or F2 dimension. The only way to improve resolution along the FI axis is to increase the number of FIDs in the experiment, which has serious practical limitations. The lines drawn in the insets help clarify the correlations. 

The HMBC experiment is just an HMQC experiment with the 1/(2J) delay set for a J value of about 10 Hz (typical for two- and three-bond 7ch) rather than 150 Hz (typical... 

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