Heteronuclear multiple-bond correlations

HMBC/GHMBC spectra are Jcu and Jch with long-range correlations observed only occasionally. In the specific case of strychnine, four couplings are typically observed in the 10 Hz optimized GHMBC spectrum. 

Structure elucidation strategies that employ long-range heteronuclear shift correlation experiments generally use the correlations observed to position quaternary atoms relative to protonated carbon fragments, e. g. in the case of a correlation from H-12 to the C-8 quaternary carbon, or to bridge heteroatoms, e.g. the correlation from H-12 to C-23 across the oxepin ether linkage, as shown above. 

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HMBC Heteronuclear multiple bond correlation, inverse CH correlation via long-range CH coupling, same format and information as described for ( C detected) CH COLOC but much more sensitive (therefore less time-consuming) because of H detection... 

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.)...

C-NMR, COSY, HMQC (heteronuclear multiple quantum coherence), and HMBC (heteronuclear multiple bond correlation).48 Furthermore, the structure of trimer was confirmed by X-ray crystallography.48 The incorporation of 13C into the indole 3a position proved valuable in these structural determinations and in documenting the ene-imine intermediate. For example, the presence of a trimer was readily determined from its 13C-NMR spectrum (Fig. 7.7). 

To be fair, we must point out that this type of experiment is extremely sensitive to the parameters chosen. Various pulse sequences are available, including the original COLOC (Correlation by means of Long range Coupling) as well as experiments variously referred to as HMBC (Heteronuclear Multiple-Bond Correlation) and HMQC (Heteronuclear Multiple-Quantum Correlation). Depending on the parameters chosen, it is often not possible to suppress correlations due to one-bond coupling ... 

W. Schoefberger, J. Schlagnitweit and N. Muller, Recent Developments in Heteronuclear Multiple-Bond Correlation Experiments, in Annual Reports on NMR Spectroscopy, G. A. Webb (ed.), Vol. 72, Academic Press, 2011, pp. 1-60. 

In the case of the 7-hydroxy-substituted compounds 44 (Scheme 2), 54 different derivatives were investigated by 13C NMR spectroscopy and, in some cases, also by 1SN NMR spectroscopy <1995JST(335)273>. With the help of proton-coupled 13C NMR spectra, semi-selective INEPT (insensitive nuclei enhanced by polarization transfer) experiments, and heteronuclear multiple bond correlation (HMBC) two-dimensional 2D-NMR spectra, all shifts could be unequivocally assigned. While the C-7 shifts did not allow the existing tautomeric situation to be determined, a clear decision could be made by H NMR spectroscopy in this respect. The 1SN NMR spectra revealed an equilibrium between the N(4)H and N(3)H tautomeric forms, which is fast on the NMR timescale. 

In order to assign the chemical shifts of the carbon atoms of the conjugated diene system of each CLA isomer, it was necessary to conduct INADEQUATE, HMBC (heteronuclear multiple bond correlation) and two-dimensional 1H-13C correlation spectroscopy (COSY) techniques on the carbon signals of the diene system of the ,Z-isomers. The results of these experiments for the CLA isomers are summarized in Table 13. 

HMBC Heteronuclear multiple-bond correlation spectroscopy To elucidate structure of organic molecules To establish long-range (i.e., multibond) heteronuclear coupling... 

Note. NMR analyses revealed that the major component of the tripeptide product synthesized from Bz-Arg-OEt and Gly-Asp-(NH2)2 was Bz-RGD(-NH2)-OH rather than Bz-RGD-(NH2)2- The primary amide group adjacent to the secondary carbon centre is hydrolysed during the reaction, whereas the primary amide adjacent to the primary carbon centre remains intact, as determined by heteronuclear multiple bond correlation. ... 

Heteronuclear multiple bond correlation (HMBC) [1, 2] is one of the most powerful NMR techniques and is now being widely used for structural analysis of complicated molecules. Since it detects long range... 

There are a number of techniques stemming from the basic two-dimensional technique, which for example allow correlation between carbon atoms and the protons attached to them and correlations of carbon atoms with protons one or two bonds removed from them, heteronuclear multiple bond correlation (HMBC). 

NMR experiments. Heteronuclear NMR experiments are represented by H C HSQC (Figure 2.1) and heteronuclear multiple bond correlation HMBC) (Figure 2.2)... 

FIGURE 2.2 heteronuclear multiple bond correlation spectrum of 8-methoxykaempferol... 

Hadden, C.E., Martin, G.E., and Krishnamurthy, V.V., Improved performance accordion heteronuclear multiple-bond correlation spectroscopy — IMPEACH-MBC, J. Magn. Reson., 140, 274, 1999. 

Product elucidation of trimethyloxonium tetrafluoroborate and methyl iodide reactions with [l,2,3]triazolo[4,5- -pyridazines (Schemes 1-3) using mainly 2-D heteronuclear multiple bond correlation (HMBC). NMR... 

The structural assignment of both 29 and 30 was accomplished through extensive two-dimensional (2-D) NMR heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple bond correlation (HMBC) spectroscopic studies <2004T8189>. In the HMBC spectrum of 29, the proton at 8.64p.p.m. shows a strong correlation Jq-h with the carbonyl carbon (C-10) at 180.9 ppm and the proton at 8.82p.p.m. with the carbonyl carbon (C-5) at 181.7 ppm. The HMBC spectrum of 30 shows a significant strong correlation Vq h of the C-5 carbonyl carbon with the H-6 proton at 8.52 ppm and the H-4 proton at 8.52p.p.m. 

Complete N NMR chemical shifts together with H- N coupling constants (referenced externally to NH4C1 in DMSO-1 6 DMSO - dimethyl sulfoxide) have been determined for toxoflavin 7 and fervenulin 8 in CDCI3 using pulsed field gradient heteronuclear multiple bond correlation (HMBC) techniques <2000H(52)811>, and are presented in Table 1. 

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