Heteronuclear shift-correlation determination

In homonuclear-shift-correlated experiments, the Ft domain corresponds to the nucleus under observation in heteronuclear-shift-correlated experiments. Ft relates to the unobserved or decoupled nucleus. It is therefore necessary to set the spectral width SW, after considering the ID spectrum of the nucleus corresponding to the Ft domain. In 2D /-resolved spectra, the value of SW depends on the magnitude of the coupling constants and the type of experiment. In both homonuclear and heteronuclear experiments, the size of the largest multiplet structure, in hertz, determines... 

In homonudear shift-correlation experiments like COSY we were concerned with the correlation of chemical shifts between nuclei of the same nuclear species, e.g., H with H. In heteronuclear shift-correlation experiments, however, the chemical shifts of nuclei belonging to different nuclear species are determined (e.g., H with C). These may be one-bond chemical shift correlations, e.g., between directly bound H and C nuclei, or they may be long-range chemical shift correlations, in which the interactions... 

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 HMBC spectrum of vasicinone displays long-range heteronuclear shift correlations between the various H/ C nuclei. These correlations are very helpful to determine the C-NMR chemical shifts of quaternary carbons and allow the interlinking of the different substructures obtained. 

Fig. 2.61. Homo- and heteronuclear shift correlations for the determination of the bonding network (the connectivities ).

For the determination of the sequence and substitution positions of the different monosaccharides of a saponin, a series of proton and carbon 2D-NMR techniques can provide valuable information about the usually crowded regions of the conventional ID spectra. Thus, integrated approaches including ID or 2D H- H homonuclear NMR shift-correlation experiments (DQF- and TQF-COSY, TOCSY or HOHAHA, NOESY, ROESY or CAMELSPIN), and H-detected H-,3C heteronuclear shift correlation (HMQC, HMBC) have proved to be extremely useful. 

There are a number of reports on the synthesis of the saturated ring system, and NMR spectra were used to determine the structure. In some cases, the stereochemistry has been established by spectroscopic analysis <90JOC2254>. Some 0x0 derivatives, such as (97), have been studied by attached proton test (APT) and heteronuclear shift correlation (HETCOR) experiments <9lJOC858>. Pyrrolidine ring conformation in two 2,3-dioxoperhydropyrrolo[l,2-c]imidazoles were analyzed from H NMR data using a generalized Karplus equation <85BSB187>. 

Figure 5.78. A 2D-heteronuclear relayed coherence transfer (RCT) spectrum of 2-acetonaphthalene. The cross peaks marked with an asterisk are due to nonrelayed magnetization, as determined by a normal heteronuclear shift correlated experiment.

D heteronuclear shift correlation (HETCOR) experiments. This method is helpful also in determining small long-range coupling constants (e.g. 4j(ii7/ii9gn,iH)), which are normally not resolved in ID H NMR spectra. 

Modern high-field H NMR techniques (correlated spectroscopy (COSY), heteronuclear chemical shift correlation (HETCOR), nuclear Overhauser enhancement (NOE), etc.), which generally permit determination of the chemical shifts and coupling constants of all protons (and connectivities between certain groups), have greatly simplified the structural determination of organic natural products (e.g., 231-235). This has certainly been the case in the field of sarpagine alkaloids. 

Due to the great complexity of this class of molecules, nuclear magnetic resonance (NMR) and mass spectroscopy (MS) are the tools most widely used to identify cucurbitacins. Both one- and two-dimensional NMR techniques have been employed for the structural elucidation of new compounds 2D NMR, 1H-NMR, 13C-NMR, correlated spectroscopy (COSY), heteronuclear chemical shift correlation (HETCOR), attached proton test (APT), distortionless enhancement by polarization transfer (DEPT), and nuclear Overhauser effect spectroscopy (NOESY) are common techniques for determining the proton and carbon chemical shifts, constants, connectivity, stereochemistry, and chirality of these compounds [1,38,45-47]. 

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