Proton-Detected HETCOR HMQC

Historically, proton-detected H—13C correlation experiments, in which only directly attached proton-carbon coupling is observed, have been fraught with experimental difficulties and therefore have lagged behind carbon-detected experiments. These problems have been overcome and the proton-detected version (also called inverse detected ) is now routine the name commonly associated with this experiment is HMQC 

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Applications Useful 2D NMR experiments for identification of surfactants are homonuclear proton correlation (COSY, TOCSY) and heteronuclear proton-carbon correlation (HETCOR, HMQC) spectroscopy [200,201]. 2D NMR experiments employing proton detection can be performed in 5 to 20 min for surfactant solutions of more than 50 mM. Van Gorkum and Jensen [238] have described several 2D NMR techniques that are often used for identification and quantification of anionic surfactants. The resonance frequencies of spin-coupled nuclei are correlated and hence give detailed information on the structure of organic molecules. 

For smaller quantities of compounds more sensitive inverse detected techniques are available, such as HMQC ( IH-I C one bond correlation via heteronuclear multiple quantum coherence, analogous to HETCOR) and HMBC (proton detected heteronuclear multiple bond correlation spectroscopy) (15). The last provide, in addition to the intraresidue multiple bond correlations, interresidue correlations between the anomeric carbon and the aglycone protons.We follow this general strategy for the structural determination of tri terpenoid saponins of Bupleurum fruticosum (16) andArdisia japonica (9). 

The HMQC spectrum is a proton-detected (inverse detection) HETCOR (Chapter 6) it shows lJ CH coupling. Table 5.9 in Chapter 5 allows us to arrange the aromatic unsubstituled carbon atoms as C-6, C-4, C-3 from left to right. The HMQC spectrum confirms the same sequence for H-6, H-4, H-3. The aromatic, unsubstituted carbon atoms can now be correlated with the firmly assigned aliphatic protons. The substituted aromatic carbon atoms cannot yet be assigned. 

Heteronuclear single-quantum correlation (HSQC) permits to obtain a 2D heteronuclear chemical shift correlation map between directly bonded H and X-heteronuclei (commonly and N). It is widely used because it is based on proton-detection, offering high sensitivity when compared with the conventional carbon-detected 2D HETCOR experiment. Similar results are obtained using the 2D HMQC experiment [77],... 

Because of the sensitivity gain from the indirect detection experiments, either HMQC or HSQC is usually the method of choice for establishing correlations between protons and 13C (as well as other heteronuclei, such as, 5N or 31P). HET-COR, the heteronuclear analog of COSY, must be used when the available (somewhat older) instrument does not have good indirect detection capability, but use of HETCOR is declining. 

The two principal H-detected, direct, heteronuclear chemical-shift correlation experiments are HMQC and HSQC. The X-nucleus-detected counterpart is HETCOR, The and X-nucleus spectral widths are reduced in each of these experiments. It is important to remember that the latter should be decreased to contain only the signals of protonated X nuclei. Quaternary carbons, for example, do not participate in these experiments, and their signals should not be included in the reduced spectral windows. 

Heteronuclear Correlation Experiments A comparison of HMQC, HSQC, and the analogous gradient experiments used to correlate one bond proton and carbon chemical shifts ( Jch) showed little differences. In all cases the signal/noise for the backbone CH2 of a methacrylate copolymer was identical within experimental error. As expected, the C detected heteronuclear detection experiment (HETCOR) showed significantly (-- 4 fold) lower s/n. 

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