Proton-detected HETCOR

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. 

The HMBC spectrum (Chapter 6) is in effect a long-range (2/ch and 3/ch) HETCOR, but with proton detection for increased sensitivity (Chapter 6). Thus, it affords correlation for quaternary carbon atoms—which of course have no lJ correlation ... 

There are other two-dimensional techniques, more sensitive than HETCOR, that make use of polarization transfer (Section 4.12.2). Even greater enhancement can be obtained if the magnetization is generated at the insensitive nucleus and then transferred back to the sensitive nucleus for detection. Procedures making use of this principle are called inverse techniques and lead to a great reduction of sample concentration or measurement time. Typical experiments involve recording spectra for insensitive nuclei such as C, Si and N, which are recorded in inverse, proton-detected procedures. The information given by such experiments is the same as that from the HETCOR experiments, but the experiments are much more sensitive and are quicker to perform. 

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],... 

We saw in Section 12.1 that APT and DEPT provide straightforward methods for differentiating among CH3, CH2, CH, and quaternary carbons. This information is almost essential in interpreting the 13C spectrum, and in some instances it can best be obtained with APT or DEPT, where 13C is observed, especially when the proton spectrum is crowded. If HETCOR and/or indirect detection of 13C is needed, these one-dimensional editing techniques provide somewhat redundant information but may still be helpful. 

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. 

Another very important experiment for spectral editing is a heterocorrelation experiment combining MQMAS (in the indirect dimension) with CP. Introducing CP at the MQMAS echo position following a split-fj MQMAS scheme creates a spin-j detected spectra (in the dimension), which is modulated by the isotropic frequency of the quadrupolar nucleus in F. HETCOR has been demonstrated on Na- P pairs, on 2 A1- P pairs and lately on a- H pairs,where proton resolution during F was obtained with the wPMLGS multiple pulse decoupling technique. ... 

As discussed later, the application of the SEMA sequence is not limited to the 2D PISEMA experiment, but it can be used to design a variety of sophisticated pulse sequences. For example, the selective transfer of magnetization between strongly coupled heteronuclei by SEMA is the key feature of the sequence that can be utilized in experiments such as HETCOR, H-detected experiments," and experiments to selectively measure relaxation parameters of a proton bonded to S nuclei. 

For heteronuclear correlations, HETCOR historically was the standard experiment used to detect proton-carbon scalar couplings. 127-129 disad-... 

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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