The HETCOR Experiment

Like COSY, HETCOR is a relatively robust sequence. Unlike COSY, it can be performed reasonably in either the absolute-value or phase-sensitive mode, although the latter gives better resolution. Since it is relatively immune to artifacts (if pulsing is not too rapid), a gradient version is largely unnecessary. Because HETCOR is a polarization transfer experiment, the relaxation delay times are a function of the H-, and not the X-nucleus, Tj s. 

There are two important delay times in the HETCOR experiment. Aj governs the defo-1 1 cusing of the C-bonded vectors and is set in the same compromise manner that we have 

The following parameters are appropriate for absolute-value HETCOR experiments  

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

pseudo-echo, sine bell, or squared sine bell weighting (all left shifted to improve sensitivity) 

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Pulse Sequence. The basic pulse sequence for the HETCOR experiment can be considered as a derivative of the pulse sequence used for the CP experiments (Figure 46). The main difference between the CP and HETCOR pulse... 

Figure 6-20 The pulse sequence for the HETCOR experiment with decoupling.

Figure 6-21 illustrates several advantages of the HETCOR experiment (1) The correlation between protons and carbons means that spectral assignment at one frequency... 

As an alternative method to achieve coherence transfer, the DEPT element can be implemented in the HETCOR experiment. This modification makes the HETCOR experiment a multiplicity selective method. 

The HETCOR experiment correlates l3C nuclei with directly attached (i.e., coupled) protons these are one-... 

We can thus assign carbon atoms on the basis of known proton chemical shifts, or we can assign protons on the basis of known carbon chemical shifts. For example, we might have a crowded proton spec-tmm but a carbon spectrum that is well resolved (or vice versa). This approach makes the HETCOR experiment particularly useful in the interpretation of the spectra of large, complex molecules. An even more powerful technique is to use results from both the HETCOR and COSY experiments together. 

What information does the HETCOR experiment provide Explain how you interpret a HETCOR plot. 

The latter technique unfortunately has two substantial disadvantages, i) significant amounts of material (at least 30 mg) are normally required, which may be impossible for a new natural product, and ii) the assignment of carbon signals with close (5 0.2 ppm) chemical shifts may be difficult because the digital resolution required cannot normally be achieved in a hetcor experiment. In addition, unless the hetcor experiment can be run under conditions where the J value emphasized is 4 - 8 Hz, no information concerning the assignment of quaternary carbons is possible. 

In the HETCOR experiment the peaks of an insensitive nucleus ( C. " N) are correlated with those of a sensitive nucleus ( H, F, " P). In Figure 27 the aliphatic part of the HETCOR spectrum of camphor (2) shows the specific resonances of the protons which are attached to each C nucleus. The relevant parts of the corresponding 1 D spectra are plotted along the axes. A correlation is observed as a cross-peak at the intersection of two lines drawn from a proton resonance and from a carbon peak, respectively. The three pairs of diastereotopic methylene protons H-3e do/exc H-5endo/e o and H-6c do/exo give individual cross peaks at the same carbon resonance, respectively. Correlations are not observed for quaternary carbon atoms. The technique is an important tool for chemical shift assignment and thus structure elucidation. 

Overlapping resonances in 2D NMR have limited protein-structure elucidation to fairly small proteins. However, three- and four-dimensional methods have been developed that enable NMR spectroscopy to be further extended to larger and larger protein structures. A third dimension can be added, for example, to spread apart a H- H two-dimensional spectrum on the basis of the chemical shift of another nucleus, such as N or - C. In most three-dimensional experiments, the most effective methods for large molecules are used. Thus, COSY is not often employed, but experiments like NOESY-TOeSY and TOCSY-HMQC are quite effective. In some cases, the three dimensions all represent different nuclei such as H- C- N. These are considered variants of the HETCOR experiment. Multidimensional NMR is now capable of providing complete solution-phase structures to complement crystal structures from X-ray crystallography. Hence. NMR spectroscopy is now an important technique for determining structure.s and orientation.s of complex molecules in solution. 

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. 

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