Heteronuclear chemical shift correlation HETCOR

NMR spectra of solids, and thus soil, are obtained by what is called magic angle spinning. The spectra obtained have broader absorption features than NMR spectra of components in solution or liquids. Numerous NMR experiments such as 3H—13C heteronuclear chemical shift correlation (HETCOR), which identifies which hydrogen atoms are attached to which carbon atoms, can also be carried out on solid samples. A great deal of useful information about the structure of components in soil can thus be obtained from NMR investigations [5,6],... 

High resolution MAS techniques of 13C, DEPT, correlated spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear chemical shift correlation (HETCOR) were used to examine conventional CBS and efficient TMTD vulcanisation of polybutadiene [37]. In conventional CBS vulcanisation, the major vulcanisate 13C NMR peak occurred at 44.9 ppm and was assigned to a trans allylic structure (-C=C-C-Sx with X=3 or 4). The efficient TMTD vulcanisation yielded as main product a 13C NMR peak at 54.0 ppm and was assigned to a cis allylic vulcanisate (-C=C-C-Sx x=l). While cyclic sulfur by-products were observed in both vulcanisation systems, the CBS formulations gave rise to a higher percentage postulated to be formed via a episulfide intermediate. 

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]. 

Heteronuclear Chemical Shift Correlation (HETCOR, HM()C).. 539 Homonuclear NOE Correlation.. . 540 NMR Spectral Collections. 

Figure 27. Aliphatic part of the heteronuclear chemical shift correlated (HETCOR) spectrum of camphor. Contour diagram (left)...

Heteronuclear Chemical Shift Correlation (HETCOR, HMQC)... 

To obtain a correlation between carbons and attached protons in a two-dimensional experiment, we must be able to plot the chemical shifts of the atoms along one axis and the chemical shifts of the protons along the other axis. A spot of intensity in this type of two-dimensional spectrum would indicate the existence of a C—H bond. The heteronuclear chemical shift correlation (HETCOR) experiment is designed to provide the desired spectrum. 

A second 2D NMR method called HETCOR (heteronuclear chemical shift correlation) is a type of COSY in which the two frequency axes are the chemical shifts for different nuclei usually H and With HETCOR it is possible to relate a peak m a C spectrum to the H signal of the protons attached to that carbon As we did with COSY we 11 use 2 hexanone to illustrate the technique... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

Heteronuclear chemical shift-correlated spectroscopy, commonly called H-X COSY or HETCOR has, as the name implies, different and F frequencies. The experiment uses polarization transfer from the nuclei to the C or X nuclei which increases the SNR. Additionally, the repetition rate can be set to 1—3 of the rather than the longer C. Using the standard C COSY, the ampHtude of the C signals are modulated by the... 

HETCOR (Section 13.19) A 2D NMR technique that correlates the H chemical shift of a proton to the C chemical shift of the carbon to which it is attached. HETCOR stands for heteronuclear chemical shift correlation. 

The heteronuclear 1H-13C chemical shift correlated (HETCOR, 2D NMR) experiments were acquired (21) with sweep widths dictated by the appearance of the 3c and H spectra by using 2K data points in the 256 free induction decays (FIDs) collected on a 3200 Hz spectral width. 

Figure 6-21 illustrates this procedure for an adamantane derivative. The H frequencies are on the vertical axis and the C frequencies are on the horizontal axis. The respective spectra are illustrated on the left and at the top. The 2D spectrum is composed only of cross peaks, each one relating a carbon to its directly bonded proton(s). There are no diagonal peaks (and no mirror symmetry associated with a diagonal), because two different nuclides are represented on the frequency dimensions. Quaternary carbons are invisible to the technique, as the fixed times A and A2 normally are set to values for one-bond couplings. This experiment often is a necessary component in the complete assignment of H and resonances. Its name, HETeronuclear chemical Shift CORrelation, usually is abbreviated as HETCOR, but other acronyms (e.g., HSC, for Heteronuclear Shift Correlation, and H, C-COSY, also are used. The method may be applied to protons coupled to many other nuclei, such as Si, and P, as well as C. 

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. 

HETCOR heteronuclear chemical shift correlated experiment... 

Clearly the development and dissemination of two-dimensional NMR techniques has had a profound impact in natural products structure elucidation. Some techniques, COSY and variants of the C-detected heteronuclear chemical shift correlation (variously referred to as HETCOR, HC-COSY, etc.) experiment, have been widely used by the natural products chemistry community. Inverse-detected heteronuclear shift correlation techniques are becoming recognized as a powerful adjunct to the COSY experiment and a replacement for their less sensitive and, in some cases, less versatile C-detected predecessor experiments (Martin and Crouch 1991). 

Fig. 10. Heteronuclear chemical shift correlation via (C,H) (HETCOR) of (+)-usnic acid in CDCI3...

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],... 

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