Shift correlation experiment, heteronuclear chemical structure

The earliest of the magnetization transfer experiments is the spin population inversion (SPI) experiment [27]. By selectively irradiating and inverting one of the 13C satellites of a proton resonance, the recorded proton spectrum is correspondingly perturbed and enhanced. Experiments of this type have been successfully utilized to solve complex structural assignments. They also form the basis for 2D-heteronuclear chemical shift correlation experiments that are discussed in more detail later in this chapter. 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

Of the many techniques available to the NMR spectroscopist in structural elucidations, none is so valuable as the indirect chemical-shift correlation experiment, such as HMBC, TOCSY (both homo- and heteronuclear varieties), and FLOCK. Once molecular fragments have been identified by the COSY and HSQC experiments, the spectroscopist attempts to combine these fragments by means of the preceding techniques. As indispensable as these methods have become to NMR spectroscopists, they nonetheless suffer a common... 

The 2D heteronuclear chemical-shift correlation experiment just described is most easily understood in the simplified-doublet form that was presented. The multiplet structure complicates the results, and the protons must be decoupled from the C nucleus by application of a 180° C pulse at the midpoint of the evolution period. Proton deeoupling can be applied during the detection period as well. 

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],... 

In this section 10.2, we review the various solid-state NMR methods used to investigate interpolymer interactions, molecular motion and the spatial structure of a polymer blend. An interaction between component polymers affects the chemical shifts and lineshapes (see Section 10.2.2.1) and the molecular motions of the component polymers (see Section 10.2.2.2). In Section 10.2.3.1, microheterogeneity from 2 to 50 nm is studied by measuring spin diffusion indirectly from its effects on H spin-lattice relaxation. The spin-diffusion processes can also be monitored by several methods based on the Goldman-Shen experiment [8] (see Section 10.2.3.2). Homonuclear and heteronuclear two-dimensional correlation experiments reveal how and to what extent component polymers interact with each other (see Section... 

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). 

It can prove very difficult to assign aU the C nuclei based only on the isotropic chemical shifts and signal intensities in ID C H CP/MAS NMR experiments of samples with natural abundance C. A more precise assignment can be obtained by correlating protons ( H) and carbons ( C) in a 2D NMR experiment, which gives direct ins ht into the chemical structure. Subsequent to excitation of the spins, a typical heteronuclear correlation... 

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