Two dimensional proton-carbon correlated

The carbon and DEPT (distortionless enhanced polarization transfer) spectra are shown in Figure 10. The HETCOR (heteronuclear two-dimensional proton-carbon correlation) spectrum is shown in Figure 11. The carbon assignments are listed in Table 5. Long-range HETCOR experiments were used to make the assignments for the thiophene carbons. 

The parent is often a useful starting point to assist in the interpretation of the NMR spectra of the unknown degradant or impurity. A sample of the parent compound ideally should be run in the same solvent used for the impurity for the most straightforward comparison, because chemical shifts and resolution may change somewhat in different solvents, particularly in the proton spectrum. Of course, solubility differences between the impurity and parent may preclude using the same solvent. Both the one-dimensional proton and carbon spectra of the parent should be obtained. Two-dimensional proton-carbon correlation spectra may also be needed to make all resonance assignments. 

HMBC //eteronuclear multiple frond correlation. A proton-detected, two-dimensional technique that correlates protons to carbons that are two and three bonds distant. Essentially, it is an HMQC that is tuned to detect smaller couplings of around 10 Hz. 

For relaxation studies of biomolecules in solution (which is no specialty of the authors of this chapter), it is often essential to use inverse-detection schemes to obtain reasonable sensitivity. Furthermore, besides problems with poor sensitivity, the carbon-13 and nitrogen-15 spectra are often too crowded to allow measurement of individual relaxation rates for different I nuclei, either by direct detection or by indirectly detecting the protons. If this is the situation, one can spread out the I nuclei signals for better resolution of individual resonances by detecting a two-dimensional H-I correlation spectrum. Relaxation experiments of this type can be considered a modification of the double polarization-transfer IS correlation experiment [7, 17] ... 

Figures 13.7 and 13.8 are two examples of two-dimensional NMR spectroscopy applied to polymers. Figure 13.7 is the proton homonuclear correlated spectroscopy (COSY) contour plot of Allied 8207A poly(amide) 6 [29]. In this experiment, the normal NMR spectrum is along the diagonal. Whenever a cross peak occurs, it is indicative of protons that are three bonds apart. Consequently, the backbone methylenes of this particular polymer can be traced through their J-coupling. Figure 13.8 is the proton-carbon correlated (HETCOR) contour plot of Nylon 6 [29]. This experiment permits the mapping of the proton resonances into the carbon-13 resonances.

Fig. 2.54 presents a two-dimensional carbon-proton shift correlation of D-lactose after mutarotational equilibration (40% a-, 60% / -D-lactose in deuterium oxide), demonstrating the good resolution of overlapping proton resonances between 3.6 and 4 ppm by means of the larger frequency dispersion of carbon-13 shifts in the second dimension. The assignment known for one nucleus - carbon-13 in this case - can be used to analyze the crowded resonances of the other nucleus. This is the significance of the two-dimensional CH shift correlation, in addition to the identification of CH bonds. For practical evaluation, the contour plot shown in Fig. 2.54(b) proves to be more useful than the stacked representation (Fig. 2.54(a)). In the case of D-lactose, selective proton decoupling between 3.6 and 4 ppm would not afford results of similiar quality. 

Key experiments useful for substructure determination by NMR include the DEPT sequence (c.. Figs. 2.44-2.46) for analysis of CH multiplicities, as well as the two-dimensional CH correlation for identification of all CH bonds (e.g. Fig. 2.55 and Table 2.2) and localization of individual proton shifts. If, in addition, vicinal and longer-range proton-proton coupling relationships are known, all CH substructures of the sample molecule can be derived. Classical identification of homonuclear proton coupling relationships involves homonuclear proton decoupling. A two-dimensional proton-proton shift correlation would be an alternative and the complementary experiment to carbon-proton shift correlation. Several methods exist [68], Of those, the COSTsequence abbreviated from Correlation spectroscopy [69] is illustrated in Fig. 2.56. 

This is the classic off-resonance decoupling experiment. If the residual splitting is measured as the proton decoupling frequency is varied, then it is possible to find the chemical shifts of the protons attached to each carbon. This approach has been replaced by two-dimensional C-H correlation. 

Figure 3.18. Improved peak resolution in a two-dimensional heteronuclear proton-carbon correlation experiment (Chapter 6) through linear prediction.

Figure 3.18. Improved peak resolution in a two-dimensional heteronuclear proton-carbon correlation experiment (Chapter 6) through linear prediction (LP). The same raw data was used in each spectrum, with the Fi (carbon) dimension processed with (a) no data extension, (b) one zero-fill and (c) LP in place of zero-filling.

Figure 3.25 The two-dimensional C- H correlation spectrum for 10 wt% poly(vinyl chloride). The inset plot shows an expansion of the correlations for the methylene protons and carbons. Reprinted with permission from ref. 4.

Two-dimensional carbon-proton shift correlation m one-bond CH coupling... 

Two-dimensional C//correlations such as C//COSY or HC HMQC and HSQC provide the Jqh connectivities, and thereby apply only to those C atoms which are linked to H and not to non-protonated C atoms. Modifications of these techniques, also applicable to quaternary C atoms, are those which are adjusted to the smaller Jqh and Jqh couplings (2-25 Hz, Tables 2.8 and 2.9) Experiments that probe these couplings include the CH COLOC (correlation via long range couplings) with carbon-13 detection (Fig. 2.16) and HC HMBC (heteronuclear multiple bond coherence) with the much more sensitive proton detection (Fig. 2.17)... 

Figure 13 The HOESY two-dimensional spectrum of micellized sodium octanoate in aqueous solution. Besides one-bond carbon-proton correlations, remote correlations are observed (marked by an arrow).

There are a number of techniques stemming from the basic two-dimensional technique, which for example allow correlation between carbon atoms and the protons attached to them and correlations of carbon atoms with protons one or two bonds removed from them, heteronuclear multiple bond correlation (HMBC). 

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