Carbon-proton coupling constants multiple-bond couplings

Fig. 12.17. Is the proton-proton coupling constant through two bonds (H-C-H), i.e., Juu positive or negative Recall that Jhh >0 (shown in Fig. 12.14a), where the induction mechanism is described. The interaction through two bonds depends on what happens at the central carbon atom are the spins of the two electrons there (one from each bond C-H) parallel or antiparallel Hund s rule suggests they prefer to be parallel This means that the situation with the two proton spins parallel is more energetically favorable, and this means JhH < 0. This rule of thumb may fail when the carbon atom participates in multiple bonds, as in ethylene. For more information, see the section Fwm the Research Fnmt," later in this drapter.

NMR is the tool most widely used to identify the structure of triterpenes. Different one-dimension and two-dimension techniques are usually used to study the structures of new compounds. Correlation via H-H coupling with square symmetry ( H- H COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC), distortionless enhancement by polarisation transfer (DEPT), incredible natural abundance double quantum transfer experiment (INADEQUATE) and nuclear Overhauser effect spectroscopy (NOESY) allow us to examine the proton and carbon chemical shift, carbon types, coupling constants, carbon-carbon and proton-carbon connectivities, and establish the relative stereochemistry of the chiral centres. 

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