Carbon-13 hydrogen direct coupling

Again, we need to define a coupling constant J to set up this experiment. Here for optimum sensitivity we have used an average value for direct (one-bond) carbon-hydrogen coupling constants of 160 Hz. This choice works well for most CH bonds, but is rather low if an acetylenic CH bond is present. 

C. Direct Carbon-Hydrogen and Carbon-Fluorine Coupling Constants References... 

An experiment that correlates spin-coupled protons and carbons. This experiment is selective for one-bond couplings and therefore provides direct carbon-hydrogen connectivity. The spectrum does not contain a diagonal proton and carbon signals are correlated via cross-peaks. 

Table II. Chemical Shifts and Directly Bonded Carbon-Hydrogen Coupling Constants for C-2 Carbon in Imidazole Derivatives °...

Unanticipated developments help to put known facts into place. Results from biochemistry drove home to organic chemists the message that it was not a chemical rarity for carbon-hydrogen bonds to be sources of hydride equivalents. Westheimer, Vennesland et al. established beyond doubt that in a redox reaction mediated by the coenzyme couple NAD(P)+/NAD(P)H the carbon-hydrogen bond of ethanol could serve directly as a hydride donor to an electron-deficient carbon of a pyridinium ion, and that this hydride equivalent could in turn be donated directly to the electropositive carbon of a carbonyl group. Thus the hydride donor capacities of carbon are also part and parcel of life. All this can occur under physiological conditions with the help of an enzyme, which somehow activates these reactants. The sequence is illustrated schematically in equation (1). In either direction hydride is transferred from carbon to carbon. 

Take note that this effect would stabilize the equatorial anomers, hence diminishes the anomeric effect of oxygenated substituents. However, there are other physical indications of the favoured delocalization of the cyclic oxygen towards the exocyclic oxygen as furnished by the direct coupling constant value Vch between the anomeric carbon and hydrogen. A comparison of the 7ch values, measured on about 20 anomeric pairs having various configurations and substitutions, shows that for each pair we observe... 

Correlations between H and 13C nuclei via scalar couplings can be shown by two-dimensional COSY spectra (4-6). This is exemplified by a heteronuclear (]H,3C) COSY spectrum of 7-methoxycoumarin (B7-4, hemiarin) shown in Fig. 3a (bottom). The cross-peaks connect the signals of hydrogen and carbon atoms directly attached. 

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