Heteronuclear Coupling of Carbon-13 to Fluorine

Heteronuclear coupling is observed when there are fluorine atoms in an organic com- 

Organic compounds that contain C, H, O, Cl, and Br wiU show only singlets when the proton decoupler is turned on. The oxygen, chlorine, and bromine atoms will not couple to a carbon-13 atom under normal conditions. However, when the organic compound has a fluorine atom attached to a carbon-13 atom, you will observe heteronuclear coupling even though the proton decou- 

204 Nuclear M netic Resonance Spectroscopy Part Two Carbon-13 Spectra 

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The spectrum of CFBrs shown in Fignre 6.21 has Hertz (Hz) values recorded on top of each peak in the doublet rather than parts-per-million (ppm) values, which is more typical. The chemical shift values for each of the peaks can be calculated by dividing the Hertz values by the field strength of the NMR spectrometer (75 MHz), giving 43.85 and 48.79 ppm. The actual chemical shift for the carbon atom would be in the center of the doublet 46.32 ppm. The coupling constant in 

Hertz is easily determined by subtracting the two Hertz values, yielding 370 Hz. This huge coupling constant is typical for direct one-bond coupling of the fluorine nucleus to a carbon-13 atom (V). 

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