OFF-RESONANCE DECOUPLING

OFF-RESONANCE DECOUPLING

	Although broad-band decoupled spectra are much simple, important information may be lost, that is the number of attached hydrogen atoms. A more advanced technique, off-resonance decoupling, can restore this information while still presenting an easily interpretation.
	In this technique, the sample is irradiated by a radio frequency generator which is either slightly upfield or downfield of normal proton resonances (i.e., off resonance). When off-resonance decoupling is used, the apparent coupling constant is greatly reduced, and peak overlap is minimized.
	Off-resonance spectra often show only singlets for each carbon atom, but the multiplicity of the peak is reported as a letter (s , d, t, or q) above the peak.

Example 1: The off-resonance proton-decoupled ¹³C NMR spectrum for ethyl phenylacetate

Example 2: ¹³C-NMR spectrum of diethyl phthalate

Advantages of Off-Resonance Spectra
       determine the number of types of carbon in a molecule
       clarify of the chemical shift
       retain multiplicities with reducing of J

Disadvantage of Off-Resonance Spectra
       if signals are closed, spectrum may be higher order and difficult to interpret