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
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
if signals are closed, spectrum may be higher order and difficult to interpret