NMR spectra can often becom quite complicated as the number of resonating protons increases due to overlap of resonating peaks and complex splitting patterns. To simplify these spectra one can use spin decoupling, often referred to as double resonance.
In double resonance experiments, a second radiofrequency signal is applied while the spectrum is scanned in the usual fashion. The radiofrequency can be set to irradiate a selected group of protons in the molecule being measured. Irradiation causes the selected protons to become decoupled from all other protons in the compound. Protons that are coupled to the irradiated group of protons will have simplified signals due to the loss of coupling with the irradiated protons. Selective irradiation will allow the spectroscopist to assign peaks more readily to a structure.
Figure 36 shows how the double resonance experiment is applied to propyl bromide.
Figure 36. NMR spectra for propyl bromide: (a) the normal spectrum at 60 MHz; (b) the effect of decoupling the a CH2 protons; (c) the effect of decoupling the b CH2 protons; (d) the effect of decoupling the g CH3 protons.
Return to: NMR Spectroscopy Page