Nuclear Magnetic Double Resonance

Nuclear Magnetic Double Resonance

Nuclear magnetic double resonance (NMDR), also called double irradiation or spin decoupling, is a technique very frequently used in the interpretation of complex NMR spectra. It is based on the principle that when one group of equivalent protons in a molecule is irradiated by applying very strong stationary radiofrequency field at the Larmor (resonance) frequency of the protons, it can no longer undergo spin-spin coupling with the neighbouring groups containing equivalent protons. As a result, the spectrum is simplified.


The above figure shows the double resonance spectrum for pure ethanol. When protons of methyl group are irradiated (Figure-B), the resonance of the methylene protons collapses to a doublet because they are coupled only with the OH proton; the coupling with methyl protons is destroyed. Similarly, when protons of the methylene group are irradiated (Figure C), the resonances of the OH and methyl protons both collapse to singlets.

Double resonance experiments can be carried out either by-
1. keeping both the applied frequency (𝜈₁) and the irradiation frequency (𝜈₂) constant, and varying the magnetic field (field sweep), or
2. by having a constant magnetic field (H.) and irradiation frequency (𝜈₂), and sweeping with the applied frequency (𝜈₁) (frequency sweep).
The irradiating frequency (𝜈₁) may be changed after each scan, permitting an extensive investigation of spin spin coupling. Nuclear triple resonance, where two irradiation frequencies are applied simultaneously and the signal due to a third nucleus examined, is valuable in the investigation of multiple and long-range coupling.
The irradiation frequency (𝜈₂), in order to decouple the nuclei completely, is a powerful r/f signal; 𝜈₁ is weak in comparison.

Heteronuclear decoupling is the decoupling of different isotopes where the difference in resonance frequencies is large, e g., ¹H and ¹⁹F. The experiment is expressed as-
¹H [¹⁹F]
Nucleus being studied
Nucleus irradiated
Homonuclear decoupling involves nuclei of the same isotope, where the difference in resonance frequencies is small. It is expressed by ¹H[¹H]. Similarly, the irradiation of the X nucleus of an ABX system is expressed as AB [X].
For effective double irradiation a good chemical shift difference between the interacting nuclei is required.
Decoupling experiments by NMDR are, generally, easy to conduct, however complete decoupling only takes place under rather restricted experimental conditions. True decoupling is only seen when the chemical shift separation is much greater than the coupling constant.