Double Resonance Techniques: Overview

Double resonance techniques manipulate two distinct nuclear spins by simultaneously applying different radiofrequency pulses that function as the observing and irradiating fields.

For example, coupling with some atoms is selectively suppressed in spin decoupling experiments,  while observing the NMR signal from other atoms.

In nonselective heteronuclear decoupling, samples are irradiated with a suitable radiofrequency range to eliminate all coupling with one nuclide.

However, higher external magnetic field strengths require stronger irradiation across wider frequency ranges. In some instruments, continuous irradiation generates enough heat to damage thermally sensitive samples.

As a result, modern methods use a series of pulses and precise timing delays, such as J-modulated spin echo pulse sequences, which can also enhance carbon signals by NOE.

An example is the attached proton test, where carbon atoms bonded to an even number of protons exhibit the opposite phase to those bonded to an odd number of protons.