Question

What does decoupling mean in NMR?

Answer 1

Decoupling is the simplification of an NMR spectrum by irradiating the sample with radio-frequencies to remove the splitting caused by protons.

Explanation:

It is especially important in `""^13"C"` NMR spectra.

Coupled Spectra

The `""^13"C"` signals are split by the H atoms attached to them.

These splittings are much larger than in `""^1"H"` spectra (`J_("C-H") = "100 – 250 Hz"`; `J_("H-H") = "0 – 15 Hz"`).

The spectra are more complicated and harder to interpret if the signals are close together.

The spectrum of diethyl phthalate, for example, shows the `"CH"_3` groups as a quartet, the `"CH"_2` groups as a triplet, and the aromatic `"CH"` groups as doublets.

Broadband Decoupling

In a "normal" `""^13"C"` spectrum, these couplings are "removed" by applying a continuous second radio frequency that excites all the `"H"` nuclei at once.

They "flip" between energy states so fast that the `"C"` atoms "see" only an average unperturbed field, and the coupling is removed.

The spectrum of diethyl phthalate reduces to a single peak for each of the six types of carbon atom.

The advantage of decoupling is that it gives a cleaner spectrum, as carbon signals are often close for complex molecules, and fewer peaks mean less overlap.

However, you lose the splitting information that can be helpful in assigning peaks.

Off-Resonance Decoupling

In off-resonance decoupling, the decoupling frequency is set at a distance from the proton frequency and the decoupling power is reduced.

The carbon nuclei see a small perturbation from the protons.

The spectrum of diethyl phthalate still shows doublets, triplets, and quartets, but the `J_("C-H")` values are reduced to 12 to 20 Hz.