Question

How do you call a spectrum with no `"C-H"` splitting?

Answer 1

A `""^13"C"` NMR spectrum with no `"C-H"` coupling is called a proton decoupled spectrum.

Explanation:

Proton-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 figure below shows the proton-coupled `""^13"C"` NMR spectrum for ethyl phenylacetate.

(from chem.sci.ubu.ac.th)

The methyl signal is a quartet, the methylene signals are triplets, and the aromatic signals are doublets.

Proton-Decoupled Spectra

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.

This means that each `"C"` signal becomes a single line.

The spectrum of ethyl phenylacetate reduces to a single peak for each of the eight types of carbon atom.

(from chem.sci.ubu.ac.th)

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.