Question

Why is `CDCl_3` a triplet in `C^13 NMR`?

Answer 1

The deuterium in the CDCl₃ splits the `""^13"C"` signal into a triplet.

Whenever you run a `""^13"C"` spectrum in CDCl₃, you always get a triplet solvent peak at 77.5 ppm.

For example, here's a spectrum of t-butyl chloride.

You see the characteristic CDCl₃ triplet at 77.5 ppm.

The reason is that `""^2"H"` has a spin quantum number `I` = 1.

A nucleus with `I = 1` has `2I + 1 = 3` possible orientations.

These orientations have the same energy in the absence of a magnetic field.

In the presence of a magnetic field, these energy levels split. Each level is labelled with a magnetic quantum number `m` = +1, 0, -1.

So a `""^13"C"` nucleus will be split into three different energy levels depending on the +1, 0, or -1 orientation of the deuterium atom.

Each orientation has the same probability, so the signal is a 1:1:1 triplet.