Question

Do halogens appear in the IR spectrum? For example, in a molecule bromine attached to a benzene, where would the bromine appear on the IR spectra?

Answer 1

The `"C-Br"` stretch in bromobenzene appears as an absorption at about `"450 cm"^-1`.

You may not be able to see it if your instrument cuts off at `"600 cm"^-1` or higher.

Answer 2

As an additional answer, if we were specifically talking about, say, `"Br"-"Br"`, `"F"-"F"`, etc., then no, those wouldn't show up.

IR-active vibrational motions, i.e. those that change the dipole moment of the molecule, show up in an IR spectrum.

So in the above diagram of `"CO"_2`, `B` (asymmetric stretch), `C` (in-the-plane scissoring), and `D` (out-of-the-paper scissoring) are IR-active, but `A` is not because `A` is totally symmetric; the changes in dipole moment cancel out in the horizontal directions.

Since diatomic halogens can only stretch one way, and that one way is totally symmetric, there is no change or production of a dipole moment. Thus, there are no IR-active vibrational motions possible.

Same with `"O"_2`, `"N"_2`, and other homonuclear diatomic species.

(Something like `"H"-"Cl"` would show up though.)