Question

How is IR spectroscopy interpreted?

Answer 1

Usually with difficulty.

Explanation:

Inorganic professors talk of sporting versus non-sporting techniques of spectroscopy. They refer to the process of duck shooting, where it is considered very unsporting to shoot a sitting bird, and it is only sporting to shoot a bird on the wing.

Non-sporting techniques of spectroscopy include NMR spectroscopy, and (especially) X-ray crystallography. These are direct methods of spectroscopy, which give you a very good idea of molecular structure and connectivity with a good degree of certainty without speculation.

On the other hand, there are the sporting techniques of spectroscopy, whose analysis requires a great deal more supposition. These include UV-vis spectroscopy, and also IR spectroscopy. IR spectroscopy is a superb probe of the `C-=O` molecule, in that this molecule possesses a very characteristic stretching frequency at or about `2100` `cm^-1`. Depending on the degree of back-donation (from the metal centre), some idea can be gained on the symmetry of a transition metal complex with `C-=O` ligands.

So what I am trying to say, is that IR spectroscopy is a bit of a black box. We can recognize a few absorptions in the spectrum, say those due to `C-H`, `C-=O`, `C=O`, `C-=N`, and `C=N`. Usually, save for `C-H`, these stick out like sore thumbs. The `C-H` or `M-H` stretching frequency may sometimes be recognized by labelling the hydrogen with the deuterium nucleus. Because the deuterium nucleus, `""^2H`, is twice as heavy as the hydrogen nucleus, its stretching frequency is predictably reduced by a `sqrt2` factor.