# How is IR spectroscopy interpreted?

May 14, 2016

#### 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 $C \equiv O$ molecule, in that this molecule possesses a very characteristic stretching frequency at or about $2100$ $c {m}^{-} 1$. 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 $C \equiv O$ 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 $C - H$, $C \equiv O$, $C = O$, $C \equiv N$, 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 $\sqrt{2}$ factor.