How is IR spectroscopy interpreted?

Question

Lucy Alessi · Accepted Answer

Usually with difficulty.

When inorganic professors discuss the differences between sporting and non-sporting spectroscopy techniques, they bring up the example of duck shooting, where it is only considered sporting to shoot a bird that is on the wing and highly unsporting to shoot a sitting bird. Non-sporting spectroscopy methods include NMR spectroscopy and (particularly) X-ray crystallography; these are direct spectroscopy methods that provide a high degree of certainty regarding molecular structure and connectivity without speculating. 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.

Santiago Alfonso · Answer

undefined IR spectroscopy is interpreted based on the absorption of infrared radiation by a molecule's chemical bonds. Each type of bond absorbs energy at specific wavelengths, resulting in characteristic peaks on the IR spectrum. These peaks correspond to different functional groups present in the molecule, allowing identification and analysis of the compound's structure.