Sometimes the most interesting peaks are the ones that aren't there

Post date: Aug 2, 2020 7:36:23 AM

This picture below might be an analogy for Infrared Spectroscopy, where sometimes it is the ABSENCE of peaks on the IR spectrum that is actually the most interesting. I teach the organic students each semester that lots of molecules show peaks immediately below (or right of) the 3000 cm^-1 line due to sp³ C-H stretching. Thus, it is the ABSENCE of peaks in that region that makes things more interesting and provides clues to the final structure. A good example of this principle that the absence of peaks is itself important information might be tetrachloroethylene, which has a noticeable absence of peaks in its IR other than the fingerprint region, but that absence of peaks (especially the C=C stretch) gives important clues as to the symmetry of the molecule.

IR spectrum of tetra-chloroethylene (modified from SDBS)

This IR spectrum is more interesting for what IR peaks are absent, then those which are present.