What does infrared spectroscopy tell you?

1 Answer
Apr 13, 2015

Infrared (IR) spectra are plots of either absorbance or % transmission vs. frequencies in wavenumbers (#cm^-1#), typically #400~"4000 cm"^-1#.

The peaks you see in IR spectra are basically results of focused, filtered light directed at molecules that is at just the right frequency of choice to try to alter the molecule's dipole moment.

These just-right (resonant) frequencies show up on the IR spectrum, because they correspond to the strongest alterations of the molecule's dipole moment, which leads to drastic alteration of the molecule's motion, and thus the resultant peaks that show up are the strongest peaks.

The IR spectra tell you what types of vibrational modes (motion) the molecule responds with after it absorbs that light, and when you figure out which peaks correspond to which motions, you can figure out what functional groups the molecule has and (almost) what the molecule is.

Note: You probably will have to use this with NMR sometime to be able to identify the molecule if it's unknown.

Molecules vibrate and wiggle all around, doing stuff like this:


and each functional group moves in a certain way.

In an IR spectrometer setup, typically, a polychromatic light source is sent through a monochromator (light-focusing gizmo), which allows for control of what frequencies of the light reaches the molecule and how wide of a frequency range (bandwidth) hits it; ideally, it's pretty narrow.

Behind the sample is a detector that receives the light that is emitted after the absorption excites the molecule. There is something called a transducer lined up behind the detector that encodes/translates the light information for a spectroscope or computer to have it plot the spectra for you.

When the frequency is just right, it is called the resonant frequency. In addition to the molecule's vibrational motion, there might be an induced change in dipole moment due to the molecule absorbing light of that particular frequency.

The resonant frequencies of the molecule show up as peaks on the IR spectrum, but only if the molecule's dipole moment has changed (even if it was nonpolar to begin with).