IR radiation is readily absorbed by molecular substances because photon energy in this region corresponds to the difference between vibrational energy states corresponding to stretching and bending of atoms on chemical bonds.
Although the force gradient is different, the mass-spring system can give useful insight into the vibrational modes along chemical bonds that give rise to infrared absorbances detected by IR spectroscopy. The relationships between electrostatic force, particle mass, and frequency are similar to the relationships between spring constant, object mass, and frequency of spring oscillation (although the force of attraction is very different). The main point is that larger masses and weaker forces give lower frequency. In general, this means that a singly bonded large atom absorbs at lower frequency than a double bonded small atom.
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