CHEM1020 Study Guide - Final Guide: Infrared Spectroscopy, Wavenumber, Transmittance
6 Nov 2018
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When a molecule is irradiated with ir radiation, the radiation is absorbed when its energy is exactly equal to the energy required to excite a molecule to a higher vibrational state. Different functional groups have different bond strengths and require different energies to bring about these transitions. Ir spectroscopy detects functional groups by the vibrations of their bonds. Weaker the bond strength the easier it is to get it to vibrate. The vibrational ir extends from 2. 5 x 10-6 m to 2. 5 x 10-5 m. the frequency of ir radiation is commonly expressed in wavenumber = the number of waves/centimeter, cm-1 = 1/ in cm/ Expressed in wavenumbers, the vibrational ir extends from 4000 cm-1 to 400 cm -1. The y- axis is % transmittance or absorbance. Atoms joined by covalent bonds undergo continuous vibrations relative to each other. The simplest vibrational motions in molecules giving rise to the absorption of infrared radiation are stretching and bending motions.
