When infrared radiation is passed through a molecule, absorption occurs if the molecule's vibration leads to a substantial change in its bond dipole moment. Transitions between vibrational energy levels, typically corresponding to infrared frequencies (4000–400 cm−1), allow absorption if the vibration significantly alters the dipole moment, making the molecule infrared active. The molecular bonds have different stretching and bending vibrations, resulting in various peaks with varying intensities in the spectrum. The strength of the IR absorptions depends on their bond's dipole moment. The dipole moment is the electric field associated with the bond. If two opposing charges are considered to be bound together by a spring, any changes in the distance separating the charges result in an alteration of the dipole moment.
Significant changes in dipole moments give strong signals, and small dipole moment changes result in weak signals. So, the oscillating electric field is an antenna for absorbing the IR radiation.
For example, the carbonyl double bond has a larger dipole moment change due to C=O stretching vibrations than the C=C double bond's stretching vibrations. As a result, the carbonyl double bond shows a stronger signal than the C=C double bond in the IR spectrum. In addition, peak intensity also depends on the sample concentration and the number of active bonds. Peak intensity increases with sample concentration increase. Symmetrical bonds are inefficient at IR absorption.
An IR spectrum can have multiple peaks with varying intensities resulting from the stretching and bending vibrations of the different bonds.
Strong signals result from efficient IR absorptions of bonds with large dipole moments. Weak peaks arise from IR absorptions of bonds with small dipole moments.
The intensity of an IR peak depends on sample concentration, the number of IR-active bonds and the dipole moment.
An increase in the sample's concentration increases the number of absorbing groups, thereby enhancing the peak intensities in the IR spectrum.
For example, a highly concentrated sample of alkane has more C–H bonds and shows stronger absorption peaks than a sample with a lower concentration and comparatively fewer C–H bonds.
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