IR Spectrum Peak Intensity: Dipole Moment

Dipole moment is the product of the partial charge on either atom and the distance between them. For two charges of equal magnitude and opposite signs, such as in hydrochloric acid, dipole moment quantifies the separation of charge within a bond. This property directly influences how strongly a bond absorbs infrared radiation.

A bond absorbs IR energy only when its vibration frequency matches the electromagnetic radiation frequency and the bond has a dipole moment. When a polar bond is placed in an electric field, it alternately stretches and compresses. If this vibration is IR-active, energy is absorbed. Symmetrical bonds with zero dipole moment show no change in dipole moment during vibration, making them IR-inactive.

The greater the change in dipole moment during vibration, the more radiation a bond absorbs, resulting in higher IR peak intensity. For example, C=O bonds have larger dipole moments than C=C bonds, causing greater dipole moment changes during vibration. This produces more intense absorption peaks for carbonyl groups compared to alkene stretches.

Trans symmetrical alkenes have zero dipole moment due to their symmetrical structure. When the C=C bond vibrates, there is no change in dipole moment, so the bond cannot absorb IR energy. This makes the C=C stretching vibration IR-inactive, resulting in the absence of a peak in the IR spectrum for these molecules.

When a polar bond is placed in an oscillating electric field, the direction of the field determines whether the bond is compressed or stretched. Electromagnetic radiation consists of an electric field component that rapidly reverses direction, causing polar bonds to alternately stretch and compress. If this oscillation matches the bond's natural vibration frequency, IR energy is absorbed.

Unsymmetrical alkenes have a dipole moment, making their C=C stretching vibrations IR-active and producing strong absorption peaks. Symmetrical alkenes lack a dipole moment, so their C=C stretching vibrations are IR-inactive and produce no peak. This difference allows IR spectroscopy to distinguish between alkene structures based on their symmetry.

A vibration is IR-active when the bond has a dipole moment and that dipole moment changes during vibration. When vibrating bond dipole matches the wave frequency of electromagnetic radiation, the bond absorbs energy. Conversely, vibrations of symmetrical bonds with zero dipole moment produce no dipole moment change, making them IR-inactive and invisible in IR spectra.