The dipole moment of a bond is the product of the partial charge on either atom and the distance between them. Dipole moments influence the efficiency of IR absorption and the peak intensity. When a bond with a dipole moment is placed in an electric field, the direction of the field determines if the bond is compressed or stretched. Electromagnetic radiation consists of an electric field component that rapidly reverses direction. It follows that polar bonds are alternately stretched and compressed.
If the frequency of this bond stretching and compressing matches the natural vibration frequency of the bond, IR energy can be absorbed by the bond. Such vibrations are said to be IR-active. When a symmetrical bond with zero dipole moment vibrates, there is no change in dipole moment, meaning that energy is not absorbed. These vibrations are considered IR-inactive. This is evident in asymmetrical alkenes, where the C=C stretching vibrations are IR-active and show strong absorption bands. However, in symmetrical alkenes with zero dipole moment, the C=C stretching band is absent.
It also follows that a greater change in the dipole moment results in stronger IR absorptions with high-intensity bands. This is why C=O bonds (with a large dipole moment) show higher intensity absorption bands than C=C bonds (with small dipole moments).
For two charges of equal magnitude and opposite signs, such as in hydrochloric acid, the dipole moment is expressed as the product of the charge on one of the atoms times the distance to the other atom.
In an electric field, depending on its direction, the bond is stretched or compressed alternatively with a simultaneous change in the dipole moment.
When the vibrating bond dipole matches the wave frequency, the bond absorbs energy. Such vibrations are called IR-active.
For instance, in unsymmetrical alkenes, the C=C stretching vibrations are IR-active and show strong absorption peaks. While the vibrating C=C in trans symmetrical alkenes—with no dipole moment—leads to an absence of the C=C stretching peak.
Moreso, the higher the change in the dipole moment, the greater the amount of radiation absorbed, and the higher the IR peak intensity.
For instance, a C=O bond, with a higher dipole moment than a vinylic C=C bond, shows a greater change in dipole moment with time, compared to a C=C, and results in a more intense peak.
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