13.13 : IR スペクトルのピーク強度: 双極子モーメント

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.