13.13
IR Spectrum Peak Intensity: Dipole Moment
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…
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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Q1: What is dipole moment and how is it calculated?
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.
Q2: Why do some bonds show IR absorption while others do not?
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.
Q3: How does dipole moment change affect IR peak intensity?
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.
Q4: Why are C=C stretches absent in trans symmetrical alkenes?
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.
Q5: What happens to a polar bond when placed in an oscillating electric field?
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.
Q6: How do unsymmetrical and symmetrical alkenes differ in IR spectroscopy?
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.
Q7: What determines whether a molecular vibration is IR-active or IR-inactive?
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.