Basic IR Spectroscopy and dipole moments

In summary: However, the IR active vibration does not have to cause a change in the dipole moment in order to be in the IR spectrum- it can also cause a change in the polarization of the molecule.
  • #1
Moogie
168
1
Hi

Why does a bond stretch or vibration have to cause a change in dipole moment to be IR active?

Presumably all vibrational modes depend on energy aborption to occur whether they cause a change in dipole or not. I'm presuming that only those vibrational modes that cause a dipole change are in the IR region of the EMR spectrum. Other vibrations absorb energy in some other region of the EMR. Is this correct?

Actually as i was typing this I've just read that the reason why vibrations that cause a change in dipole lead to absorptions in the IR region is due to the mechanism by which the photon transfers its energy to the molecule, which is outside the scope of the article i saw.

Can anyone offer a simple explanation to this?

Kind regards
 
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  • #3
Hi

That's a too involved for my needs but thank-you for your trouble.

Does an IR active vibrational mode have to cause a dipole moment or cause a change in dipole moment? For example if the molecular already had a dipole and a particular vibration occurred but this did not change the dipole would this be IR active, or would the vibration only be IR active if it changed the existing dipole

many thanks
 
  • #4
Moogie said:
For example if the molecular already had a dipole and a particular vibration occurred but this did not change the dipole would this be IR active, or would the vibration only be IR active if it changed the existing dipole
Typically, IR active modes are only those associated with a changing dipole moment.
 
  • #5
Moogie said:
Hi
Does an IR active vibrational mode have to cause a dipole moment or cause a change in dipole moment? For example if the molecular already had a dipole and a particular vibration occurred but this did not change the dipole would this be IR active, or would the vibration only be IR active if it changed the existing dipole

If the vibrational mode doesn't involve a change in dipole moment it will not be observed in the infrared spectrum. If that vibration causes a change in the polarization of the molecule (the electron distribution) then it should be observed in the Raman spectrum.
 
  • #6
cameronforde said:
If the vibrational mode doesn't involve a change in dipole moment it will not be observed in the infrared spectrum. If that vibration causes a change in the polarization of the molecule (the electron distribution) then it should be observed in the Raman spectrum.

Just a quick clarification for the benefit of the readers: Infrared absorption spectrum and Raman emission spectrum, respectively.
The observed Raman spectrum can of course be in the IR energy range.
 

1. What is the purpose of IR spectroscopy?

IR spectroscopy is used to identify and analyze the chemical bonds present in a molecule. It can provide information about the functional groups, molecular structure, and chemical composition of a compound.

2. How does IR spectroscopy work?

IR spectroscopy involves passing infrared light through a sample and measuring the absorption of different wavelengths. The absorption peaks correspond to the vibrational frequencies of the chemical bonds in the sample, providing information about the types of bonds present.

3. What is a dipole moment?

A dipole moment is a measure of the separation of positive and negative charges within a molecule. It is a vector quantity that indicates the polarity of a bond or molecule. A larger dipole moment indicates a more polar bond or molecule.

4. How is dipole moment related to IR spectroscopy?

In IR spectroscopy, the intensity of the absorption peaks is directly proportional to the change in dipole moment during a bond's vibration. This allows for the identification of different types of bonds and their relative strengths based on the intensity of their absorption peaks.

5. What are some applications of IR spectroscopy?

IR spectroscopy has a wide range of applications in various fields such as chemistry, biochemistry, forensics, and pharmaceuticals. It is commonly used for the analysis of organic compounds, identifying unknown substances, and monitoring chemical reactions. It is also used in quality control and environmental testing.

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