Basic IR Spectroscopy and dipole moments

[Moogie]

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

 

[Gokul43201]

This is primarily due to the fact that the vibrational frequencies of most molecules are in the IR range, but that's not a complete story. Here's a simplified explanation: https://www.physicsforums.com/showpost.php?p=903416&postcount=6

 

[Moogie]

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

 

[Gokul43201]

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.

 

[cameronforde]

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.

 

[alxm]

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.