Why Don't All Molecular Vibrations Lead to Observable Infrared Absorptions?

[Moogie]

Hi

I'm a little bit confused about infrared spectroscopy. My basic understanding (which was obviously wrong) was that molecules absorbed EMR in the infrared region and that this caused vibrations in the molecule. Some vibrations/stretching cause a change in charge distribution and for reasons not to known to me, only those that cause a change in charge show up on the IR spectra.


However I have just read:

Not all molecular vibrations lead to observable infrared absorptions. In general, a vibration must cause a change in the charge distribution within a molecule to absorb infrared light. The greater the change in charge distribution, the stronger the absorption.


This explains why the spectra only shows vibrations that cause a change in charge, but what I don't understand is why "not all vibrations lead to observable infrared absorbtions." How can any vibration occur without absorbing energy. I thought the molecules absorbed the energy which caused the vibration, not that the vibration caused energy to be absorbed.

I would appreciate it if someone could clarify this for me

Kind regards

 

[ehild]

Only those vibrations can interact with the electric field of the electromagnetic radiation which are accompanied with change of dipole moment.

If you have a carbon monoxide (CO) molecule for example, the oxygen atom is a bit negative, the carbon atom is a bit positive, and they will move in opposite direction in an oscillating electric field. If the frequency of the field is close or identical with the natural vibration frequency of the molecule, the amplitude of the vibration increases, the molecule gains energy from the field, it absorbs radiation.

In case of a H2 molecule, the two atoms are identical, they experience the same force in the electric field, so the bond length between them will not change, no vibration is excited, H2 does not absorb in the infrared range.

This is the strongest interaction between the electric field and a molecular vibration, through the induced dipole moment. There are other interactions.
The electric field can interact also with a rotating molecule if it has a permanent dipole moment. So CO has both rotational and vibrational absorption spectrum, but at different frequency ranges.

ehild

 

[Moogie]

Only those vibrations can interact with the electric field of the electromagnetic radiation which are accompanied with change of dipole moment.

What do you mean by this sentence please?

Thanks kindly for your answer

 

[ehild]

This is the sentence you can read in textbooks on infrared spectroscopy:) and I tried to explain it. It is connected to quantum mechanics, but I wanted to give a simpler explanation

Again: The electric field changes with the frequency of the radiation at the position of the molecule, according to a SHM : E= E₀sin(wt).
It exerts qE force on the charges in the molecule. You can imagine that all atoms carry some charge and they are surrounded by the cloud of valence electrons. The atom with the positive charge moves in the direction of the electric field, the negatively charged atom moves in the opposite direction. As the electric field changes, they follow the change of the field, so they vibrate around their equilibrium position. I tried to draw a picture showing a vibrating molecule.

ehild