A question concerning the interaction of infra-red radiation and matter

[orpheus]

Hello. This is my first post ever here, and I'm pretty sure it's not in the correct section, and I apologize for that.

 

So the atoms in a molecule vibrate in relation to each other. And when IR radiation is absorbed by a molecule these vibrations increase. Apparently absorption only happens if there is a change in dipole moment. How do the streching and bending effects change dipole moment? Is it that streching adjusts the distance between the atoms or what?

[big314mp]

Is it that streching adjusts the distance between the atoms or what?

 

That's basically how it works.

 

Consider water. It is a bent structure, and so it has a dipole moment. If it is hit with an IR photon, it will bend out of shape. Let's say that it straightens out, so that there is no dipole moment. This resonance will have an associated frequency that can be seen as an absorption band in an IR spectrometer.

 

This article describes the effects pretty well:

 

http://en.wikipedia.org/wiki/Infrared_spectroscopy

[swansont]

Classically light looks like radiation you'd get from an oscillating electric dipole, and so in order to conserve angular momentum, the systems have to change their dipole moment. Quantum-mechanically this shows up as selection rules based on the photon having one unit of angular momentum.

[orpheus]

Hey. Thanks to you both. Now, as you suspected, my question originates from attempting to understand exactly what IR spectroscopy is based on. I realized that the absorption rate and frequency would be unique for certain sructures, and therefore the method can be used as detection. But what I was not sure about was what happens on the more basic level.

 

And now I believe, if I am not mistaken, that when the dipole receives the photon, it receives energy. Then it compensate the energy overload with either streching or bending. These compensate for the energy load because the atoms move to positions that that would not otherwise be energy favourable? Is this it?

 

How does one calculate the amout of energy? For instance, if my sample is anhydrous hydrochloric acid (a dipole) and it is bombed with IR. The diatomic HCl can be considered a harmonic oscillator, as mentioned above, and the potential energy depends on the distance of the poles. These then can change either beause of geometrical (bending) reasons or because of the streching (distance). Shoud it not be calculable then?

[big314mp]

In chemistry class, we used a program called "Spartan" that did some mysterious QM calculations and gave back those answers. It would tell the approximate absorption, the location of the peak, and what type of motion the atom was undergoing.

 

It was a slow approach, and very computationally intensive (running on fast workstations, a "rough estimate" calculation of diethyl ether took 10+ minutes).

 

Stretching vibrations can be roughly modeled using the spring equation. It's only an approximation, but it holds true reasonably well. It can give some insights into why some frequencies are lower or higher than others, etc.

[orpheus]

There's a little bit more information on this here:

 

http://www.spectroscopynow.com/coi/cda/detail.cda?id=18428&type=EducationFeature&chId=2&page=1