[harkkam]

MY teacher told me that molecular vibrations cause the IR spec to give us readings. I wasn't happy with that definition so I went on wikipedia to do my own reading and gained some more knowledge but still unclear of a few things.

So IR spec works by shooting diff wavelengths of light and measuring the amount of light that transmits through the sample and plotting it on a graph.

Now is it the IR Beam that cause's the functional group to vibrate in certain ways like wagging or shaking or w/e which then absorb the IR energy and gives off a signal to the machine.

Or are molecules that have a dipole naturally on their own just always stretching and bending at certain frequencies, and it is this vibration that causes the molecule to absorb the IR energy.

Thank you.... 

[Borek]

IR of the proper length (energy, frequency) is absorbed by the molecule (bond) - and it is almost immediately emitted back. Trick is, other frequencies go straight through the sample, these absorbed - when emitted - go in all possible directions. That's why the intesity of the radiation going through is slightly lower, that's also why you can observe radiation emited to sides.

Molecules always vibrate, however, every bond have many charactersitic vibrations (just like guitar string). If the amount of energy is correct, bond 'jumps' to the higher vibration frequency.

[harkkam]

Okay. i see

So the molecules are constantly vibrating and certain radiation will cause the energy in the bond to jump to a higher level.

When the IR beam passes through, some energy goes right through will other energy is absorbed and the energy level of the bond is jumped and then the radiation is emitted to all sides.

This is recorded on the computer.

I hope i got it right

[Squirmy]

I think you have it right...just some extra comments/questions for you.

Or are molecules that have a dipole naturally on their own just always stretching and bending at certain frequencies, and it is this vibration that causes the molecule to absorb the IR energy.

I would say they are naturally vibrating at their ground state, but the absorption (and excitation to higher vibrational state) is a response to the IR radiation rather than the vibrations causing absorption.

Note that there doesn't have to be a molecular dipole for IR absorption to occur, but a change in dipole when the vibration occurs. CO₂ has no net dipole, but it is IR active (btw, that's what makes it a "greenhouse" gas). The reason is that it can vibrate asymmetrically...for instance, one oxygen moving away from the central carbon while the other moves toward it.

Something to think about: why does IR absorption require a change in dipole moment?  What happens on a molecular level when the electromagnetic radiation (light) is absorbed?

[harkkam]

Hmm...well I think that a dipole is needed for IR absorption because,

with no dipole there is symmetry and when the IR energy is absorbed it does not shoot the particles to the sides of the machine were they are to be detected.

With a dipole perhaps the molecule can assume unsymmetrical shapes and with IR energy fired toward it creates more vibration that shoots the radiation in all different directions causing a net decrease in radiation absorbed at the reader plate.

Thats my guess.

When electromagentic energy is absorbed I believe that since energy is quantized that only certain wavelengths have the right amount of energy to take the electrons from a lower energy shell and promote them to higher shells further away from the nucleus imparting energy to the system as a whole.

[Squirmy]

Well, like everything else in chemistry the reason ultimately comes down to quantum mechanics. The explanation is usually a bunch of Hamiltonians and other math I've long since forgotten followed by a statement like, "So, clearly IR absorption requires a change in dipole moment"

Fortunately, you can look at it in a more classical mechanics way. As the name implies, electromagnetic radiation is comprised of an electric field and a magnetic field, both of which are sinusoidal/oscillating waves.

The partial charges in the molecule are either attracted or repulsed by the electric component, but since it's an oscillating wave, the molecule also oscillates. We just call it stretching, bending, wagging, etc. depending on how the molecule is distorted in the oscillation.

[harkkam]

hmmm...

so The IR beam is oscilating and when the naturaly vibrating molecule absorbs the IR energy the molecule oscilates at a higher frequency.

The IR beam is carrying charge I assume then?

[Squirmy]

No, light doesn't carry a charge. Check out the intro in this wiki, particularly the first and third paragraphs: http://en.wikipedia.org/wiki/Electric_field

[FeLiXe]

by the way, another addition to what happens with the absorbed energy: I don't think it is true that most of it is emitted again to different directions. the molecule will collide with other molecules on a shorter time scale than emission. and there should be a lot of energy exchange with different molecules and the energy is distributed this way and the sample heated up. of course the sample will emit some thermal radiation. but the thermal radiation should be related to the temperature of the sample independent of the way it was heated up