[xshadow]

Hi!!!

How can i understand quickly if a vibrational mode is active or not in IR???
I mean a "graphic method" seeing the vibrational mode of the molecule...

For example if the vibrational modes of CO2 are:


How can say if the bending mode is IR active  or not??
I should see if there is a change in the molecular dipole momentum vector? but how can i see it in this figure (for the CO2 bending mode) 

And the change must be in the module of the vector or also a  direction  variation is worth for a IR active mode??

Please help me...is important !!!
Thaaanks

[Irlanur]

First of all, one (degenerate) mode is missing.

How do you determine whether a molecule has a permanent dipole?

[xshadow]

First of all, one (degenerate) mode is missing.

How do you determine whether a molecule has a permanent dipole?

HI!!!

TO determine if a molecule has a permanent dipole i need to see :

1) the chemical bond polarity...for example in H2O  the two bond H1-O and H2-O are polar covalent bond
 because O and H have a different electronegativity

So there are two dipole momentum vectors

2) Now to see if there is a permanent dipole i have to SUM all dipole vector  in the molecule.
If the sum isn't  zero there's a permanent dipole
In the case of H2O there are two momentum  dipole vectors BUT the sum is zero (due to the symmetry)

Now for CO2 i haven't a  permanent dipole in the "standard condition" (when it's linear) due to the symmetry.

But when CO2 vibrates i think  there is a change in the  electric dipole vector of the molecule... or not??? HOw can i imagine this looking that figure??
Thanks!

[Corribus]

Whether or not there is a permanent dipole doesn't matter. What matter is whether there is a mode of vibration that changes the molecular dipole moment. As often as not, this is easily determined via symmetry considerations.

[xshadow]

Ok!!

1) but how can i understand if the  bending vibrational mode of that figure  is active or  not in IR only looking that figure (without using the theory group) ??

2) What does it mean that a vibrational mode of a molecule isn't active in IR

That if a send a photon of the wrong frequency v_0 (ie the vibrational frequency of the harmonic oscillator associated with the vibrational mode )  the "vibrational mode" doesn't interact with  photon ?  ie the photon isn't adsorbed by the molecule so i don't see anything in the spectrum (for THIS vibrational mode at this frequency)

THANKS!!!

[Corribus]

1) but how can i understand if the  bending vibrational mode of that figure  is active or  not in IR only looking that figure (without using the theory group) ??

Imagine CO2 in its unbent state, such that it is linear. O=C=O. It should be easy to see that there is no dipole moment for this conformation. Now imagine you bend it, such that the two O's go up and the C goes down - like it's shaped like water, say. Can you see that there is now a dipole moment? So for a bending mode, you go from no dipole moment to a nonzero dipole moment. The dipole moment has changed. Therefore the bending mode is IR active.

Note: it's a doubly degenerate mode, because the bend can either be up/down (in the plane of the computer screen) or in/out (perpendicular to that plane).

2) What does it mean that a vibrational mode of a molecule isn't active in IR

That if a send a photon of the wrong frequency v_0 (ie the vibrational frequency of the harmonic oscillator associated with the vibrational mode )  the "vibrational mode" doesn't interact with  photon ?  ie the photon isn't adsorbed by the molecule so i don't see anything in the spectrum (for THIS vibrational mode at this frequency)

There are basically two conditions that must be met for a molecule to couple to (absorb) a photon - at least, there are two that apply here: 1. the energy must be exactly (in a first approximation) the same as the energy separating the lower and upper states. 2. The change induced by the absorption event must be spatially compatible with the directional orientation of the oscillating electromagnetic field of the photon. I have described what this means in another thread.

Here is a link to a description of what selection rules mean in a physical sense.

http://www.chemicalforums.com/index.php?topic=76737.msg279617#msg279617

Here is a link to this line of thought applied to CO₂ specifically. Although, there is some discussion of group theory in that thread, so it may be hard for you to follow.

http://www.chemicalforums.com/index.php?topic=73681.msg267339#msg267339

[xshadow]

Understand

Thanks a lot!!!