Main Question or Discussion Point
Why is it not possible to obtain a pure vibrational spectrum(IR-spectrum) of a molecule? (vibrational spectrum always contains lines of rotational energies)
But rotational energies or in microwave region and vibrational are in infrared!Gokul43201 said:That's because typical rotational energies (or energy eigenvalues) are of the same order of magnitude as the energies for vibrational and bending (or "breathing") modes for most common organic molecules.
Why is it so that rotation must be excited inorder to excite vibration?Gokul43201 said:How do you excite a molecule to undergo vibrations without undergoing rotation ? If I'm not mistaken, in most cases, that would require a violation of angular momentum conservation.
What are some typical values for the energy/wavelength of the principal rotational mode vs. the principal vibrational mode. I thought they were maybe, an order of magnitude apart, not much more. Remember, the IR region and microwave region actually have an overlap.photon79 said:But rotational energies or in microwave region and vibrational are in infrared!
I think, and my recollection is not great, that the main concern here is conserving angular momentum. Imagine an large model of an asymmetric molecule like H2O, floating in the air. Now fire a ball at it and hit the molecule, so that the ball essentially stops after hitting the molecule. Depending on the orintation at which the ball hits the molecule, it will excite different vibrational modes, but will also set the molecule spinning. If the molecule didn't spin, angular momentum would not be conserved.Why is it so that rotation must be excited inorder to excite vibration?
Finally how this angular momentum is conserved by the excitation of rotation?Gokul43201 said:I think, and my recollection is not great, that the main concern here is conserving angular momentum.