why rotational in vibrational??

[photon79]

Hello,
I recently had an exam in Molecular physics, in which one of the unanswered questions asked by my proff was "How come there are rotational transition lines in a vibrational spectrum? because we observe vibrational spectrum in infrared level radiation and rotational in micro wave level ,,then how rotational lines are incorporated in vibrational spectrum"??

please let me know because I have to give this exam again!!!

[Creator]

Hello,
I recently had an exam in Molecular physics, in which one of the unanswered questions asked by my proff was "How come there are rotational transition lines in a vibrational spectrum? because we observe vibrational spectrum in infrared level radiation and rotational in micro wave level ,,then how rotational lines are incorporated in vibrational spectrum"??



Because rotational transitions occur at the same time as vibrational. As you've indicated, rotational energies are an order of magnitude lower that vibrational; so with sensitive enough equipment (with high enough resolution) one can observe a 'fine structure' (rotational energies) within the vibrational sprectrum.

See here for explanation:http://www.answers.com/topic/rotational-spectroscopy

Creator

[Claude Bile]

I would suggest looking at the energy level diagram and rationalise what peaks correspond to what transition.

When the molecules jump vibrational levels, they are, at the same time jumping rotational levels as well. In addition to being bound by vibrational selection rules, they are also bound by rotational selection rules for every transition that is made.

Essentially a purely rotational transition occurs in the microwave region, BUT a combined rotational and vibrational transition (or rovibrational) occurs in the infrared (commonly, there are exceptions).

I hope that sort of answers your question. If you have further questions, post them here.

Claude.