MadSci Network: Chemistry
Query:

Re: What does in plane and out of plane mean

Date: Tue Nov 16 09:30:56 1999
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 942288307.Ch
Message:

What does in plane and out of plane mean?

When referring to the infrared spectrum of Benzene (aromatics) the "in plane hydrogen wag" occurs at approxamately 1030 and the "out of plane hydrogen wag" occurs at approxamately 660. What do these actually represent?


The in-plane and out-of-plane wags are bending motions of the C-H bond which are respectively in the plane of the molecule and perpendicular to the plane. They are shown schematically below:
in-plane wag out-of-plane wag
in-plane wag out-of-plane wag

Actually, the whole notion of individual bond vibrations is a crude approximation (but one that works pretty well). Molecules have vibrational modes which involve all their atoms and are governed by the symmetry of the molecule. In some vibrational modes certain atoms don't move; but in no case (with very rare exceptions) do you see a vibration involving only two atoms. See Cotton's Chemical Applications of Group Theory, which is almost certainly beyond the scope of your course!

Dan Berger
Bluffton College
http://cs.bluffton.edu/~berger



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