Vibrations of Free Molecules - 2. Forms of Vibration of the Methyl Group in Propene


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Vibrations of Free Molecules - 2. Forms of Vibration of the Methyl Group in Propene
Alternative Title
Schwingungen freier Moleküle - 2. Schwingungsformen der Methylgruppe in Propen
Schrader, Bernhard
Schneider, Richard
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C 1210
IWF (Göttingen)
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Film, 16 mm, LT, 71 m ; F, 6 1/2 min

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The movement of the atoms during the more important normal vibrations of propene are shown in perspective representation: the stretching vibrations of the C=C double bond and the stretching and bending vibrations of the CH[3]-group. The in-phase and out-of-phase movements as well as the concept of characteristic vibrations become clear in the process.
molecular structure
oscillations / in molecules
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Computer animation
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Vibrations of Free Molecules - Vibration of the Methyl Group in Propene This is the formula of a more complicated molecule: propene. Since the molecule contains nine atoms, 21 normal vibrations are possible. In each
propene molecule the nearest hydrogen atom of the methyl group lies in the plane of the molecule.The C= C double
bond stretching vibration is infrared- and Ramanactive.You may observe the
large variations in the length
of the C= C double bond and also the simultaneous changes in many bond angles.The methyl group has nine possible vibrations: There are three stretching vibrations, three scissors vibrations, and two rocking vibrations. Two of each type of vibrations are degenerate; that is, they have the same frequency. There is also one twisting vibration.At about 3000 wave numbers we find a symmetric and two degenerate
stretching vibrations of the methyl
group. Let us now look at one of the degenerate stretching vibrations. The motions of the CH-bonds nearest to and farthest from us are outof-phase.The
CH 3 scissors vibrations occur at about 1400 wave numbers. We
shall first observe the symmetric scissors vibration.You can clearly see that the length of the neighbouring C-C bond also vary. All the atoms move a little.This is one of the degenerate deformations of the methyl group.While the angle between the two CH-bonds pointing away from
us increases, the angle with the CH--bond pointing towards us
decreases and vice versa. The rocking vibration ist only visible in the infrared spectrum.Apart from
the rocking motion of the CH 3 group, you can sec that the CH 2 group on
the left moves in the
opposite direction. The intensity of
the torsional vibration of the CH 3 group is extremely small in the infrared and the Raman spectrum.You can see clearly
that the left und right
hand parts of the molecule execute a torsional motion in opposite directions.


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