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

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.