Vibrations of Free Molecules - 1. Stretching and Deformation Vibrations of Ethylene

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Vibrations of Free Molecules - 1. Stretching and Deformation Vibrations of Ethylene
Alternative Title
Schwingungen freier Moleküle - 1. Valenz- und Deformationsschwingungen in Äthylen
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Film, 16 mm, LT, 78 m ; F, 7 min

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The movements of the atoms during the more important normal vibrations of ethylene are shown: the stretching vibrations of the C=C double bond and the stretching and bending vibrations of the CH[2]-groups. The in-phase and out-of-phase movements as well as the concept of characteristic vibrations become clear in the process.
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Vibrations of Free Molecules - Stretching and Deformation Vibrations of Ethylene The ethylene molecule has six atoms. Therefore there are 3 x 6 - 6 = 12 normal vibrations.Above is the infrared spectrum of ethylene, and below it you see the Raman spectrum. Five of the normal vibrations can be seen as bands in the infrared spectrum. A further six are visible in the Raman spectrum. The position of one of these bands is uncertain. The twelfth normal vibration is inactive in both spectra.The vibrations can be divided into stretching vibrations and bending vibrations. There is one C-C stretching vibration and fhere are four C-H stretching vibrations. In addition there are four in-plane bending vibrations and three out-of-plane ben-ding vibrations of the CH 2-groups.The stretching vibration of the carbon-carbon double bond
is only visible in the Raman spectrum. It occurs at
1623 wave numbers.Here is the ethylene-molecule drawn in perspective. You will now see vibrations of this molecule, excited by one energy quantum h x n; for the sake of clarity the amplitudes of the vibrations
have been magnified by a factor of five.You can see the large variations in the distance between the carbon atoms ... In phase with this motion the angle between the CH-bonds in the CH 2-group also varies.The four CH-stretching vibrations are
visible as four bands in the infrared and Raman spectra at about 3000 wave numbers.The two symmetric stretching vibrations of the CH 2-group occur at lower
frequencies than the antisymmetric vibrations.Here
is the Raman-active symmetric stretching vibration of the CH 2-group.In this vibration, all the CH-bonds stretch simultaneously. The bond between the carbon atoms also stretches slightly,
but out-of-phase with the motions
of the CH-bonds.Two of the in-plane bending vibrations - the
so-called scissors - occur at about 1400 wave numbers in the Raman and infrared spectra. This is the Raman-active scissors
vibration.In this vibration the CH 2-angles in both groups vary simultaneously. The length of the double
bond also varies, but out-of-phase.The
two other in-plane bending vibrations
are called rocking vibrations. Both
give only very weak bands. Here is the . Raman-active rocking vibration. In this vibration,
only bond angles in the plane of the molecule change. The wagging vibrations are very strong in the infrared, but very weak in the Raman spectrum. You will now see
the infrared active vibration. The
wagging vibration also causes only changes in bond angles, but in this case perpendicular to
the plane of the molecule.


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