Basics of vibrational spectroscopy

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CH stretchings


Alkanes: νCH < 3000 cm-1

Alkanes are characterized by 2 specific stretchings.

For CH3 : V asym = 2962 +/- 10 cm-1, V sym = 2872 +/- 10 cm-1. For CH2 : V asym = 2926 +/- 10 cm-1, V sym = 2852 +/- 10 cm-1
Pictogram information
Vibrations are shifted if there is an O or N atom directly bound to the carbon.

In order to confirm the presence of an alkane group, its stretching must be correlated to bending vibrations.

Bending vibrations

Illustration of the angular deformations. For CH3 : Umbrella : sym = 1375 cm-1, asym = 1460 +/- 10 cm-1. For CH2 : shear = 1455 +/- 10 cm-1, rocking = 720 +/- 10 cm-1 (only present if more than 4 CH2 in a row.
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