Basics of vibrational spectroscopy

Navigation menu

CH stretchings

Alkanes: ν_CH < 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.

Click on each framed text for more information