The Basis of Orientation Measurements by Infrared Spectroscopy

The absorption of infrared radiation occurs to a maximum degree when the direction of the electric vector of the radiation is parallel to the direction of the dipole moment changes involved in the various vibrational modes of the absorbing molecule. If the direction of the electric vector lies at an angle to the direction of the dipole movement change, the component of the former resolved along the latter direction is involved in the absorption process, which thus occurs less strongly. This is the directional property that makes infrared spectroscopy useful for orientation studies. 

This directional property is not usually a pertinent factor in determining the intensities of the bands in an infrared spectrum, because the radiation is not polarized and the direction of the electric vector is random in the plane perpendicular to the propagation direction. Furthermore, in solutions, and in many solid samples, the absorbing molecules are randomly oriented. 

Fisore 7.1 The rdationship between the directions of incidence and the vibrator in a polymer 

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