Polarized Infra-Red Radiation as an Aid to Crystal Analysis

The optical properties of crystals have been widely used as subsidiary data in X-ray structure analysis. Recent advances in infra-red spectroscopy now make it possible to extend the optical measurements into the region of molecular vibrational frequencies. Since certain infra-red absorption bands of organic molecules can be assigned to vibrations largely localized in certain substituent groups, or to vibrations of the molecule as a whole, a study of infra-red dichroism gives information about the orientation of the groups or molecules in the crystal. For example, if all the molecules in a crystal are equally oriented with respect to the crystal axes, then the minimum absorption of polarized infra-red radiation will be observed when the plane of polarization contains the direction of the change of electric moment produced by the molecular vibration responsible for the absorption. If the crystal contains two sets of mutually inclined molecules, the maximum absorption occurs when the plane of polarization bisects the obtuse angle between the two planes containing the directions of the electric moments and the incident beam. From the ratio of the maximum to minimum optical densities, the angle between the planes can be calculated. Information of this sort can be obtained with the aid of an extremely simple polarizing attachment to a standard infra-red spectro-photometer.