Measurements with crystals using polarized infra-red radiation

A method is described for the measurement of the infra-red absorption spectra of crystals and other crystalline materials using polarized radiation. From the variation in intensity of vibration bands as the crystals are rotated with respect to the incident radiation vector, a partial determination of the crystal structure can be made, and the results can be compared with those obtained from X-ray measurements. Alternatively, in some cases, an assignment of vibration frequencies to particular classes of normal modes can be made. The conclusions agree with those reached by X-ray analysis in most cases, but in para-dinitrobenzene crystals it is suggested that the nitro groups are slightly twisted out of the plane of the aromatic rings. Some general factors affecting the scope of this method are considered.