Infrared Studies of Crystal Benzene. IV. Absolute Intensities

Abstract

The intensity of the absorption due to v20 at 1036 cm—1 in solid benzene at —170°C has been measured relative to the intensity of this band measured with the same benzene film in the liquid phase at 25°C. From this ratio and from the value for the absolute intensity of v20 in liquid benzene reported by Hisatsune and Jayadevappa, a value of 670±90 darks for the absolute intensity of v20 in solid benzene has been obtained. Using the relative intensity measurements reported earlier, the absolute intensities of all the gasphase-allowed fundamentals have been estimated. All the intensities in the crystal are lower than the values in the liquid phase, but the behavior of the bands at 1036 and 1480 cm—1 is not too abnormal. However, the intensities of the fundamentals at 687 and 3060 cm—1 decrease markedly from the gas-phase values, the former by a factor greater than two, the latter by a factor of seven. Thus, it is concluded that the interactions in the solid which cause the intensity to change are quite specifically limited to these two normal modes. Analysis of the data using the isotope intensity sum rules modified for use with crystals reveals that the intensity changes observed are much greater than those expected due to simple distortion of the norma coordinates by the crystal field.