THE INFRARED ABSORPTION ANISOTROPY OF $IODOFORM^{*}$

Abstract

The polarized infrared spectrum of crystalline iodoform has been obtained using crystals of thicknesses from 50 to 200 microns. Spectra were recorded with the crystals oriented with respect to the electric vector by an x-ray diffraction type goniometer, and the anisotropy of the spectrum was studied carefully as a function of angle of orientation. A complete assignment of all fundamentals was possible, including at least two of the lattice frequencies of the crystal. The anisotropic behavior of $v_{1}$, the $C-H$ stretching fundamental, is such that it is possible to state that the Pimentel’s ``oriented gas model” holds within well-defined limits. It is demonstrated that a much better procedure of locating transition moment directions in crystals than the use of ``dichroic ratios” is the complete goniometric study of the absorption anisotropy. Finally the threefold degenerate first overtone of $v_{4}$, the C-H bending mode, was split by the local (site group) field and the intensity ratio of its members is reported.