Infrared Studies of Mixed Crystals: Naphthalene in Anthracene

Abstract

Polarized infrared studies have been made using single crystals of dilute solid solutions of naphthalene in anthracene in the spectral region 755 to 830 cm−1. The ab plane at normal incidence was studied with the plane of polarization either parallel or perpendicular to the b axis. The mole fraction of naphthalene was varied from 0.009 to 0.029. The contour of the naphthalene band at 787 cm−1 shows a dependence upon concentration, approaching the contour observed in pure naphthalene. There is an increasing dichroic splitting of the band which is attributed to space group coupling as the number of nearest identical neighbors increases. It is likely that the magnitude of this splitting in naphthalene is only about 5–10 cm−1 and that the crystal forces in naphthalene as well as in the mixed crystals of naphthalene in anthracene are sufficiently weak that the ``oriented gas'' model is applicable. Using this model, the 787 cm−1 band of naphthalene is assigned as a single fundamental of either B1u or B3u class.