Infrared Studies of Mixed Crystals: Naphthalene-d8 in Naphthalene

Polarized infrared studies in the region 615 to 645 cm−1 have been made using dilute solid solutions of naphthalene-d8 in naphthalene. The ab plane at normal incidence was studied with light polarized either parallel or perpendicular to the b axis. The mole fraction of C10D8 was varied from 0.10 to 0.30. The dichroic splitting of the 632-cm−1 band observed in pure C10D8 decreases with concentration. This is the splitting predicted in the factor group analysis and is attributed to intermolecular coupling. This is a further indication that the magnitude of this splitting in molecular crystals such as C10H8 is only a few cm−1, and that the ``oriented gas'' model is applicable. Using this model, the 632-cm−1 band of C10D8 is assigned as a single fundamental of the B1u or B3u class.