Calculation of Birefringence Changes in Anthracene and Phenanthrene Crystals

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Abstract
Birefringence changes due to thermal expansion have been calculated for anthracene and phenanthrene. The method assumes that the effective molecular polarizability calculated at room temperature is constant in the molecular axis system for small changes of molecular orientation and separation. For anthracene, the calculations reproduce the observed birefringence change quantitatively. For phenanthrene, two thirds of the observed change is due to thermal expansion, the rest to structural alterations at the 70°C phase transition; transition to a Pa structure is ruled out. Birefringence calculations could be used to test other high-temperature structures for phenanthrene which might be suggested by lattice-energy calculations.