Calculated molecular orientational disorder in anthracene crystals

Abstract

Calculations based on an atom–atom intermolecular potential suggest that there are at least three crystal structures of anthracene close in energy to the observed P2₁/a phase. Bulk concentrations of misoriented molecules up to 10^–2–10^–3 mole per mole at room temperature are found by crude calculations and are associated with vacancies and dislocations. The misoriented molecules form strings of 10–100 units along the [010] direction. In zones of mixed packing photodimerisation, perhaps of vibrationally excited molecules, appears to be sterically possible.