Structure and dynamics of a new phase of anthracene

Abstract

A new triclinic phase of anthracene (II), which cannot be isolated in the pure state, has been identified by analysis of the intensities and symmetry of the electron diffraction patterns (recorded at low temperature) exhibited by stressed specimens of the monoclinic, thermodynamically stable form (I). A number of trial structures were derived on the basis of both packing calculations (using the “atom–atom” approach) and observed topotactic relationships between II and I. Lattice dynamical computations were used to refine the structure and, in particular, it was possible to compute the i.r. and Raman active vibrations of the two forms. The atomic coordinates and temperature factors of the new structure (P1, a= 8.342 Å, b= 5.892 Å, c= 11.282 Å, α= 123.34°, β= 96.70°, γ= 85.91°, z= 2, V= 460.07 ų) are given and so are some of the thermodynamic properties, also computed, of the phase II. The influence of temperature and pressure upon phase II (which has a cohesive energy only 2 kJ mol^–1 less than that of I), along with its importance in the solid-state photodimerization process, is also discussed.