Determination of crystal structure of metastable anthracene by a novel method

Abstract

A combination of transmission electron microscopy (TEM) and ‘atom–atom’ computational procedures^1,2 developed for molecular crystals has led to the discovery and the determination of the crystal structure and lattice dynamics of a metastable, triclinic phase (hereafter designated, II) of anthracene. The new form, which is only 2 kJ mol⁻¹ less stable than the monoclinic, thermodynamically stable parent phase (I), may be generated by the application of compressive force or shear stress approximately perpendicualr to the basal plane of I. In essence, the size and shape of the unit cell into which molecules of anthracene are packed is determined from the electron diffraction patterns of the new phase. From the symmetry and space group, determined by TEM, assuming that the individual molecules are as undistorted in II as in I, A ‘trial structure’ of the ‘new’ phase may be computed by minimising the total energy as a function of the moleculear packing characteristics, knowing the empirically determined³ atom–atom potentials between all pairs of non-bonded atoms. The structure is refined by varying the molecular coordinates, as described below, so as to arrive at a set of lattice vibrations^4,5 all of which are real in the entire Brillouin zone. The enhanced photo reactivity of phase II compared with I is explicable in terms of the resulting crystal structure.