Computational approach to the study of extended defects in molecular crystals. Part 2.—Structural changes at planar faults—their importance in facilitating photodimerization and in governing stacking fault energies

Abstract

Using an approach based on the pairwise evaluation of non-bonded interactions it has proved possible to estimate the extent of structural changes that occur at an extended planar fault (e.g., stacking fault or antiphase boundary) of a prescribed type in organic molecular crystals. In so doing it is also possible to estimate the magnitude of the extra energy that has to be invested into a crystal in order to accommodate such a fault. The calculations are illustrated by reference to a substituted anthracene, 1,8-dichloro-9-methylanthracene on which some experimental information relating to extended defects was already available. The way in which the fault energy is diminished as a result of allowing the molecules in the vicinity of the fault to “relax” and the lattice to expand is demonstrated. In particular there are indications that for a (100) fault plane the lowest energy is achieved by incorporating a translation vector of 1/10 [250] and a small degree of folding of the constituent molecules in and adjacent to the plane of the fault. Such structural relaxation gives rise to incipient dimer pairs in which contiguous molecules, oriented in trans registry and with the C₉—C_10′ distance in the range 4.5–4.7 Å are “preformed”. These facts help to explain the facile solid-state photodimerization of 1,8-dichloro-9-methylanthracene to yield the trans dimer, since the crystal structure of the perfect solid implies that this material should be light stable (the C₉—C_9′ or C₁₀—C_10′ distances being > 7 Å and contiguous molecules along 〈100〉 direction are in cis registry).

The general application and limitations of the computational approach to the elucidation of extended structural faults in organic solids generally are also discussed.