Conformational polymorphism. 5. Crystal energetics of an isomorphic system including disorder

The methodology for investigating the influence of crystal forces on molecular conformation has been extended to an isomorphous system, involving chemically similar species which crystallize in essentially identical structures. The model systems chosen for this study are p=methyl-N-(p-chlorobenzylidene)aniline (CLME) and p-chloro-N-(p-methylbenzylidene)aniline (MECL), which crystallize in the monoclinic space group P2₁/a with two molecules in the cell. Calculations involving the lattice energy minimization of the two structures were carried out to understand why the unstable planar conformation is stabilized by the lattice, although there are dimethyl- (MEME) and dichloro-substituted (CLCL) N-benzylideneanilines (BA) that contain lower energy molecular conformations. Furthermore, lattice energy calculations have been applied to hypothetical structures which are based on computationally substituting MEME and CLCL molecules into the CLME and MECL structures to determine whether it is possible that these analogues will pack as isomorphs of the system studied here, and to reveal the role of the substituents on disorder. Two different potential functions were applied (6-12 and 6-exp), both of them yielding lower energies for MECL than for the hypothetical structures and the lowest energies in comparison with minimized lattice energies of all BA compounds investigated to date. Analysis of the partitioned partial atomic energies was carried out to examine the similarities and differences in packing between the two isomorphs and the hypothetical structures. The relative stability of MECL arises from the favorable energetic environment of the ring, especially due to methyl and chlorine substituents.