Conformational polymorphism. 4. Crystal energetics of a trimorphic system including disorder

Lattice energy minimization techniques have been applied to study the influence of crystal forces as well as orientational and positional disorder on molecular conformation. We have extended the methodology developed previously, which took advantage of the phenomenon of conformational polymorphism - the existence of crystal forms of the same molecule in significantly different conformations. The compound chosen for this study is the trimorphic p-methyl-N-(p-methylbenzylidine)aniline, which adopts different conformations in the different polymorphs. Two of the polymorphs (I and III) exhibit disorder of two different types, while polymorph II is not disordered. Lattice energy calculations on the three polymorphs were performed in order to account for the relative stability of forms I and III, in which the unstable planar conformation has been adopted. Furthermore, we evaluate the influence of disorder on molecular conformation in this system. The minimizations were carried out with three different potential functions and yield the highest energy for form II, with much lower energies for the other two forms. The minimized lattice energies are analyzed in terms of partial atomic energy contributions to the total energy, and it is shown that the relative energetic contributions of various groups of atoms to the total energy are the same for the three polymorphs, indicating that the energetic environments of these groups in these polymorphs are similar. The stability of form I and form III with respect to form II is due to the relatively favorable environment of the bridge atoms, demonstrating the important role of the disorder to the stabilization of the enegetically less favorable planar conformation.