ChemInform Abstract: CONFORMATIONAL POLYMORPHISM. THE INFLUENCE OF CRYSTAL STRUCTURE ON MOLECULAR CONFORMATION

Abstract

A methodology for the study of the influence of crystal forces on molecular conformation has been developed and ap- plied. As part of this study the phenomenon of conformationalpolymorphism, in which a molecule adopts significantly differ- ent conformations in different crystal polymorphs, is elucidated. A combination of quantitative analysis of the molecular pack- ing in different space groups along with ab initio molecular orbital calculations is used to analyze these phenomena. The model system p-(N-chlorobenzy1idene)-p-chloroaniline (a Schiff base) was chosen for study. Lattice energy calculations involving the minimization of the energy of the triclinic and orthorhombic crystal forms of this molecule were carried out in order to ex- plain the stability of the former lattice in which the unstable planar conformation of the molecule obtains. Three different po- tential functions were employed (6-9, 6- 12, and 6-exp), in order to avoid potential-dependent conclusions. All potentials yielded lower (more negative), for the triclinic lattice, in agreement with the experimental observation. The conforma- tional energy of the isolated molecule was studied by molecular orbital methods using both minimal and split valance basis sets. The energy differences obtained from these calculations are in good agreement with the differences in lattice obtained from the crystal calculations. A of the lattice energy into partial atomic energies was performed in order to carry out a detailed analysis of the packing differences between the polymorphs. The introduction of this partitioning proved to be a powerful probe for analyses of the energetics of different crystal packing modes. The approach employed, including packing analysis and crystal energetic studies to conformational polymorphs, yields much information as to the nature of the crystal forces in the different polymorphs and promises to be a useful tool in the investigation of the role of these forces in in- fluencing molecular conformation.