THE IMPORTANCE OF CONSIDERING GROWTH-INDUCED CONFORMATIONAL CHANGE IN PREDICTING THE MORPHOLOGY OF BENZOPHENONE

The underlying crystal-growth theory and structural molecular chemistry important in our understanding of the methodology behind the theoretical prediction of crystal morphology is presented together with its application to the molecular solid benzophenone. Benzophenone crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a well defined morphology dominated by large {110} faces with smaller {021}, {011}, {101}, {111}, {002} and {020} faces, and forms in a habit elongated along the c crystallographic axis. A comparison of this observed morphology with that predicted from lattice geometry, PBC analysis, attachment energy and Ising models reveals a much more squat habit with only the {110}, {011} and {101} forms predicted. Calculations of the Ising temperatures reveal that only the {110} crystal form should grow below the roughening transition, in direct contradiction to the experimental data. These discrepancies are rationalized through a consideration of the change in the molecular conformation experienced by the benzophenone molecule during the growth process, as revealed from a comparison between the crystallographic structure and that calculated using semi-empirical molecular-orbital methods for the free molecule. Examination of the molecular packing in the solid state reveals that this conformational change is easier to accommodate in the {hk0} and {00l} faces where the surface binding sites are unconstrained and where the number of significant atom-atom interactions is small. This is in contrast to the pyramidic {hkl} faces where the conformational change appears to result in stronger surface adsorption which leads, in turn, to an underestimation of the surface attachment energy if the molecular arrangement in the solid-state structure is assumed. The implication of this additional conformation-change enthalpy involved in crystal growth at the crystal-solution interface for the proportionality, surface/bulk equivalence and equivalent wetting assumptions used in modelling theory are discussed.