THE IMPORTANCE OF CONSIDERING GROWTH-INDUCED CONFORMATIONAL CHANGE INPREDICTING THE MORPHOLOGY OF BENZOPHENONE

The underlying crystal-growth theory and structural molecular chemistr y important in our understanding of the methodology behind the theoret ical prediction of crystal morphology is presented together with its a pplication to the molecular solid benzophenone. Benzophenone crystalli zes 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 alo ng the c crystallographic axis. A comparison of this observed morpholo gy 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 tem peratures reveal that only the {110} crystal form should grow below th e roughening transition, in direct contradiction to the experimental d ata. These discrepancies are rationalized through a consideration of t he change in the molecular conformation experienced by the benzophenon e molecule during the growth process, as revealed from a comparison be tween the crystallographic structure and that calculated using semi-em pirical molecular-orbital methods for the free molecule. Examination o f the molecular packing in the solid state reveals that this conformat ional change is easier to accommodate in the {hk0} and {00l} faces whe re the surface binding sites are unconstrained and where the number of significant atom-atom interactions is small. This is in contrast to t he pyramidic {hkl} faces where the conformational change appears to re sult in stronger surface adsorption which leads, in turn, to an undere stimation of the surface attachment energy if the molecular arrangemen t in the solid-state structure is assumed. The implication of this add itional conformation-change enthalpy involved in crystal growth at the crystal-solution interface for the proportionality, surface/bulk equi valence and equivalent wetting assumptions used in modelling theory ar e discussed.