Abstract
Abstract The drug benzocaine is bimorphic at 300K. Form (I) is monoclinic and form (II) is orthorhombic. Both show highly structured diffuse scattering. X-ray data were analysed using MC modeling, assuming the scattering to be purely thermal in origin. This indicated the presence of highly correlated molecular motions. In both forms broad diffuse streaks were observed in the 0kl section indicating longitudinal displacement correlations extending over ~50 Å. In the hk0 section streaks between Bragg peaks indicate correlated motions of hydrogen-bonded coplanar pairs of chains forming molecular ribbons. The main difference between the two forms is in the dynamical behaviour of these ribbons and how they are able to slide relative to each other. For form (I) a model involving harmonic springs was able to describe the motion satisfactorily but for form (II) inclusion of anharmonic effects was necessary. These proved to be precursors of a new low temperature phase, form (III).
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