Evidence for a size dependent nucleation mechanism in solid state polymorph transformations

This study applies aimless shooting and likelihood maximization to determine the molecular mechanism in the solid state polymorph transformation in terephthalic acid from over 500 candidate order parameters. The crystals examined here extend the range of crystal sizes considered in our previous work (J. Amer. Chem. Soc. 2007, 129, 4714) and reveal a change in the mechanism with increasing system size. As the crystal size increases beyond that studied in our previous work, the polymorph transformation mechanism changes from a global distortion of the crystal to a local corner nucleation mechanism. In the corner nucleation mechanism, the interfacial area between the two polymorphs is minimized for a given nucleus size. However, this mechanism differs from classical nucleation theory in that the molecular level details are essential to describe the nucleation process, which involves nonspherical domains at the corner of the crystal. These new findings suggest that there is a range of sizes for which corner nucleation is the dominant mechanism of polymorph transitions, thus implying that different mechanistic regimes exist for nucleation based on crystal size. From a computational standpoint, this study demonstrates the utility of aimless shooting and likelihood maximization to identify nonintuitive reaction coordinates in complex systems.