Rationalising the difference in crystallisability of two sulflowers using efficient in silico methods

Abstract

The molecular structures of the first and second generation sulflowers, sulflower and persulfurated coronene (PSC), are remarkably similar: carbon ring structures decorated with sulfur atoms, without any additional moiety. However, their crystallisability is starkly different, with sulflower easily forming well-characterised crystals, but with PSC only resulting in amorphous forms, despite extensive experimental efforts. Here this phenomenon is investigated using crystal structure prediction (CSP) methods to generate plausible structures on the lattice energy surface for both systems, followed by molecular dynamics and well-tempered metadynamics to investigate their persistence at finite temperature. Coherently with experimental observations the sulflower experimental form emerges as exceptionally stable under ambient conditions and persists in all dynamic simulations. However, all PSC structures transition to amorphous phases when subjected to a small amount of work. While CSP methods are commonly used to identify a shortlist of structures that a molecule could plausibly crystallise into, this work demonstrates, for the first time, the ability of in silico methods to predict whether a molecule can crystallise into any structure at all.