A study of the high-pressure polymorphs of L-serine using Ab initio structures and PIXEL calculations

Abstract

Polymorphs of L-serine have been studied using ab initio density functional theory for pressures up to8.1 GPa. The SIESTA code was used to perform geometry optimisations starting from the coordinatesderived from high-pressure neutron powder diffraction. Between 0 and 8.1 GPa two phase transitionsoccur, the first of which takes place between 4.5 and 5.2 GPa and the second between 7.3 and 8.1GPa.A change in molecular conformation occurs during the I-to-II transition, resulting in a stabilisation inintramolecular energy of 40 kJ mol1. There is good agreement between the theoretical andexperimental coordinates, and the largest root-mean-square deviation between experimental andoptimised structures is 0.121 A . Analysis of the effect of pressure on the intermolecular interactionsusing the PIXEL method showed that none becomes significantly destabilising as the phase-I structureis compressed. It is proposed that the phase transition is driven by attainment of a more stableconformation and from the reduction in the molecular volume. The second phase transition occurs withonly a small change in the hydrogen bonding pattern and no substantial difference in molecular conformation. The effect on the energies of attraction between molecules suggests that this transition isdriven by the bifurcation of a short OH/O interaction.