The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges.

The solid-state phase transitions between the α, β and γ forms of DL-norleucine were studied using DSC, thermal stage polarization microscopy and solid-state NMR. Since the crystals consist of 2D hydrogen-bonded bilayers with van der Waals interactions between consecutive bilayers, the transitions occur in a layer-wise fashion with a propagating transformation front. The α↔γ transition at 390 K is a clear example of a first order transition with a relatively large enthalpy difference between the polymorphs and a small hysteresis, indicating the kinetic barrier for this transition is relatively small. In contrast, the α↔β transition is not reproducible in similar crystals and the enthalpy difference is very small. Both the α and β polymorphic forms can coexist in a "single crystal" over a large temperature range, apparently without enforcing stress, while the α↔γ transition propagates fast to relieve stress from the volume and conformational change. Moreover, the kinetics of the α↔β transition are much faster in single crystals than in powders, which is attributed to the inhibitory effect of defects on cooperative motion. The thermodynamic transition temperature of the α↔β transition is estimated between 253 and 268 K. This work also shows that traditional methods of polymorph screening might overlook some solid-state phase transitions similar to the α↔β transition in DL-norleucine.

We study the effects of temperature and sliding velocity on superlubricity in numerical simulations of the Frenkel-Kontorova model. We show that resonant excitations of the phonons in an incommensurate sliding body lead to an effective friction and to thermal equilibrium with energy distributed over the internal degrees of freedom. For finite temperature, the effective friction can be described well in terms of a viscous damping force, with a damping coefficient that emerges naturally from the microscopic dynamics. This damping coefficient is a non-monotonic function of the sliding velocity which peaks around resonant velocities and increases with temperature. At low velocities, it remains finite and nonzero, indicating the preservation of superlubricity in the zero-velocity limit. Finally, we propose experimental systems in which our results could be verified.

Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. DL-Norleucine is a molecular crystal exhibiting two enantiotropic phase transitions. The high temperature a 4 g transition has been shown to proceed through nucleation and growth [Mnyukh et al., J. Phys. Chem. Solids, 1975, 36, 127]. We focus on the low temperature b 4 a transition in a combined computational and experimental study.The temperature dependence of the structural and energetic properties of both polymorphic forms is nearly identical. Molecular dynamics simulations and nudged elastic band calculations of the transition process itself, suggest that the transition is governed by cooperative movements of bilayers over relatively large energy barriers.

Understanding solid-solid polymorphic transitions within molecular crystals on the molecular scale is a challenging task. It is, however, crucial for the understanding of transitions that are thought to occur through cooperative motion, which offer an interesting perspective for future applications. In this paper, we study the energy barriers and mechanisms involved in the β → α DL-norleucine transition at the molecular scale by applying different computational techniques. We conclude that the mechanism of the transition is a cooperative movement of bilayers through an intermediate state. The results indicate that local fluctuations in the conformations of the aliphatic chains play a crucial role in keeping the cooperative mechanism sustainable at large length scales. We have characterized the intermediate state.