Dynamics of the α and β Polymorphs of dl-Norleucine at Different Temperatures: Sliding to a Partial Phase Transition

Abstract

The enantiotropically related α and β polymorphs of dl-norleucine are an interesting probe for polymorphism in molecular crystals. We present the results of quantitative molecular dynamics simulations of these two polymorphs at both stable and metastable temperatures. Because of a fully flexible force field, we can judge the differences in crystal and molecular properties between the polymorphs. In simulations of the β polymorph at 350 K, we observe partial phase transitions which we could follow with the help of specifically designed order parameters. The transitions are exclusively occurring along b′. This indicates a possible transformation mechanism in which first shifts of bilayers occur in this direction, followed by shifts along a′. The transformed lattice parameters and molecular properties behave identically with temperature for the two polymorphs. Consequently, the polymorphs only differ in the orientation of the molecular bilayers, which explains the ease of transitions between them. The partial transitions consist of different types which differ in the number of interfaces involved and in the amount of distortion of the crystal. At the time scale of the simulations, we did not observe a full polymorphic phase transition.