Motion in the solid state studied by NMR and extended time scale MD simulation

Abstract

The ability to characterise dynamic processes in the solid state is crucial to our understanding of many important materials. Nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulations are two, highly complementary techniques that can be used in this pursuit, with NMR providing robust measurements of kinetic parameters across a large range of time-scales while MD can give insight into the form that the motion takes. The aim of the work presented in this thesis has been to demonstrate the extent of this complementarity by combining both techniques to investigate interesting systems, and also to expand upon it by implementing extended time scale MD methods that allow slower dynamic processes to be accurately simulated.

Atomistic simulations of a urea inclusion compound and of octafluoronaphthalene (OFN), a molecular solid, illustrate the dynamic range and complicated nature of motions that can be present in solid phases of matter, and the inherent difficulty of modeling them without explicit knowledge of their form. Whilst the MD simulation can provide this information, there are computational limits to the range of time-scales it can conventionally access. The OFN system provides an example of this limit, as slow molecular motions observed by NMR experiments are shown to be inaccessible to long, ambient temperature simulations. To combat this deficiency, the metadynamics extended time scale technique has been implemented, allowing rare dynamic events to be observed in very short simulations, and the effects of complex correlated motions to be explored.