The imaging of molecular motions has long been a gedanken experiment in the development of models explaining structure-function relationships and chemical reactivity. In this thesis, a roadmap to molecular movies proposes an interdisciplinary approach to these experiments, combining the three experimental techniques of gas electron diffraction (GED), time-resolved electron diffraction (TRED), and ultrafast electron diffraction (UED), and the computational fields of quantum chemistry, and software development.
At York a time-averaged GED apparatus, the only one of its kind in the UK, was relocated and recommissioned, allowing the equilibrium ground-state structures of 1,2-dithiane and 4-(N,N-dimethylamino)benzonitrile in the gas phase to be resolved. Furthermore, our TRED apparatus was upgraded with a new ultrafast laser system, custom-built solenoid lens, and optimised detector geometry, resulting in a 60% increase in the signal-to-noise ratio and improved spatial resolution.
Through collaboration with the UED group at the Stanford Linear Accelerator Center (SLAC), the ring opening dynamics of 1,2-dithiane were captured with sub- 200 fs temporal resolution, using the MeV UED apparatus at SLAC. Combined with non-adiabatic multi-reference molecular dynamics simulations, these results revealed an oscillatory ring-opening motion with a period of ~400 fs, and the presence of transient straight-chain species.
Molecular dynamics simulations were also used to investigate the photostability of asparagusic acid, the ring-opening motion of which was found to be mediated by the dynamics of the carboxylic acid group. Similar studies for 1,2- dibromotetrafluoroethane predicted the sub-100 fs cleavage of the C–Br and the formation of non-bridging radical intermediate, followed by a secondary C–Br cleavage of the anti conformers.
A suite of software tools has been designed to extract experimental data, analyse computational results, and combine the experimental and simulated domains into interpretable descriptions of molecular motion. This synergistic relationship between experimental and computational chemistry has allowed the capture of previously unseen motions.
