Application of DFT and EMS to the study of strained organic molecules

Electron momentum spectroscopy (EMS) studies of the valence shells of [1.1.1]propellane, 1,3-butadiene, ethylene oxide and cubane are reviewed. Binding energy spectra were measured in the energy regime of 3.5-46.5 eV over a range of different target electron moments, so that momentum distributions (MDs) could be determined for each ion state. Each experimental electron momentum distribution is compared with those calculated in the plane wave impulse approximation (PWIA) using both a triple-zeta plus polarisation level self-consistent field (SCF) wave function and a further range of basis sets as calculated using density functional theory (DFT). A critical comparison between the experimental and theoretical momentum distributions allows us to determine the 'optimum' wave function for each molecule from the basis sets we studied. This 'optimum' wave function then allows us to investigate chemically or biologically significant molecular properties of these molecules. EMS-DFT also shows promise in elucidating the character of molecular orbitals and the hybridisation state of atoms.