EMS studies of larger molecules of chemical and biochemical interest

The challenges involved in extending electron momentum spectroscopy (EMS) studies beyond small polyatomic molecules to more complicated systems are discussed. EMS results for the highest occupied (frontier) molecular orbitals of glycine (NH2CH2COOH) and dimethoxymethane ((CH3O)2CH2) demonstrate possible approaches to overcoming such challenges as closely spaced valence orbitals, low volatility, and the conformational mobility of the target compound. The increased sensitivity available from recently developed multichannel electron momentum spectrometers is a key factor in overcoming these challenges and making such EMS studies feasible. The utility of Kohn–Sham density functional theory (DFT) for EMS calculations on larger molecules such as glycine and dimethoxymethane using the recently formulated target Kohn–Sham approximation is also investigated as experimental momentum profiles are compared with theoretical momentum profiles generated via Kohn–Sham DFT and a range of Hartree–Fock calculations. The Kohn–Sham DFT calculations provide better agreement with experiment for the frontier orbitals of glycine and dimethoxymethane than even the near Hartree–Fock limit results.