Molecular conformation of dimethyl oxalate studied by electron momentum spectroscopy

The shapes of molecular orbitals convey conformational information about the molecule. Electron momentum spectroscopy (EMS) is a method that measures individual molecular orbitals in the momentum space, and it may thus offer a tool for examining molecular conformation that cannot be fully determined by conventional structure-analysis techniques. In consideration of this, EMS measurements were conducted on dimethyl oxalate, the molecular structure of which in the gas phase has long been the subject of considerable debate. The obtained momentum profiles were found to be unexplained by theoretical calculations with a fixed CC torsional angle, φ. The deviations from the experiment were almost resolved by incorporating the influence of thermal-induced CC torsional motion, inferred from theoretical potential energy curves (PECs) along φ. The findings demonstrate that the CC torsional angle is distributed over the entire range from 0° to 180° at room temperature. Furthermore, the energy of the trans-structure is revealed to be lower than that of the cis-structure. For the highest occupied orbital, a small but discernible variation was observed in the theoretical momentum profile, depending on the PEC used. A theoretical analysis reveals that the PEC dependence is enhanced by decreasing the temperature, indicating that EMS experiments at low temperatures may provide a valuable opportunity to examine the torsional potential, whose reliable theoretical prediction has been hindered by the delicate balance between π bonding effects and lone pair-lone pair repulsion.