Assigning a Structural Motif using Spontaneous Molecular Dipole Orientation in Thin Films

Spontaneous orientation of molecular dipoles has been observed to produce bulk electric fields, termed ‘spontelectric’ fields, in a broad variety of molecular solid thin films formed by condensation from the gas phase. Such spontelectric fields are found in cis-methyl formate (cis-MF) and the present work combines observation of these fields with high quality ab initio studies of cis-MF monomers and dimers. This enables a prediction of the structural motif within the unit cell of the crystalline phase of solid cis-MF, showing it to be a non-polar dimer. Dimer formation at deposition temperatures of >90 K is cited to contribute to the observed collapse of the spontelectric field at these temperatures. This is the first time that such a structural prediction has been made, using observations of spontelectric behaviour as a key indicator.