Structural investigation of liquid formic acid by X-ray and neutron scattering, ab initio calculations and molecular dynamics simulations

A structural investigation of liquid formic acid (FA) is performed at room temperature and atmospheric pressure by neutron and X-ray scattering, ab initio calculations and classical Molecular Dynamics (MD) simulations. Ab initio calculations using the 6-311G** basis set are performed on the one hand to study the structure of the two possible conformers cis and trans; on the other hand, to examine some possible clusters including cis, trans, and cis-trans conformers which may describe the intermolecular arrangement in the liquid. Our investigation clearly shows the existence of clusters other than crystal ones. The MD simulated results, using five force fields, show that X-ray and neutron scattering data are less reproduced by Q-BP potential. The formic deuterium can participate in the hydrogen-bond network of the liquid; however, the C-D…O interactions are weaker than the O-D…O ones. The complementarity between X-ray and neutron scattering is also highlighted.