The Structure of the Formic Acid Molecule in the Liquid State from Neutron Diffraction Measurements Involving Five Isotopically Different Species

Neutron diffraction experiments on liquid formic acid of the five isotopically different forms DCOOD, DCOOH, HCOOD, 0.36 DCOOD + 0.64 DCOOH, and 0.36 DCOOD + 0.64 HCOOD were performed. Inelastic scattering corrections had to be performed in a particularly careful way considering that H is a strongly incoherent scatterer. From the five corrected coherent cross sections and the corresponding weighted sums of pair correlation functions a first set of intramolecular structure data is derived. A suitable decomposition of the primary experimental material in selected groups of pair correlation functions served to work out further structural details which showed that formic acid in the liquid state is a molecule for which the OH group is rotated by about 50° out of the molecular plane.