STRUCTURAL INVESTIGATION OF LIQUID FORMIC-ACID - X-RAY AND NEUTRON-DIFFRACTION, AND REVERSE MONTE-CARLO STUDY

The structure of liquid formic acid was studied by X-ray and neutron d iffraction experiments. The intramolecular geometry of the formic acid molecule was determined fom the X-ray diffraction data, and the resul ts proved to be in good agreement with similar results from neutron di ffraction. A reverse Monte Carlo calculation, fitting simultaneously t wo total structure functions resulting from these experiments, and a s ubsequent detailed analysis of the intermolecular structure was perfor med. Several total radial distribution functions, corresponding to var ious isotopic substitutions on the two hydrogen atoms, were also calcu lated. The calculated total radial distribution functions of those iso topic substituted systems, which have already been measured, were comp ared to the experimental functions. The agreement was found to be quit e good, except in the range where the intra- and intermolecular distan ces are overlapping. The geometry of the hydrogen bonds, the hydrogen bonded clusters and the spatial distribution of the hydrogen bonded ne ighbours were also examined. We found that in the liquid phase the for mic acid molecules prefer to form short branched hydrogen bonded chain s, where the hydrogen bonds can be formed either between two hydroxyli c oxygen atoms or between a hydroxylic and a carbonyl oxygen atom. No significant amount of cyclic hydrogen bonded dimers was found.