Structure of liquid formic acid investigated by first principle and classical molecular dynamics simulations

The structure of liquid formic acid has been investigated by Car-Parrinello and classical molecular dynamics simulations, focusing on the characterization of the H-bond network and on the mutual arrangement of pairs of bonded molecules. In agreement with previous computational studies, two levels of H-bonded structures have been found. Small clusters, characterized by O-H center dot center dot center dot O bonds, are held together by weak C-H center dot center dot center dot O bonds to form large branched structures. From the ab initio simulation we infer the importance of cyclic H-bond dimer configurations, typical of the gas phase. Most of these dimer structures are however found to be embedded into H-bonded chains. When only O-H center dot center dot center dot O bonds are taken into account, linear H-bond chains are detected as basic structures of the liquid. More branched structures occur when C-H center dot center dot center dot O bonds are also considered. Regarding the arrangement of molecular pairs, we observed that O-H center dot center dot center dot O bonds favor the occurrence of configurations with parallel molecular planes, whereas no preferential orientation is observed for molecules forming C-H center dot center dot center dot O bonds.