Impeded Dimer Formation in the High-Pressure Crystal Structure of Formic Acid

We have determined the high-pressure crystal structure of formic acid, including the positions of the hydrogen atoms, using a combination of single-crystal x-ray diffraction techniques and ab initio density functional calculations. The molecules are arranged in infinite hydrogen-bonded chains which adopt a structural conformation that is distinctly different from that of the low-temperature form. In the high-pressure structure the molecules, which lie on symmetrically flat planes, adopt both the cis and trans conformations while in the low-temperature structure only trans molecules are formed. The molecules in the high-pressure structure are also oriented so that only relatively modest structural changes would be required to produce dimers, or a hydrogen-bonded molecular network, at higher pressure.