High-pressure far-infrared spectroscopic studies of hydrogen bonding in formic acid.

Simple molecules such as HCOOH, or formic acid, are suggested to have played important roles in planetary physics due to their possibility for high pressure and temperature chemistry under impact conditions. In this study, we have investigated the effect of pressure (up to 50 GPa) on H-bonding and reactivity of formic acid using synchrotron far infrared spectroscopy. Based on the pressure-induced changes to H-bond ν(O-H···O) stretching and γ(O-H···O) deformations, we observe significant reorganization of H-bonding network beginning at ~20 GPa. This is in good agreement with reports of symmetrization of H-bonds reported at 16-21 GPa from X-ray diffraction and Raman spectroscopy studies as well as molecular dynamics simulations. With further increase in pressure, beyond 35 GPa, formic acid undergoes a polymerization process that is complete beyond 45 GPa. Remarkably, upon decompression, the polymeric phase reverts to the crystalline high-pressure phase at 8 GPa.