High-Pressure Far-Infrared Spectroscopic Studies of Hydrogen Bonding in Formic Acid

Abstract

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.

Theoretical Study of Shocked Formic Acid: Born–Oppenheimer MD Calculations of the Shock Hugoniot and Early-Stage Chemistry

“…We observed two instances of complexation formation via C−C pair interactions ("hot" trajectory only), 62 instances via C−hydroxyl O interactions (43 for the "cold" and 19 for the "hot"), and 78 instances via C−carbonyl O interactions (49 for the "cold" and 29 for the "hot"). These interactions are similar to those seen in experimental 2,38 and theoretical studies 2 of formic acid under high pressure static compression, where both papers observed hydrogen bond symmetrization at pressures of ∼20 GPa and polymerization occurring at ∼40 GPa, although those studies were performed at room temperature. As stated earlier, only 26 molecules displayed CO or CH scission reactions (for both the "cold" and "hot" trajectories), while there were 142 instances in which complexes were formed.…”

Terahertz and Mid-infrared Spectra of Cold Formic Acid Aerosol Particles

“…17 At very high pressures, above 35 GPa, a band just over 300 cm −1 is attributed to amorphous formic acid. 2,78 The aerosol band position at 242 cm −1 is more similar to that of formic acid "glass" (249 cm −1 ) 33 and to that of the cyclic dimer at 251 cm −1 (see Figure 1). Noting the similarity of the broad far-IR band of amorphous formic acid to this dimer feature, Iopollo et al 2 assigned it to a rocking mode of the cyclic dimer.…”

“…One previous study displays a spectrum attributed to amorphous formic acid: it is somewhat noisy and limited to the region below 250 cm –1 but has a band with position given as 227 cm –1 . ,, Another study cites a band at ∼235 cm –1 for formic acid deposited at 77 K prior to annealing . At very high pressures, above 35 GPa, a band just over 300 cm –1 is attributed to amorphous formic acid. , The aerosol band position at 242 cm –1 is more similar to that of formic acid “glass” (249 cm –1 ) and to that of the cyclic dimer at 251 cm –1 (see Figure ). Noting the similarity of the broad far-IR band of amorphous formic acid to this dimer feature, Iopollo et al assigned it to a rocking mode of the cyclic dimer.…”

Polyether phases of formic acid revealed under high pressure

“…2, the medium intensity IR modes 44,46 can be observed near 227 cm −1 and 480 cm −1 in ref. 38 above 20 GPa.…”

“…This evolution explains the soft mode behavior of the OH-stretching IR mode reported up to 12.2 GPa. 2 To the best of our knowledge, the mid-IR absorption studies of FA have not been conducted at higher pressures, owing to absorption saturation, 38 which evaded crucial molecular information under high pressures. Here, we have measured the mid-IR reflectance of FA above 12 GPa, which systematically follow the reported data up to this pressure, 2 presented in Fig.…”