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

Abstract

Fourier transform infrared spectra of formic acid aerosol particles in situ generated in a collisional cooling cell at temperatures ranging between 90 and 210 K are recorded in the mid-infrared and THz/far-infrared regions. Infrared spectroscopic features are used to identify the formic acid dimer above 200 K, the crystalline β 1 phase in the 110− 200 K temperature range, and amorphous solid formic acid at ∼90 K. Density functional calculations of discrete clusters and the full periodic β 1 crystalline structure help to assign the low wavenumber intermolecular modes by comparison with experimental far IR and Raman data from the literature. Refractive indices of the formic acid crystal in the mid-IR range are retrieved. These data are essential for radiative transfer modeling of solid formic acid condensed in the terrestrial atmosphere. THz/far IR spectra, in addition to the well documented mid-infrared spectra, may be useful for the identification of solid formic acid in astrophysical environments such as the young stellar object W33A.

Vibrational mode analysis of hydrogen-bonded organic frameworks (HOFs): synchrotron infrared studies

“…Periodic Density Functional Theory (p-DFT) was employed for the computational models and harmonic frequency analysis of the hydrogen-bonded systems using the CRYSTAL17 (C17) package. 51 This code has been previously used by our workgroup to calculate vibrational frequencies for molecular crystals 31 and flexible networks involving hydrogen-bonded carboxyl dimer synthons. 42 Resultant synthetic far-IR spectra have been demonstrated of sufficient accuracy 52,53 to assign features originating from large-amplitude, low-frequency intermolecular lattice modes and intramolecular torsion vibrations that are prevalent in HOF systems but can otherwise be difficult to model using non-periodic descriptions.…”

“…For comparison to experimental vibrational spectra, a Â0.965 scaling factor was applied to p-DFT computed frequencies, as previously benchmarked for hydrogen-bonded molecular crystal systems. 57…”

“…23 Vibrational spectroscopy has long been applied to probe hydrogen-bonding in molecular systems from gas-phase complexes and clusters, [24][25][26] polar solvent interactions including those in aqueous media, 27,28 and for molecular solids such as water ice. [29][30][31][32] Such studies have employed a suite of broadband infrared sources and laser methods for spectroscopic studies at equilibrium and for system dynamics, respectively. 33,34 The most common approach is to identify the shifts in frequency associated with proton donor group modes, such as O-H and N-H stretching modes, in the mid-IR region.…”

Subtle Influences of a Flexible Tecton on an R22(8) Carboxyl Dimer Synthon: From Molecular Threading to 2D → 3D Interpenetration

“…Tight thresholds for gradient (RMS: 0.0003 au) and displacement (RMS: 0.0012 au) were applied to ensure that the unit cells converged to a local minimum during geometry optimization. Finally, for the calculation of vibrational frequencies (with wavenumber linear scaling of ×0.965 as utilized for hydrogen-bonded and organic nitrile molecular crystals), the harmonic approximation approach was invoked, where the vibrational motion of the lattice was observed through the visualization package CHEMCRAFT…”