Concerted Pair Motion Due to Double Hydrogen Bonding: The Formic Acid Dimer Case

Abstract

| Formic acid dimer as the prototypical doubly hydrogenbonded gas-phase species is discussed from the perspective of the three translational and the three rotational degrees of freedom which are lost when two formic acid molecules form a stable complex. The experimental characterisation of these strongly hindered translations and rotations is reviewed, as are attempts to describe the associated fundamental vibrations, their combinations, and their thermal shifts by different electronic structure calculations and vibrational models. A remarkable match is confirmed for the combination of a CCSD(T)-level harmonic treatment and an MP2-level anharmonic VPT2 correction. Qualitatively correct thermal shifts of the vibrational spectra can be obtained from classical molecular dynamics in CCSD(T)-quality force fields. A detailed analysis suggests that this agreement between experiment and composite theoretical treatment is not strongly affected by fortuitous error cancellation but fully converged variational treatments of the six pair or intermolecular modes and their overtones and combinations in this model system would be welcome.

Transfer learned potential energy surfaces: accurate anharmonic vibrational dynamics and dissociation energies for the formic acid monomer and dimer

“…Contrary to that, a 24-mode VSCF/VCI calculation on a PES calculated at the CCSD(T)-F12a/haTZ level of theory and fit to permutationally invariant polynomials 40 correctly describes the spectroscopy and increased width of the OH-stretch band. 43 The present work also allows to make contact with a recent assessment 35 of various sources of errors in the spectroscopy and thermodynamics of FAD which reported that ''More often than not, the overestimation of harmonic downshifts in DFT is qualitatively compensated by the inability of classical dynamics to sample the anharmonic region probed by the quantum nature of the hydrogen atom. This frequently provides right answers for the wrong reasons whenever high-frequency XH stretching spectra are simulated, because in this case, temperatures of several 1000 K would be needed to sample the relevant fundamental vibrational displacements.''…”

“…This confirms earlier findings and is related to the fact that MD simulations at 300 K do not sample the mechanical anharmonicity of XH stretch or any other strongly anharmonic modes sufficiently to give reliable frequencies from such an approach. Contrary to that, a 24-mode VSCF/VCI calculation on a PES calculated at the CCSD(T)-F12a/haTZ level of theory and fit to permutationally invariant polynomials 40 correctly describes the spectroscopy and increased width of the OH-stretch band.present work also allows to make contact with a recent assessment35 of various sources of errors in the spectroscopy and thermodynamics of FAD which reported that "More often than not, the overestimation of harmonic downshifts in DFT is qualitatively compensated by24 Page 24 of 42 Physical Chemistry Chemical Physics Physical Chemistry Chemical Physics Accepted Manuscript Open Access Article. Published on 26 October 2021.…”

“…24 It is also a promising fuel and H 2 carrier 25,26 which can be produced by electrocatalytic CO 2 reduction reactions and, thus, may contribute to decreasing atmospheric CO 2 levels. 26,27 Formic acid monomer (FAM) and its dimer (formic acid dimer, FAD) have been the subject of several experimental [28][29][30][31][32][33][34][35] and theoretical 34,[36][37][38][39][40][41][42][43][44][45] studies. In the vapor phase, formic acid exists as hydrogen bonded dimers 46 making it a prototype for complexes with hydrogen bonds such as enzymes or DNA base pairs.…”

Vibrational Stark Fields in Carboxylic Acid Dimers

“…However, the interpretation of the IR spectra, especially in the O-H stretching region has been difficult due to vibrational coupling, tunneling-splitting, and anharmonicity. [4][5][6][7][8][9][10][11] One of the interesting models to interpret the IR spectra in the O-H stretching region of carboxylic acid dimers is monomers in the dimer model. 12 However, the role of hydrogen bonding in the interpretation of these spectra has been reduced to tuning the OH stretching vibration in and out of resonance with combinations of the remaining vibrational modes of the COOH functional group.…”

Vibrational Stark fields in carboxylic acid dimers

“…However, the interpretation of the IR spectra, especially in the O-H stretching region, has been difficult due to vibrational coupling, tunneling-splitting, and anharmonicity. [4][5][6][7][8][9][10][11] One of the interesting models to interpret the IR spectra in the O-H stretching region of carboxylic acid dimers is monomers in the dimer model. 12 However, the role of hydrogen bonding in the interpretation of these spectra has been reduced to tuning the OH stretching vibration in and out of resonance with combinations of the remaining vibrational modes of the COOH functional group.…”