Formic Acid Dimerization: Evidence for Species Diversity from First Principles Simulations

Abstract

Dimerization of formic acid has been simulated using ab initio molecular dynamics at conditions mimicking rare gas matrix isolation experiments. Aggregation product distributions and the corresponding reaction pathways have been studied as a function of temperature. At higher temperatures, the cyclic, C(2h) symmetric, global minimum structure A with two O-H...O horizontal lineC hydrogen bonds predominates over the second most stable, acyclic, local minimum isomer B with one O-H...O horizontal lineC and one C-H...O horizontal lineC hydrogen bond, whereas the latter is the main species at low temperature. Significant concentrations of two additional, less stable, local minimum species, C exhibiting an O-H...O horizontal lineC and an O-H...O(H)-C hydrogen bond, and D with an O-H...O(H)-C and a C-H...O horizontal lineC hydrogen bond, which should be detectable in experiment, are predicted at low temperature. Theoretical vibrational spectra are provided to guide the experimental search for these species. Furthermore, free-energy barriers for thermal interconversion of different dimer species have been calculated using targeted molecular dynamics in conjunction with thermodynamic integration. The small barrier for the C --> A reaction of 7.0 kJ/mol indicates that the C species can only be stabilized at ultracold conditions. The data presented here thus hold important clues for the execution and interpretation of low-temperature vibrational spectroscopy at matrix isolation or ultracold helium droplet conditions.

Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates

“…Stretching vibrations of H-bonds have a high diagnostic value for determination of the nature and strength of these bonds [ 97 , 98 ]. Previously, the spectral manifestations of dimerization and isotopic effects on spectroscopic observables were studied for different molecular systems [ 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 ] including carboxylic acid dimers [ 109 , 110 ]. Upon carboxylic acid dimerization, the structure of the OH stretching band in IR spectra changes most prominently: The narrow band of monomers changes to a broad, intensive, and complex substructured band of dimers shifted to lower wavenumbers.…”

Supramolecular Control of Spin Exchange in a Spin‐Labelled [2]Rotaxane Incorporating a Tetrathiafulvalene Unit

“…The effect of shifting the stretching vibrational frequency is known to be present in small molecules. If we consider the structure of the formic acid dimer, the Δ ν CO shift, calculated as the difference between the ν CO stretching frequency for the dimer and monomer, is observed even when the CO group is not directly involved in the hydrogen bond 48. The flow of the electronic density in the molecule after bond formation in one part of the system causes structural changes in the non‐interacting parts.…”

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

“…It may be too ambitious to aim at a quantitative description of the complex coupling pattern in the OH stretching region 36,41,[95][96][97] , but the more isolated couplings in the monomer fingerprint up to the carbonyl stretching region 81 are waiting to be understood in detail for FAD. Between these carbonyl stretching fundamentals and the OH stretching region, the PES starts to open up for other, metastable isomers 20,79,[98][99][100][101][102][103] , which need to be characterised. The enormous sensitivity of matrix isolation spectroscopy 104 can also be exploited towards this goal once the subtleties of spectral matrix shifts 23,24,96,105 are better Figure 7: Some diagonal-( x i,i ) and off-diagonal ( x i,j ) anharmonicity matrix elements for the six intermolecular modes, computed according to Eqs.…”