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Issue 43, 2016
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Deprotonation of formic acid in collisions with a liquid water surface studied by molecular dynamics and metadynamics simulations

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Abstract

Deprotonation of organic acids at aqueous surfaces has important implications in atmospheric chemistry and other disciplines, yet it is not well-characterized or understood. This article explores the interactions of formic acid (FA), including ionization, in collisions at the air–water interface. Ab initio molecular dynamics simulations with dispersion-corrected density functional theory were used. The 8–50 picosecond duration trajectories all resulted in the adsorption of FA within the interfacial region, with no scattering, absorption into the bulk or desorption into the vapor. Despite the known weak acidity of FA, spontaneous deprotonation of the acid was observed at the interface on a broad picosecond timescale, ranging from a few picoseconds typical for stronger acids to tens of picoseconds. Deprotonation occurred in 4% of the trajectories, and was followed by Grotthuss proton transfer through adjacent water molecules. Both sequential and ultrafast concerted proton transfer were observed. The formation of contact ion pairs and solvent-separated ion pairs, and finally the reformation of neutral FA, both trans and cis conformers, occurred in different stages of the dynamics. To better understand the deprotonation mechanisms at the interface compared with the process in bulk water, we used well-tempered metadynamics to obtain deprotonation free energy profiles. While in bulk water FA deprotonation has a free energy barrier of 14.8 kJ mol−1, in fair agreement with the earlier work, the barrier at the interface is only 7.5 kJ mol−1. Thus, at the air–water interface, FA may dissociate more rapidly than in the bulk. This finding can be rationalized with reference to the dissimilar aqueous solvation and hydrogen-bonding environments in the interface compared to those in bulk liquid water.

Graphical abstract: Deprotonation of formic acid in collisions with a liquid water surface studied by molecular dynamics and metadynamics simulations

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Publication details

The article was received on 02 Sep 2016, accepted on 30 Sep 2016 and first published on 10 Oct 2016


Article type: Paper
DOI: 10.1039/C6CP06071D
Citation: Phys. Chem. Chem. Phys., 2016,18, 29756-29770
  • Open access: Creative Commons BY-NC license
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    Deprotonation of formic acid in collisions with a liquid water surface studied by molecular dynamics and metadynamics simulations

    G. Murdachaew, G. M. Nathanson, R. Benny Gerber and L. Halonen, Phys. Chem. Chem. Phys., 2016, 18, 29756
    DOI: 10.1039/C6CP06071D

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