Proton transfer from water to anion: Free energy profile from first principles metadynamics simulations

- The effect of shape and size of the network of water molecules on intracluster proton transfer was explored.
- Beyond the critical size of hydrated cluster, one of the adjacent water molecules gives its proton to anion.
- The free energy barrier of the proton transfer reaction was estimated using first principles based metadynamics simulation.

We elucidate the characteristic proton transfer process from water to aniline anion, and the free energy profile of the process in aqueous solution from the first principles simulations. We have explored the structure and energetic of hydrated clusters of aniline anion with up to three water molecules by gas phase electronic structure calculations. Our results indicate that the proton transfer in hydrated clusters proceeds more easily as the number of water molecule increases. The presence of minimum three water molecules in the gas phase cluster is sufficient to facilitate the proton transfer process from water to anion. The analysis of trajectories obtained from density functional theory based first principles molecular dynamics simulations of aqueous solution of anion do not show any evident of complete transfer of proton from water to anion due to energy barrier, and cooperativity of other surrounding water molecules. Using biasing potential through first principles metadynamics simulations, we report the observation of proton transfer reaction from water to aniline anion with a barrier height of 27.7 kJ/mol. In the aqueous solution of the anion, and beyond the critical number of solvent water molecules in hydrated clusters, one of the adjacent water molecules becomes more acidic and gives its proton to anion. Our observations are in agreement with the observation from experimental study.

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