p^Ka of silicic acid in presence of La³⁺ using single sweep method coupled to DFT-based molecular dynamics

Riccardo Spezia (1) , Rodolphe Vuilleumier (2)

1 LAMBE - Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement
2 PASTEUR - Processus d'Activation Sélective par Transfert d'Energie Uni-électronique ou Radiatif (UMR 8640)

In this paper we show how it is possible to obtain the pK_a of a weak acid (H₄SiO₄) bound to a highly charged cation (La³⁺) from DFT-based molecular dynamics in a relatively simple, tunable and efficient way. This was done by combining molecular dynamics in bulk water of a La³⁺/H₄SiO₄ complex with the single sweep method. At this end, we have first performed molecular dynamics runs where a high temperature is assigned on the defined reaction coordinate, here the O–H distance. Then we performed constrained molecular dynamics simulations for selected values of the reaction coordinate along the observed dissociation pathway. The average forces obtained from those constrained simulations are then interpolated to get a final free energy curve. The pK_a is obtained by integrating the resulting free energy curve and that of a reference calculation, the autodissociation of water. We then show that the pK_a of the silanol groups away from La³⁺ is close to the first pK_a of silisic acid, while the pK_a of the silanol groups interacting with La³⁺ is about three units lower.

Mots clés: temperature-accelerated molecular dynamics; free energy calculation; PKa

Domaines: Chimie-Physique [physics.chem-ph]

Riccardo Spezia, Rodolphe Vuilleumier. p^Ka of silicic acid in presence of La³⁺ using single sweep method coupled to DFT-based molecular dynamics. Molecular Physics, 2013, 111 (22-23), pp.3478-3485. ⟨10.1080/00268976.2013.848300⟩. ⟨hal-00918858⟩