Dissociation Mechanism of Acetic Acid in Water

Abstract

The reaction mechanism for acetic acid dissociation in aqueous solution has been investigated by combining the metadynamics method with transition path sampling (TPS). By using collective variables that describe not only the deprotonation of the acid but also the solvation state of the hydronium ion and its distance from the acetate, a reactive trajectory in which stable separated ions were produced was obtained. More transition trajectories were sampled by using the TPS technique, taking the metadynamics trajectory as the initial trajectory. Two different dissociation reaction pathways were determined, one driven by the breaking of an H-bond formed by the water molecule in contact with the hydroxyl and involving the formation of a metastable contact ion pair and the other characterized by a direct transfer of the proton to the solution through an unstable Zundel-ion-like intermediate.

Brønsted basicity of the air–water interface

“…Of course, exoergic proton transfer could nevertheless be hindered by a significant kinetic barrier that would prevent R2 from proceeding fast enough during CH 3 COOH(g) collisions with the surface of water (42,43). Reaction R3, in contrast, is exoergic as written, spontaneous both in gas phase and in aqueous solution, and therefore expected to proceed readily at the interface.…”

Aqueous solutions: state of the art in ab initio molecular dynamics

“…As such, it is internally implemented in CPMD (http://www.cpmd.org/, Copyright c IBM Corp. 1990-2008, Copyright c MPI für Festkörperforschung Stuttgart 1997-2001) and CP2k (http://cp2k.berlios.de) and can be easily interfaced to other AIMD codes through the PLUMED external plugin [236]. AIMD-MetaD has been applied to the study of the dissociation of acetic acid [237], the condensation of tungstate ions [238,239] and the hydrolysis of formamide [240,241], ethyl formate [242] and cisplatin [243] in water.…”

Dynamisches Protonierungsgleichgewicht der in Wasser gelösten Essigsäure

“…CPMD kombiniert mit Metadynamik und "transition path sampling" wurde weiterhin verwendet, um Profile der freien Enthalpie für die Deprotonierung von Essigsäure in Wasser zu berechnen. [30] Theoretische Methoden wurden auch zur Untersuchung wichtiger PT-Reaktionen in Proteinsystemen genutzt, z. B. für den PT in Bakteriorhodopsin, [31] in Gramicidin A, [32,33] oder entlang einer Kette von Wassermolekülen im D-Pfad in COX [34] und die Protonentranslokation in Carboanhydrase.…”

“…[30] In manchen Fällen des schnellen Protonenaustausches hüpft das Proton jedoch nicht zur Essigsäure zurück, sondern entweicht zu einem anderen Wassermolekül, das ebenfalls eine H-Bindung mit dem Hydroniumion bildet. Setzt sich dieser Prozess fort, fängt das Proton an, in der Wasserbox umherzuwandern (Spitzen in Abbildung 1 a).…”