Can the pH value of water solutions be estimated by quantum chemical calculations of small water clusters?

Abstract

In the study, various water clusters were explored from the point of view of the proton transfer between H-bonded neighbors. A relatively modest approach--the MP26-31++G(d,p) level--was chosen as acceptable considering the fact that also larger systems have to be included. The tight ion-pair model (with usually three fixed O-O distances) was adopted for the autodissociation process. First, cluster-estimated pH values rapidly decrease as cluster size increases from 2 to 6. For larger clusters in gas phase, the topology of H bonds plays an important role, varying pH from 7 to 13 in hexamers and from 5 to 15 in octamer clusters. The relationship energy/distance was quantified, too. Enhancing our model with the conductorlike screening model (COSMO) approach brought significant improvement in description of the autodissociation reaction with a stable zwitterionic structure. However, when the O-O restrictions were released, the small barrier for backward reaction disappeared, reforming neutral cluster spontaneously. Also Boltzmann weighting procedure was applied, and for the explored clusters in vacuo, the series of pH 25-18-14-13-10 was obtained for cluster sizes n=2, 3, 4, 6, 8. Using the COSMO approach, the analogous series is 15-14-12-10-9. The limit of the series is still about two to three units above the experimentally known pH. In order to reach the size-independent (bulk) value, larger clusters are needed. However, the situation is far from hopeless since (as it was proven in the study) four-coordinated molecules are not involved in the proton transfer process directly; they can only be a part of the surrounding environment.

To address accuracy and precision using methods from analytical chemistry and computational physics

“…Another way is to apply implicit solvation procedures, such as based on continuum models. The usefulness to combine methods on implicit and explicit considerations has also been subject to various studies (Bashford and Case 2000;Bandyopadhyay et al 2002;Chaudhari et al 2004;Ikeda et al 2006;Mrázek and Burda 2006). Sound works have pointed out that certain important properties of water can not be taken into account to a desired extent by implicit solvation models.…”

Mass and charge transfer within a floating water bridge

“…12,13 ). The gap at mesoscopic scale is bridged by a number of theories including quantum mechanical entanglement and coherent structures in water, theories which are currently discussed (e.g.…”

Inelastic UV Scattering in a Floating Water Bridge

“…At macroscopic scale several of these phenomena can be explained by modern electrohydrodynamics, analyzing the motion of fluids in electric fields (e.g., the text book on Electrohydrodynamics by Castellanos [9]). On the molecular scale water can be described by quantum mechanics [10,11]. The gap at mesoscopic scale is bridged by a number of theories including quantum mechanical entanglement and coherent structures in water, theories which are currently discussed (e.g., Refs.…”