Proton and protonic entities in solid heteropoly compounds: An ab initio calculation of the environmental effect on the H<inf>5</inf> O<inf>2</inf>⁺ ion

Abstract

Systematic experimental investigations of heteropoly compounds, particularly their structure and activity, led to the conclusion that most of their characteristics are governed by the presence of protons and protonic entities. Special attention has been paid to two forms of 12-tungstophosphoric acid: hexahydrate (WPA-6) and dehydrated phase (WPA-0). It was postulated that in WPA-6 dynamic equilibrium of protonic entities exists, and that dehydrated phase is stabilized by protons. To confirm the role of the "free protons" or "proton gas" derived on the basis of thermal, structural and spectroscopic experimental studies, we carried out also ab initio calculations on a number of systems containing H5 O2+ ion. We were not able to perform direct calculations on the real systems investigated experimentally since the structure of heteropoly compounds is too complex. However, it has been found that H5 O2+ ion in WPA-6 definitely is not planar and the results obtained indirectly support the postulated dynamic equilibrium, i.e. possibility of existing of free protons. © 2007 Elsevier B.V. All rights reserved.