Proton transfer processes in polar regions of humic substances initiated by aqueous aluminum cation bridges: A computational study

Abstract

An overview of the variety of processes induced by the aluminum cation interacting with carboxyl and carboxylate groups is given by means of quantum chemical density functional theory (DFT) calculations. Different hydration states of Al3+ ranging from the hexaaquo complex down to the unhydrated cation and direct/indirect bonding with the polar groups are considered. The calculations reflect the amphoteric character of the hydrated aluminum complex showing in most cases its acidic character via proton transfer from the water molecules of the hydration shell to the carboxylate group, but in some cases also deprotonation of the carboxyl group. Several additional processes are observed such as interconversion of bidentate and monodentate bonding by the carboxyl/carboxylate groups and strong hydrogen bonding between proton transfer partners. Comparison with analogous previous investigations on cation bridges induced by calcium and sodium shows the pronounced activity of the triply charged aluminum cation. The importance of the strong polarizing and bridging power of the aluminum cation for soil organic matter with low exchange capacities and a low concentration of charged groups is discussed.