Metal-ion solvation and abundance in atmospheric media before strong earthquakes

Abstract

Gas phase metal-ions form hydrated complexes, which have characteristic solvation enthalpies ranging from around -150kJ/mol for monohydrates to around -40kJ/mol for larger water clusters. Previous reports on seismo-ionospheric coupling have suggested that select metal-ions M+, such as sodium and iron, are dragged by anomalous electric fields up to E-layer altitudes before earthquakes and are responsible for sporadic ionospheric metal-ion layers [Pulinets,1997 Adv.SpaceRes.,20,2173]. This report applies quantum chemical methods to calculate the geometries, energetics, thermochemistry and ionospheric abundances of a suite of solvated alkali and ionic transition metal clusters with up to six water molecules. Structural and energetic properties of ion-water clusters are calculated using Møller-Plesset perturbation theory (MP2) with a large number of different basis sets. Calculated solvation enthalpies for Na+, K+, Fe+ and Cu+ reported here are in excellent agreement with experimental mass spectrometric data. For instance, the calculated solvation enthalpy for groundstate 6D Fe+(H2O) is -108.9 kJ/mol and is in good agreement with the experimental value of -120±12 kJ/mol [Magnera,JACS,111,4100]. The subsequent hydration step toward Fe+(H2O)2, which is accompanied by a spin change to a 4A1 ground state, is exothermic by -198.0 kJ/mol. The experimental enthalpy ΔH1,2 for water attachment onto Fe+(H2O) is -170.7 [Marinelli, 1998,JACS,111,4101]. Interestingly, results from both this ab initio study and mass spectrometry demonstrate that Fe+ binds a second water molecule somewhat more strongly than the first one. The subsequent solvation steps asymptotically approach values of around -44 kJ/mol, showing a trend toward the bulk water limit. An analysis of calculated clustering equilibria K indicates that ionospheric metal ion-water cluster abundances are governed by the shifting balance between the temperature induced changes of K and water monomer abundances. Results from ab initio calculations presented here indicate that H2O attachment onto alkali and transition metals is a thermodynamically favorable process, such that hydrated metal clusters would form a significant ionospheric repository.