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Thermochemistry of silicic acid deprotonation



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Geochimica et Cosmochimica Acta Vol 65 No 24 pp 4435 4443 2001 Copyright 2001 Elsevier Science Ltd Printed in the USA All rights reserved 0016 7037 01 20 00 00 Pergamon PII S0016 7037 01 00739 6 Thermochemistry of silicic acid deprotonation Comparison of gas phase and solvated DFT calculations to experiment J SEFCIK1 and W A GODDARD III2 1 2 Laboratorium fu r Technische Chemie ETH Zu rich Universita tstr 6 CH 8092 Zu rich Switzerland Materials and Process Simulation Center Beckman Institute 139 74 California Institute of Technology Pasadena CA 91125 USA Received October 23 2000 accepted in revised form June 8 2001 Abstract Theoretical approaches to the thermochemistry of silicate anions have so far focused on gas phase molecular orbital and density functional theory DFT calculations These calculations predict that in the presence of hydroxide ligands the most stable singly charged anion of the silicic acid H4SiO4 is the five coordinated anion H5SiO 5 However experimental evidence from in situ nuclear magnetic resonance NMR experiments clearly shows that deprotonated silicic acid in alkaline aqueous solutions is fourcoordinated H3SiO 4 We compare gas phase and solvated DFT calculations of monomeric anions of silicic acid in order to assess solvent effects on the thermochemistry of silicic acid deprotonation We show that appropriate inclusion of solvation in quantum chemical calculations is critical for correct prediction of coordination and thermochemistry of silicate anions in aqueous solutions Multiply charged anions of silicic acid are found to be electronically unstable in the gas phase and thus it is not possible to use thermodynamic cycles involving these species in thermodynamic calculations However a high dielectric constant solvent is sufficient to stabilize these anions and solvated calculations can be used to directly compute their thermodynamic quantities When we include the zero point energy ZPE and statistical mechanics contributions to the Gibbs free



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