A theoretical kinetic model of the temperature and pH dependent dimerization of orthosilicic acid in aqueous solution

Abstract

The first steps in a pH- and temperature-dependent theoretical kinetic model of silicate polymerization and dissolution are examined in this work with a combined ab initio and transition state theory based study of the dimerization of H(4)SiO(4). The role of solvation has been of primary concern in this work, and its influence on theoretical activation energies and pre-exponential factors has been thoroughly benchmarked. Relatively inexpensive MP2/6-31+G(d)//HF/6-31+G(d) calculations of octahydrate clusters, with conductor-like polarizable continuum model corrections obtained in the MP2-level single-point calculations, have been shown to lead to a good description of the limited experimentally determined energetics of dimerization for most elementary reactions. Pre-exponential factors computed from this level of theory are found to be relatively insensitive to the level of theory utilized for geometry optimizations, the number of explicit waters, hindered rotor corrections, and variational effects arising from the minimization of rate constants. Within this framework, a kinetic model of the chemistry of H(4)SiO(4) and H(3)SiO(4)(-), forming H(6)Si(2)O(7) and H(5)Si(2)O(7)(-), has been compiled. Numerical simulations over pH = 3-12 show that a number of pH- and temperature dependent trends in reaction rates and positions of equilibrium are well described with this simple dimerization model. More specifically to the dimerization process, we obtain dimerization constants, log K(dim), of 1.85 and -7.15 for the formation of H(6)Si(2)O(7) and H(5)Si(2)O(7)(-) respectively, which compare well with experimentally determined values of 1.2 and -8.5, respectively.

Correct Symmetry Treatment for X + X Reactions Prevents Large Errors in Predicted Isotope Enrichment

“…A widely-cited review 4 recommends multiplying the reactant symmetry numbers by a factor of two, referencing Pollak and Pechukas, though many studies involving identical reactants do not mention, and likely do not include, this factor of two. [5][6][7][8][9] Though a factor of two mistake can impact many phenomena, it causes huge errors in isotopic enrichment since the disputed factor of two is much larger than most non-hydrogen kinetic isotope effects. 10 This paper uses two methods to explain proper symmetry treatment in reaction kinetics.…”

Kinetics of Alkoxysilanes and Organoalkoxysilanes Polymerization: A Review

“…It has generally been reported that hydrolysis, condensation, and cyclization reactions are endothermic under different conditions [2,17,70,71,72,73], while solvation and hydrogen bonding are slightly exothermic [17]. The polymerization reaction of silanes obeys the Arrhenius equation (kh=Ae−EaRT), where k h is the rate constant, E a is the activation energy, R is the gas constant, and T is the absolute temperature.…”

Equilibrium and kinetic Si isotope fractionation factors and their implications for Si isotope distributions in the Earth’s surface environments

“…With the equilibrium Si isotope fractionation factors provided here, we could further explore the molecular-level clay mineral precipitation mechanisms by distinguishing the equilibrium isotope fractionations from the other processes. Previous theoretical works (He and Liu 2015;Rimstidt and Barnes 1980;McIntosh 2012;Nangia and Garrison 2008;Zhang and Liu 2014) all suggested that the polymerization of H 4 SiO 4 molecules needed to overcome a rather high activation energy barrier. Besides, the Si kinetic isotope fractionation of about -1.1 % determined by Geilert et al (2014) was identical to the Si equilibrium isotope fractionation between the monodentate 1 V[Fe 2 OSi(OH) 3 and the H 4 SiO 4 solution (He and Liu, 2015).…”