Nucleophilic substitution reaction mechanisms: An atomic-molecular perspective on chemical speciation and transport properties in silicate melts

Abstract

There has been no systematic attempt to determine if nucleophilic substitution (SN) reactions proceed in silicate melts, even though they commonly occur in gaseous and liquid phases containing C, Si, P, and Ge centered tetrahedra. The oversight is here rectified by providing such an analysis. Conditions required for nucleophilic substitution reactions to occur are: (1) the presence of nucleophiles (Lewis bases) which in silicate melts are bridging oxygen (BO), non-bridging oxygen (NBO−) and free oxygen (O2−); (2) the presence of tetrahedra (Q species) with strongly electrophilic centers (i.e., Si atoms); (3) the presence of Si transition species containing pentahedrally coordinated Si (i.e., VSi species); (4) rapid reaction rates among tetrahedral species. All conditions are met for silicate melts. For example, the strong nucleophile, NBO− exists at high temperatures in binary alkali and alkaline earths silicate melts due primarily to thermal agitation whereby some Si-NBO-M bonds are ruptured to produce the nucleophilic Si-NBO− moiety. This nucleophile attacks the Si center of an adjacent tetrahedron to form a SiO bond thereby producing a VSi transition species. The transition species decomposes by rupture of another SiO bond located on the polar opposite side of the transition species. Three types of SN reaction are recognized and all involve VSi transition species. They are NBO-BO exchange reactions (e.g., Q3 + Q4 → Q4 + Q3), disproportionation reactions (e.g., 2Q3 → Q4 + Q2) and polymerization reactions (e.g., Q3 → Q4 + 1/2O2−). H2O and OH− are also nucleophiles and their reaction with Q species proceeds via SN reaction mechanisms, and may cause depolymerization of melts. Hydrogen bonding of H2O to BO and NBO may also occur, as in ice and water, thereby enhancing H2O solubilities in melts. These latter reactions should neither depolymerize melts nor affect NBO/T values.The diffusivity of Si and O in melts, anionic conductivity and chemical speciation (e.g. Q species abundances) proceed via one or other SN reaction, with the transition species assuming a critical role in diffusivity, conductivity and viscosity. The SN reaction mechanism, coupled with transition state theory, provides explanations for: (1) the formation of pentahedrally coordinated Si (VSi) and its apparent restriction to highly siliceous glasses; (2) the remarkably similar diffusivities of Si and O in silicate melts; (3) the ‘jump distance’ (α) of the Eyring equation, which by the SN mechanism is ~3.5 Å (i.e., diameter of the VSi transition species); (4) the minimum number of monomeric units (i.e., Q species) involved in the ‘cooperative region’ of the Adam-Gibbs equation; and (5) the quantitative distribution of Q species in Na silicate glasses up to ~50 mol% Na2O.