Molecular orbital models of ring expansion mechanisms in the silica-carbon monoxide system

The development of a zero net shrinkage dental restorative material based upon a polymer-bioactive glass composite requires a second-phase material that expands. This study details the mechanisms of silica ring expansion by reaction with carbon monoxide. Carbon monoxide was used as a model adduct to represent potentially active sites on the polymer phase of the dental restorative. Silica rings were used to model the bioactive-glass phase of the composite. The 3-, 4-, 5-, and 6-"member" silica rings have been modeled using the Austin Method (AM1) semi-empirical molecular orbital calculations. The reaction pathways were determined for carbon monoxide (CO) reaction addition to each of the rings. The activation barriers (Ea) for the ring expansions were determined from the transition state geometries wherein only one imaginary eigenvalue in the vibration spectrum existed (a true saddle point). In each case the reaction pathway included the hydrogen bonding of CO with a silicon, exothermic pentacoordinate bonding to silicon by the CO and weakening of the Si-O bridging bonds of the ring, and, finally, the incorporation of CO into the ring, forming a silica-carbonate ring. The activation for the ring expansions are +4.3, +6.1, +7.0, and -2.9 Kcal/mol for 3-, 4-, 5-, and 6-"member" silica rings, respectively. The volumetric expansion of the silica was estimated based upon the dilation of adjacent silicon-silicon atomic distances. The dimensional change was calculated to be 3.9%, 21.3%, 19.4%, and 24.2% for 3-, 4-, 5-, and 6-membered silica-carbonate rings, respectively.