MOLECULAR-ORBITAL MODELS OF RING EXPANSION MECHANISMS IN THE SILICA-CARBON MONOXIDE SYSTEM

The development of a zero net shrinkage dental restorative material ba sed upon a polymer-bioactive glass composite requires a second-phase m aterial 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-e mpirical molecular orbital calculations. The reaction pathways were de termined for carbon monoxide (CO) reaction addition to each of the rin gs. The activation barriers (Ea) for the ring expansions were determin ed from the transition state geometries wherein only one imaginary eig envalue in the vibration spectrum existed (a true saddle point). In ea ch 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 i ncorporation 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 Kca l/mol for 3-, 4-, 5-, and 6-''member'' silica rings, respectively. The volumetric expansion of the silica was estimated based upon the dilat ion of adjacent silicon-silicon atomic distances. The dimensional chan ge was calculated to be 3.9%, 21.3%, 19.4%, and 24.2% for 3-, 4-, 5-, and 6-membered silica-carbonate rings, respectively. (C) 1997 John Wil ey & Sons, Inc.