MOLECULAR-ORBITAL STUDY OF AN ENVIRONMENTALLY ENHANCED CRACK-GROWTH PROCESS IN SILICA

We have used molecular orbital calculations to investigate effects of environmental substances and strain on fracture of the Si-O bond in an H6Si2O7 molecule. We believe that the tendencies observed simulate cr ack growth in silica in the vicinity of a crack tip. This study is foc used on the initial stage of environmental enhancement, in which a mol ecule of a substance from the environment approaches a reaction site i n silica. Five environmental substances-ammonia, water, formamide, nit rogen, and argon-were considered. The total energy of each H6Si2O7-env ironmental molecule system was calculated with H6Si2O7 in each of two strain conditions and with the environmental molecule at two different distances from the bridging 0 in H6Si2O7. This provides estimates of the average force required to move an environmental molecule toward th e reaction site in silica. The energy difference due to environmental molecule position is relatively small for ammonia and water, and is re latively large for nitrogen and argon. Experimentally, ammonia and wat er have shown the greatest tendency to enhance crack growth in silica, whereas nitrogen has shown virtually no such tendency. The tendency f or a substance to enhance crack growth therefore appears to be at leas t partly an inverse function of the energy required to move a molecule of that substance toward the reaction site on the silica surface. Ele ctron population analyses indicate that oxygen atoms adjacent to the r eaction site engage in antibonding interactions with certain environme ntal-molecule atoms. These interactions are much smaller in magnitude when H6Si2O7 is strained than when it is unstrained. It therefore appe ars possible that in a crack in silica, where an environmental molecul e would experience restricted mobility, strain reduces steric hindranc es to the approach of environmental molecules to the reaction site. St raining H6Si2O7 also increases the magnitudes of the charges on Si and 0 at the reaction site, probably leading to increased electrostatic a ttractions of environmental molecules and H6Si2O7.