SILICA FRACTURE .3. 5-FOLD AND 6-FOLD RING CONTRACTION MODELS

In part I, of this series, a ring contraction model was proposed as th e basic mechanism of slow crack growth in silica glass. AM1 molecular orbital theory running on a CAChe workstation was used to find the tra nsition state for the contraction of a 4-fold ring into a 3-fold ring. Using the same AM1 method, the predicted transition state has been fo und for the contraction of a 5-fold ring into a 4-fold ring. The activ ation barrier to fracture for this contraction is E(f) = + 7.9 Kcal mo l(-1) using Unrestricted Hartree Fock(UHF) theory. As would be expecte d, the barrier calculated for Restricted Hartree Fock (RHF) was a litt le higher at E(f) = + 14.8 Kcal mol(-1). This confirms our initial hyp othesis that ring contraction can lead to much lower fracture energies than expected from simple Si-O bond breaking. Several different schem es of ring contractions are possible for both 5-fold and 6-fold ring s tructures. All contraction paths have different intermediate structure s that lead to the same end point of slow crack growth. The various ba rriers to fracture range from + 8 to + 52 Kcal mol(-1) for the 5-fold ring contractions and from + 9 to + 41 Kcal mol(-1) for 6-fold ring co ntractions.