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In part I of this series, a ring contraction model was proposed as the basic mechanism of slow crack growth in silica glass. AM1 molecular orbital theory running on a CAChe workstation was used to find the transition 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 found for the contraction of a 5-fold ring into a 4-fold ring. The activation barrier to fracture for this contraction is Ef = +7.9 Kcal mol−1 using Unrestricted Hartree Fock (UHF) theory. As would be expected, the barrier calculated for Restricted Hartree Fock (RHF) was a little higher at Ef = +14.8 Kcal mol−1. This confirms our initial hypothesis that ring contraction can lead to much lower fracture energies than expected from simple Si-O bond breaking. Several different schemes of ring contractions are possible for both 5-fold and 6-fold ring structures. All contraction paths have different intermediate structures that lead to the same end point of slow crack growth. The various barriers 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 contractions.
KeywordsFracture Energy Activation Barrier Silica Glass Bond Breaking Orbital Theory
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