Topological disorder and reactivity of borosilicate glasses:: Quantum chemical calculations and 17O and 11B NMR study

We present O-17 and I IB NMR and quantum chemical calculations based on Hartree-Fock (HF) and density functional theory to investigate the site connectivity, the degree of chemical short-range order (CSRO), and the topological short-range order in binary borosilicate glasses, which can serve as a simple model system for more complex borosilicates and other covalent oxide glasses and melts. We also calculate the enthalpy of hydrolysis for each oxygen cluster in such glasses. The stability of Si-[4]-O-B-[3] was evaluated from the relative energy differences among the bridging oxygen clusters (Si-[4]-O-Si-[4], Si-[4]-O-B-[3], and B-[3]-O-B-[3]) calculated at the B3LYP/6-311+G(2d,p)//HF/3-21G and B3LYP/6-311+G(2d,p) levels of theory as well as data from O-17 3QMAS NMR. The fraction of Si-[4]-O-B-[3] at a boron mole fraction of 0.57 is about 37% (+/-1), suggesting a significant inter-dispersion of B-[3] and Si-[4], where a random distribution of Si-[4] and B-[3] and complete phase separation lead to 50 and 0% of Si-[4]-O-B-[3], respectively. B-11 MAS NMR shows that the fraction of boroxol rings increases with increasing boron content. The bond angle distribution function G(alpha) for each cluster was obtained from the lattice energy variation with angle. The calculation for Si-[4]-O-Si-[4], shows close similarity to that obtained by recent high energy photon diffraction experiments. We 4 also predicted the effects of composition, fictive temperature and degree of disorder among framework cations on G(alpha). The calculated enthalpies of hydrolysis of Si-[4]-O-Si-[4], Si-[4]-O-B-[3], and B-[3]-O-B-[3] are -16.98, -3.03, and 16.61 kJ/mol, respectively, implying differential reactivity with water and that Si-[4]-O-B-[3] clusters should have intermediate kinetic stability. Quantum chemical simulations also suggest a possible mechanism for the formation of the oxygen tricluster, related to the presence of five-membered rings.