Topological Disorder and Reactivity of Borosilicate Glasses: Quantum Chemical Calculations and 17O and 11B NMR Study

We present 17O and 11B 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 [4]Si−O−[3]B was evaluated from the relative energy differences among the bridging oxygen clusters ([4]Si−O−[4]Si, [4]Si−O−[3]B, and [3]B−O−[3]B) 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 17O 3QMAS NMR. The fraction of [4]Si−O−[3]B at a boron mole fraction of 0.57 is about 37% (±1), suggesting a significant inter-dispersion of [3]B and [4]Si, where a random distribution of [4]Si and [3]B and complete phase separation lead to 50 and 0% of [4]Si−O−[3]B, re...