Molecular Orbital Approach to Structures and Basicity of Binary Borate Glasses

Abstract

Structures and basicity of binary borate glass were studied by applying cluster approximation and MNDO semiempirical SCF-MO method. Optimized geometries of the characteristic structural unit in borate glass, pentaborate, triborate and diborate clusters showed good agreement with the experimentally determined ones. The bond length variations arising along with the formations of four fold coordinated boron (⁴B) or non bridging oxygen (NBO) reflect the intramolecular charge rearrangement and conform well to the pred iction of Gutmann's bDnd length variation rule. Ten isomers of molecular formula [H₈B₁₂O₂₃]^2-, were constructed in order to represent the structures in various composition of binary borate glass. Delocalization energy resulting from molecular orbital interaction, sparkle affinity and proton affinity were estimated as the measures of the basicity in binary borate glass. On the basis of the hard and soft acid and base (HSAB) theory, BO₄^- unit is classified rather into a hard base and NBO is classified rather into a soft one. A hard dopant cation is stabilized in the electrostatic field around BO₄^- and a soft one coordinated by NBO and taken into the borate network. In the case where H₂O is the counter oxide, proton forms the OH terminal in the low H₂O content range and BO₄^- appears in the medium H₂O content range. Thus, the proton behaves differently from the alkali metal cations.