Ab initio calculation of B and O nuclear quadrupole coupling constants and NMR shielding constants in molecular models for B 2O 3 glass

Abstract

Ab initio Hartree-Fock-Roothaan calculations with large polarized basis sets yield 17O nuclear quadrupole coupling constants, e 2 q O Q O/ h, in planar H 2BOBH 2, a molecular model for bridging oxygens in B 2O 3, which vary strongly with angle. The calculated e 2 q O Q O/ h values of 4.8 and 6.1 MHz for ∠BOB of 120° and 132°, respectively, are in good agreement with experimental values of 4.7 and 5.8 MHz for the two inequivalent oxygens in vitreous B 2O 3, for which ∠BOB of 120° (boroxol rings) and 128–132° (BO 3 triangles linked to boroxol rings) have been obtained by X-ray and neutron diffraction. The calculated 17O NMR shielding constant, σ, changes from −91 ppm for H 2BO − to 87 ppm for H 2BOBH 2 (BOB = 132°) and increases with ∠BOB at a rate of about 1 ppm/deg. Less accurate smaller basis set calculations on H 3B 3O 6 rings and on the H 2BOBH 2 molecule in lower symmetries indicate that the 17O q values are not changed significantly by ring closure but are affected by nonzero dihedral angles between the BH 2 planes. 17O and central T atom values of σ and q are also calculated for the series BO 3 3−, CO 3 2− and NO 3 − and compared witt experiment. The O in NO 3 − is calculated for the deshielded by 325 ppm with respect to H 2O(g), consistent with an experimental chemical shift of about 384. The calculated anisotropy in σ T increases along the BO 3 3− −NO 3 − series, consistent with experiment. These results indicate that accurate calculations on simple molecular models can reproduce the trends in q and σ observed in borate glasses and other solids.