Ab initio calculation of B and O nuclear quadrupole coupling constants and NMR shielding constants in molecular models for B_2O_3 glass

Ab initio Hartree-Fock-Roothaan calculations with large polarized basis sets yield 17O nuclear quadrupole coupling constants, e2qOQO/h, in planar H2BOBH2, a molecular model for bridging oxygens in B2O3, which vary strongly with angle. The calculated e2qOQO/h values of 4.8 and 6.1 MHz for angleB---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 B2O3, for which angleB---O---B of 120° (boroxol rings) and 128–132° (BO3 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 H2BO− to 87 ppm for H2BOBH2 (B---O---B = 132°) and increases with angleB---O---B at a rate of about 1 ppm/deg. Less accurate smaller basis set calculations on H3B3O6 rings and on the H2BOBH2 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 BH2 planes. 17O and central T atom values of σ and q are also calculated for the series BO33−, CO32− and NO3− and compared witt experiment. The O in NO3− is calculated for the deshielded by 325 ppm with respect to H2O(g), consistent with an experimental chemical shift of about 384. The calculated anisotropy in σT increases along the BO33− −NO3− 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.