Calculation of the NMR shielding of O in MgO at high temperature

Abstract

17O NMR studies indicate that the O in crystalline MgO becomes less shielded magnetically as the temperature is increased. Over a range of about 1300 °C the shielding decreases by about 10 ppm. Increasing the temperature in MgO also leads to a small increase in the average Mg-O bond length and a large increase in the isotropic temperature factor, corresponding to a large increase in the root mean square deviation of the Mg-O distance from its equilibrium value. Using the gauge-including-atomic-orbital (GIAO) version of coupled Hartree-Fock perturbation theory, implemented at the self-consistent-field (SCF) level using a polarized split valence basis set, we have calculated the NMR shielding at the central O of a cluster OMg6(OH)12 −2, Oh symmetry, for several values of the Mg-O distance within a range of 30% about our calculated equilibrium value. We have also considered some distorted geometries for this cluster. We find that the O NMR shielding decreases nonlinearly as the central Mg-O distance is decreased. For the shortest Mg-O distance studied (1.391 A) in the symmetric geometry the calculated O NMR shielding was 229.4 ppm, while at the equilibrium Mg-O distance calculated for the cluster (1.987 A) the calculated shielding was 317.8 ppm. If the average 17O shielding is estimated by weighting the calculated NMR shieldings at various Mg-O distances by the probability of that distance, as obtained from the experimental isotropic temperature factors, at least a part of the observed deshielding at high temperatures can be reproduced. Calculations on a C4v symmetry OMg6(OH)12 −2 cluster, with the central O displaced from the centre, show even stronger deshielding. Therefore, consideration of such asymmetric motions within the central OMg6 unit could further reduce the calculated 17O shielding.