A computational investigation of 17O quadrupolar coupling parameters and structure in alpha-quartz phase GeO2.

Ab initio band-structure calculations based on density functional theory have been completed for alpha-quartz phase GeO2 to obtain electric-field gradients (efg) for oxygen atoms, including those for GeO2 at elevated pressure and temperature. To interpret the resulting efg values and examine correlations between structure and 17O quadrupolar coupling parameters, additional ab initio self-consistent Hartree-Fock molecular orbital calculations were completed. The quadrupolar coupling constant was found to have a strong dependence on Ge-O distance and angleGe-O-Ge, with the quadrupolar asymmetry parameter being primarily dependent on angleGe-O-Ge. Analytical expressions describing these dependencies consistent with earlier investigations of analogous silicate compounds are also reported.

Modeling the (13)C chemical-shift tensor in organic single crystals by quantum mechanical methods: finite basis set effects.

The influence of using finite basis sets to calculate (13)C magnetic shieldings were explored using the Hartree-Fock and the B3LYP hybrid density functional methods. The shielding values were compared in a linear least-squares fashion for a test group of 102 (13)C complete chemical-shift tensors determined from 14 organic single crystals. Pople's basis sets allow for the addition of polarization and diffuse functions in a straightforward way, allowing the examination of 81 combinations at the double and triple zeta level. Dunning's correlation-consistent basis sets were explored as well. The errors associated with predicting the shielding values were found to be largely systematic as revealed by the analysis of the determined regression parameters between calculated chemical shieldings and experimental chemical shifts. Expansion of the basis set leads to a convergence of these regression parameters to their ideal values. The random errors, however, do not decrease by employing larger basis sets; therefore, given the appropriate regression parameters, a small basis description such as 3-21G can be adequate in predicting the relative magnetic-shielding values, i.e. the chemical shifts. Furthermore, in certain cases the inclusion of unbalanced diffuse and polarization functions can significantly degrade the predicted shielding rmsd. Unless employed carefully, these functions do not justify their computational expense. The chemical-shift distance is used to evaluate shielding predictions in individual tensor components. The analysis of the chemical-shift's distance between calculated and experimental data indicates an orientational dependence on the magnitude of errors and suggests the use of the shift anisotropy as a useful fiduciary mark to optimize model chemistries for magnetic-shielding calculations.

Modeling NMR chemical shift: A survey of density functional theory approaches for calculating tensor properties.

The NMR chemical shift, a six-parameter tensor property, is highly sensitive to the position of the atoms in a molecule. To extract structural parameters from chemical shifts, one must rely on theoretical models. Therefore, a high quality group of shift tensors that serve as benchmarks to test the validity of these models is warranted and necessary to highlight existing computational limitations. Here, a set of 102 13C chemical-shift tensors measured in single crystals, from a series of aromatic and saccharide molecules for which neutron diffraction data are available, is used to survey models based on the density functional (DFT) and Hartree-Fock (HF) theories. The quality of the models is assessed by their least-squares linear regression parameters. It is observed that in general DFT outperforms restricted HF theory. For instance, Becke's three-parameter exchange method and mpw1pw91 generally provide the best predicted shieldings for this group of tensors. However, this performance is not universal, as none of the DFT functionals can predict the saccharide tensors better than HF theory. Both the orientations of the principal axis system and the magnitude of the shielding were compared using the chemical-shift distance to evaluate the quality of the calculated individual tensor components in units of ppm. Systematic shortcomings in the prediction of the principal components were observed, but the theory predicts the corresponding isotropic value more accurately. This is because these systematic errors cancel, thereby indicating that the theoretical assessment of shielding predictions based on the isotropic shift should be avoided.