Origin of NMR shielding in fluorides
In this work, we analyze in detail the relation between electronic structure and fluorine nuclear magnetic resonance (NMR) shielding in a series of solid state alkali fluorides (LiF, NaF, KF, RbF, and CsF). For that purpose, we use solid-state NMR calculations implemented in the density functional theory full potential WIEN2K code (APW + lo). Both measurements and calculations show that the NMR shielding varies across the series by approximately 200 ppm. We focus our discussion on an explanation of the origin of the observed trend, and we show that the variation is mainly determined by contributions from “semicore” metal-p and valence F-p bands. More specifically, the trend in fluorine shielding can be related to the small but significant change in the hybridization of these states. A second important ingredient determining the value of the shielding is the presence and position of metal-d states in the unoccupied part of the Kohn-Sham bands. Although the present analysis has been demonstrated for the 19F nucleus in alkali fluorides, the main results are more general and can explain similar trends observed in other solids and for other nuclei.
NMR Shielding in Metals Using the Augmented Plane Wave Method
We present calculations of solid state NMR magnetic shielding in metals, which includes both the orbital and the complete spin response of the system in a consistent way. The latter contains an induced spin-polarization of the core states and needs an all-electron self-consistent treatment. In particular, for transition metals, the spin hyperfine field originates not only from the polarization of the valence s-electrons, but the induced magnetic moment of the d-electrons polarizes the core s-states in opposite direction. The method is based on DFT and the augmented plane wave approach as implemented in the WIEN2k code. A comparison between calculated and measured NMR shifts indicates that first-principle calculations can obtain converged results and are more reliable than initially concluded based on previous publications. Nevertheless large k-meshes (up to 2 000 000 k-points in the full Brillouin-zone) and some Fermi-broadening are necessary. Our results show that, in general, both spin and orbital components of the NMR shielding must be evaluated in order to reproduce experimental shifts, because the orbital part cancels the shift of the usually highly ionic reference compound only for simple sp-elements but not for transition metals. This development paves the way for routine NMR calculations of metallic systems.
Chemical Shift, Relaxation Time, and Electric Quadrupole Coupling in Alkali Halides
Chemical shift and spin lattice relaxation times are measured for Cl 35 in all the alkali chloride crystals and for Br 81 and I 127 in some of the halides. These measured values cannot be explained by using Kondo-Yamashita's theory alone which takes into account the effect of overlap between the central ion and the nearest neighbors. For the chemical shift, it seems that the overlap with the second nearest neighbors is an essential cause of the discrepancy and that the covalent and the electrostatic effects are rather small. If these conclusions are admitted, there must be a large chemical shift (about -2∼-3×10 -4 ) of chlorine ion in aqueous solution relative to the free chlorine ion. A model combining the electrostatic and the overlap effect is proposed to explain reasonably the relaxation times in the crystals. The electric quadrupole coupling in free molecules of alkali halides is also discussed from the present viewpoint. The combination of the electrostatic and the overlap effects seems to give a re...