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.
Understanding of 33 S NMR Shielding in Inorganic Sul fi des and Sulfates
Density functional theory (DFT) calculations of the magnetic shielding in solid state nuclear magnetic resonance (NMR) experiments provide an important contribution for the understanding of the experimentally observed chemical shifts. In this work we focus on the relation between atomic and orbital character of the valence and conduction band wave functions and the S NMR shielding in sulfides and sulfates. This allows us to understand the origin of the observed large (over 1000 ppm) variation of the chemical shifts measured at the sulfur nucleus. We show that the variation of the NMR chemical shifts in sulfides is mostly related to the presence of metal d states and their variation in the energy position within the conduction bands.
SCIENCE AND EMPIRICISM IN PILE FOUNDATION DESIGN
Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design—that of estimating the axial capacity—relies heavily upon ...