We
use the gauge-including projector augmented-waves (GIPAW) method
to report, for the first time, theoretical 27Al Knight
shifts in metallic systems other than metallic Al. We consider metallic
Al and a set of six intermetallic compounds, for which experimental
chemical shifts were recently made available in the literature. The
orbital and spin components of the chemical shielding tensors are
computed from the same ground-state spin-polarized electronic structure,
converged under the influence of a uniform external magnetic field.
A linear response formalism is used to compute the orbital part, while
the spin part is approximated by the linear relationship between the
external field and the Fermi contact contribution to the induced magnetic
hyperfine field at the nuclear position. Core spin-polarization effects
are taken into account by means of a perturbative approach. Our results
show that the GIPAW approach yields reasonably acceptable chemical
shifts with affordable k-meshes in the irreducible
Brillouin zone, enabling separation between contributions to metallic
shifts from the electron orbital and from the electron spin susceptibilities.
The reaction of HF molecules with brucite, Mg(OH)(2), leading to the formation of Mg(OH)(2-x)F(x), was theoretically studied by ab initio density functional theory (DFT) with periodic boundary conditions. We proposed as mechanism for this reaction four elementary steps: adsorption of the HF molecule, OH(-) liberation from brucite as a water molecule, desorption of the newly formed H(2)O, and rearrangement of the F(-) anion into a hydroxyl position. For the Mg(OH)(2-x)F(x) formation, with x = 1/9, the final product, outcome from an initially adsorbed HF molecule, we computed the Helmholtz free energy variation DeltaF = -23 kcal/mol. The calculated frequency for the most intense infrared band, a Mg-F stretching mode, was 342 cm(-1). Two transition states, corresponding to the hydroxyl reacting with a proton forming a water molecule and migration of a fluoride anion into a hydroxyl vacancy, were computed. The calculated reaction barriers indicate that the reaction between Mg(OH)(2) layers and HF molecules is slow and irreversible.
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