New perspectives in the PAW/GIPAW approach: JP-O-Si coupling constants, antisymmetric parts of shift tensors and NQR predictions

Abstract

In 2001, Pickard and Mauri implemented the gauge including projected augmented wave (GIPAW) protocol for first-principles calculations of NMR parameters using periodic boundary conditions (chemical shift anisotropy and electric field gradient tensors). In this paper, three potentially interesting perspectives in connection with PAW/GIPAW in solid-state NMR and pure nuclear quadrupole resonance (NQR) are presented: (i) the calculation of J coupling tensors in inorganic solids; (ii) the calculation of the antisymmetric part of chemical shift tensors and (iii) the prediction of (14)N and (35)Cl pure NQR resonances including dynamics. We believe that these topics should open new insights in the combination of GIPAW, NMR/NQR crystallography, temperature effects and dynamics. Points (i), (ii) and (iii) will be illustrated by selected examples: (i) chemical shift tensors and heteronuclear (2)J(P-O-Si) coupling constants in the case of silicophosphates and calcium phosphates [Si(5)O(PO(4))(6), SiP(2)O(7) polymorphs and α-Ca(PO(3))(2)]; (ii) antisymmetric chemical shift tensors in cyclopropene derivatives, C(3)X(4) (X = H, Cl, F) and (iii) (14)N and (35)Cl NQR predictions in the case of RDX (C(3)H(6)N(6)O(6)), β-HMX (C(4)H(8)N(8)O(8)), α-NTO (C(2)H(2)N(4)O(3)) and AlOPCl(6). RDX, β-HMX and α-NTO are explosive compounds.

Magnesium incorporation into hydroxyapatite

“…The PBE generalised gradient approximation was used and the valence electrons were described by norm conserving pseudopotentials [72] 43 Ca and 25 Mg, the quadrupolar parameters C Q and η were also analysed. [38] It should be noted that based on previous studies and our own experience, the error on calculated 1 H and 31 P isotropic chemical shifts is estimated at ~0.4 [77] and 0.7 ppm, [78] respectively, whereas it is ~3 to 5 ppm on 43 Ca and 25 Mg isotropic chemical shifts, and ~0.5 to 2 MHz on 43 Ca and 25 Mg C Q values, [37,39] respectively. The four Ca(I) ions in the unit cell are usually referred to as "columnar calcium sites", because they form single atomic columns perpendicular to the basal plane.…”

Structural Insight into Layered Silicon Hydrogen Phosphates Containing [SiO6] Octahedra Prepared by Different Reaction Routes

“…Appropriate k-point meshes for crystalline models and optimize forces to lower than to 5 meV/Å were chosen. 29 Si and 31 P chemical shifts were calibrated based on the experimentally measured chemical shifts [60] for AIV-SiP 2 O 7 , [61] HexI-SiP 2 O 7 , [62] and Si 5 O(PO 4 ) 6 [63] structures. For those we chose an energy cutoff of 600 eV and found the 29 Si NMR chemical shifts converged to better than 0.2 ppm.…”

Prediction of NMR J-coupling in solids with the planewave pseudopotential approach

“…In a similar vein, care must be taken in comparing J involving different species. For example, 31 P– 17 O couplings are typically larger than 29 Si– 17 O couplings 11. Part of this difference is due to the fact that γ P ≈ 2γ Si , and it would be reasonable to expect that calculated errors in J for 31 P‐ 17 O couplings would be twice as large as for 29 Si– 17 O couplings.…”

“…Experimentally only the total tensor can be measured, however, the contributions (sometime called the Ramsey terms) can be useful in interpreting the observed J . Quantum chemical studies have shown that several general observations can be made,25 and so far these have been found to also apply to solid‐state systems:8–11 the DSO term is generally very small; the SD term is usually small, but can play a significant role certain couplings—e.g. those involving 19 F; for one bond couplings the PSO and FC terms are dominant; and for longer range couplings usually the FC is the dominant mechanism.…”

“…More recent work has enabled J tensors in solids to be computed within this framework 8. Only a few initial applications of this approach have been published 8–11. However, we take this early opportunity to highlight some issues which should be considered when making the comparison both with experiments and with quantum chemical approaches applied to cluster models of solids.…”