¹¹B MAS NMR and First-Principles Study of the [OBO₃] Pyramids in Borates

Abstract

Borates are built from the [Bϕ3] planar triangles and the [Bϕ4] tetrahedral groups, where ϕ denotes O or OH. However, the [Bϕ4] groups in some borates are highly distorted to include three normal B-O bonds and one anomalously long B-O bond and, therefore, are best described as the [OBO3] pyramids. Four synthetic borates of the boracite-type structures (Mg3B7O13Br, Cu3B7O13Br, Zn3B7O13Cl, and Mg3B7O13Cl) containing a range of [OBO3] pyramids were investigated by multifield (7.05, 14.1, and 21.1 T) (11)B magic-angle spinning nuclear magnetic resonance (MAS NMR), triple quantum (3Q) MAS NMR experiments, as well as density functional theory calculations. The high-resolution (11)B MAS NMR spectra supported by theoretical predictions show that the [OBO3] pyramids are characterized by isotropic chemical shifts δiso((11)B) from 1.4(1) to 4.9(1) ppm and nuclear quadrupole parameters CQ((11)B) up to 1.3(1) MHz, both significantly different from those of the [BO4] and [BO3] groups in borates. These δiso((11)B) and CQ((11)B) values indicate that the [OBO3] pyramids represent an intermediate state between the [BO4] tetrahedra and [BO3] triangles and demonstrate that the (11)B NMR parameters of four-coordinate boron oxyanions are sensitive to local structural environments. The orientation of the calculated unique electronic field gradient tensor element Vzz of the [OBO3] pyramids is aligned approximately along the direction of the anomalously long B-O bond, corresponding to B-2pz with the lowest electron density.

Ultrahigh-Field 25Mg NMR and DFT Study of Magnesium Borate Minerals

“…The structural model built from X-ray diffraction results underwent a geometrical optimization prior to carrying out NMR calculations. In our previous borate studies, ,,,, this approach was necessary to improve the calculated results because it is difficult to accurately locate the positions of light elements such as H, B, and F, via X-ray diffraction. ,, As such, a comparison of the theoretically calculated NMR parameters (chemical shielding and EFG tensors) with the experimentally measured NMR parameters (δ iso , C Q , η) may provide refinements of the structural models obtained via diffraction methods.…”

“…The synthetic series studied includes boracite, cubic-Mg 3 B 7 O 13 Br, MgB 4 O 7 , kotoite, and suanite. Boracite and cubic-Mg 3 B 7 O 13 Br samples were synthesized via the boric acid flux method . Briefly, for the sample of boracite this involved heating 1 g of MgCl 2 between two 0.5 g layers of H 3 BO 3 in a Teflon-lined stainless steel autoclave at 240 °C for 72 h; for the Mg 3 B 7 O 13 Br sample, 0.3 g of MgBr 2 and 0.5 g B 2 O 3 were heated to 500 °C in an evacuated silica tube for 72 h. Synthesis of MgB 4 O 7 involved heating a MgO:B 2 O 3 mixture at the molar ratio of 1:3 in a platinum crucible at 750 °C and atmospheric pressure for 24 h. Kotoite and suanite were synthesized by dissolving MgCl 2 ·6H 2 O and H 3 BO 3 in deionized water at stoichiometric proportions (3:2 for kotoite and 1:1 for suanite), followed by heating of the solutions to 110 °C until the stoichiometric gel products formed by evaporation (∼24 h).…”

“…The retention of the unit cell parameters from room-temperature XRD experiments versus a default thermal relaxation to 0 K was intended to facilitate direct comparison with data obtained from room-temperature NMR spectra . Calculations using the optimized structures of inderite and kurnakovite yielded significantly more accurate C Q and η values than those calculated from the original XRD structures in our previous study, hence results from this research were performed with XRD crystal structures − ,,,− that underwent a full optimization of atom positions while the unit cell parameters were kept constant. Calculations were performed on the Grex SGI Altix XE 1300 cluster, which consists of 316 compute nodes, each with two 6-core Intel Xeon X5650 2.66 GHz processors (Westgrid, Compute Canada, University of Manitoba).…”

“…Magnesium, as a strongly lithophilic element, combines readily with oxygen to form diverse minerals (e.g., borates, carbonates, oxides/hydroxides, silicates, and sulfates) on Earth’s surface. For example, magnesium borates are common minerals in salt spring deposits and evaporites and are known to feature great structural diversity arising from the complex linkages of [3] B and [4] B groups and from the presence of different Mg coordination environments. − …”

“…For several nuclei, for example, 11 B, 23 Na, 27 Al, and 73 Ge, correlations between nuclear quadrupole coupling constants, C Q , and |α| or |ψ| have been documented, − suggesting that effects from the local chemical environment dominate the electric field gradient (EFG) at the observed nucleus. To date, empirical correlations between 25 Mg NMR parameters and the Mg local structural environments have been documented for several families of Mg compounds such as phosphates, aluminates, oxides, silicates, sulfates, nitrates, and carboxylates. ,,,, However, there have been relatively few 25 Mg NMR studies of borates. , Borates are known to feature diverse Mg coordination polyhedra with a wide variety of nearest-neighbor ligands (O 2– , OH – , H 2 O, F – , Cl – , and Br – ), including unusual structures such as the extremely elongated [MgO 4 Br 2 ] tetragonal dipyramid with MgBr and MgO bond lengths of 3.009 and 2.015 Å, respectively, in cubic Mg 3 B 7 O 13 Br; and the highly asymmetrical [MgO 4 Cl] and [MgO 5 ] polyhedra in boracite (Mg 3 B 7 O 13 Cl) and MgB 4 O 7 , respectively. ,, …”