Correlations between 11B NMR parameters and structural characters in borate and borosilicate minerals investigated by high-resolution MAS NMR and ab initio calculations

Abstract

Borates consisting of diverse fundamental building blocks (FBB) formed from complex polymerization of planar triangular [B/ 3 ] groups and tetrahedral [B/ 4 ] groups, where / = O and OH, provide an excellent opportunity for investigation of correlations between the NMR parameters and local structures. However, previous studies suggested that the 11 B NMR parameters in borates are insensitive to local structural environments other than the B coordination number, in contrast to those documented for 29 Si, 23 Na and 27 Al in silicates, and no correlation between 11 B chemical shifts and the sum of bond valences has been established for borate minerals with hydroxyl groups or molecular water in the structures. In this study, high-resolution NMR spectra have been acquired at the ultra high field of 21 T as well as at 14 T for selected borate and borosilicate minerals, and have been used to extract high-precision NMR parameters by using combined ab initio theoretical calculations and spectral simulations. These new NMR parameters reveal subtle correlations with various structural characters, especially the effects of the 11 B chemical shifts from the bridging oxygen atom(s), site symmetry, symmetry of FBB, the sum of bond valences, as well as the next-nearest-neighbor cations and hydrogen bonding. Also, these results provide new insights into the shielding mechanism for 11 B in borate and borosilicate minerals. In particular, this study demonstrates that the small variation in 11 B chemical shifts can still be used to probe the local structural environments and that the established correlations can be used to investigate the structural details in borates and amorphous materials.

Plasticizing and crosslinking effects of borate additives on the structure and properties of poly(vinyl acetate)

“…2a, it can be seen that there are two types of boron sites present in the dried borax powder, one is tetra-coordinated boron [B4 4 ] denoted in region A (approximately 1 ppm), and the other is tri-coordinated boron [B4 3 ] shown in region B (approximately 15 ppm), where 4 refers to either O or OH. 29 In Fig. 2b, one can see that there are two new resonance lines in the PVAc/borate samples in addition to regions A and B, which is attributed to the crosslinked tetracoordinated boron species (region C, approximately 5 ppm) and exchanging boron atoms between boric acid and borate ions (region D, approximately 10 ppm), consistent with previous reports.…”

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.…”

“…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. − …”

Computational simulations of solid state NMR spectra: a new era in structure determination of oxide glasses

“…This can be achieved either empirically by reference to materials of a known structure or theoretically using ab initio calculations. The first approach has been extensively applied in silicates for 29 Si NMR, 43 17 O NMR, 13,44,45 and 23 Na NMR, 12,[46][47][48][49] for 27 Al NMR, 50,51 11 B NMR 52,53 and other nuclei. 36 However, crystalline systems generally exhibit a limited diversity of local structures in contrast to the disorder and variety of structural units (different coordination numbers and Q n distributions) present in multicomponent glasses.…”