Correlations between 11B NMR parameters and structural characters in borate and borosilicate minerals investigated by high-resolution MAS NMR and ab initio calculations |
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| Authors: | Bing Zhou Zhaohua Sun Yefeng Yao Yuanming Pan |
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| Institution: | (1) Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Beijing, 810008, Qinghai, China;(2) College of Materials Science and Engineering, Tongji University, Tongji, China;(3) Shanghai Key Laboratory of Magnetic Resonance, Shanghai, China;(4) Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, S7N 5E2, Canada; |
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| Abstract: | Borates consisting of diverse fundamental building blocks (FBB) formed from complex polymerization of planar triangular Bϕ3] groups and tetrahedral Bϕ4] groups, where ϕ = O and OH, provide an excellent opportunity for investigation of correlations between the NMR parameters
and local structures. However, previous studies suggested that the 11B NMR parameters in borates are insensitive to local structural environments other than the B coordination number, in contrast
to those documented for 29Si, 23Na and 27Al in silicates, and no correlation between 11B 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 11B 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 11B in borate and borosilicate minerals. In particular, this study demonstrates that the small variation in 11B 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. |
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