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2018
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D.J.Srivastava, J.H.Baltisberger, P.Florian, F.Fayon, R.A.Shakhovoy, M.Deschamps, N.Sadiki, P.J.Grandinetti, 'Bond Angle Distribution in Silica Glass from 2D J-Resolved Spectroscopy', Phys. Rev. B 98 134202 (2018) doi:10.1103/physrevb.98.134202A 2D J-resolved magic-angle spinning NMR spectrum of silica glass at 29Si natu- ral abundance levels, 4.7%, was measured using the Shifted-Echo Phase Incremented Echo Train Acquisition (SE-PIETA) pulse sequence. At 29Si natural abundance levels the JSi-O-Si couplings splittings appear as overlapping doublet patterns arising from isolated 29Si-O-29Si linkages. The experimental 2D J-resolved spectrum is analyzed to obtain a bi-variate probability distribution correlating the central Si-O-Si angle of a Q4{Q4 linkage to its mean Si-O-Si angle (seven angles) using relationships be- tween 29Si isotropic chemical shifts and geminal JSi-O-Si coupling of a Q4{Q4 to its local structure. To obtain a self-consistent bi-variate probability distribution it was necessary to introduce an additional dependence of the 29Si chemical shift of a Q4 on mean Si-O distance as well as mean Si-O-Si angle. The implication of this neces- sary modication is that there appears to be a positive correlation between Si-O-Si angle and Si-O distance in the silica glass, running opposite to the trend generally found in crystalline silica polymorphs. From the analysis of the 29Si 2D J-resolved spectrum we determine a Si-O-Si bond angle distribution in silica glass as having a mean at 147:8, a mode at 147 and a standard deviation of 10:7. The Si-O-Si angle distribution determined from the spectrum is consistent with previous high-energy and neutron diraction studies. Our statistical model for analyzing the experimental 29Si 2D J-resolved spectrum also indicates that the individual Si-O-Si bond angle distributions are relatively uncorrelated. |