The 11B quadrupole interaction and nonbridging oxygens in crystalline borates

Coordination of boron in nominally boron-free rock forming silicates: Evidence for incorporation of BO3 groups in clinopyroxene

“…Whereas tetrahedral B environments are generally associated with small quadrupolar coupling constants C Q 6 1 MHz and isotropic chemical shifts d iso $ 0 ppm, the less symmetrical trigonal coordination leads to significantly quadrupolar-broadened NMR signals (C Q $ 2.4-3.0 MHz), typically originating in the less shielded spectral region d iso > 12 ppm (MacKenzie and Smith, 2002;Kriz and Bray, 1971;Turner et al, 1986;van Wü llen and Mü ller-Warmuth, 1993;Kroeker and Stebbins, 2001).…”

“…They assumed a single 11 BO 3 site associated with nearly identical NMR parameters for Ba8 and Ba17 (see Experimental section). The relatively high value g % 0.65 of the EFG asymmetry parameter is suggestive of either one or two non-bridging oxygen (NBO) atoms at the BO 3 groups; the more symmetric trigonal environments associated solely with bridging oxygen (BO) or NBO species normally display low asymmetry parameter-values g < 0.3 (Kriz and Bray, 1971;Kroeker and Stebbins, 2001). Due to the minute amounts of boron relative to silicon in the structure, all bridging oxygen atoms at the B sites are expected to involve B-O-Si bonds, implying B(OSi) 2 O À or B(OSi)(O À ) 2 trigonal moieties.…”

Boron speciation and non-bridging oxygens in high-alkali borate glasses

“…Foremost among this poorly charted territory is the reliable characterization of non-bridging oxygens in high-alkali-content glasses. Efforts to distinguish amongst three-coordinate borons with zero (T 3 ), one (T 2 ), two (T 1 ) or three (T 0 ) non-bridging oxygens by NMR have focussed on characterizing the quadrupolar parameters [1] and chemical shifts [2] in crystalline phases of known structure. Nevertheless, broad signals from the sizeable second-order quadrupolar interactions -exacerbated by inherent structural disorder -result in unavoidable peak overlap in one-dimensional magic-angle spinning (MAS) spectra.…”

g-Factor calculations for the species occurring in borate glasses

“…In the region R $ 1, the fourfold coordinated units coexist with three-fold coordinated units [23][24][25][26][27][28][29][30]. Raman spectroscopy suggested the coexistence between pyroborate and orthoborate units [31].…”