Quantitative study of the short range order in B 2O 3 and B 2S 3 by MAS and two-dimensional triple-quantum MAS 11B NMR

Abstract

Two-dimensional multiple-quantum magic angle spinning (MQMAS) NMR and MAS NMR of 11B at various magnetic fields, were applied to elucidate the structure of vitreous (glassy) boron trioxide (v-B 2O 3), vitreous boron trisulfide (v-B 2S 3) and crystalline boron trisulfide (c-B 2S 3). These techniques, when combined with computer simulations of the resulting spectra, provide the isotropic chemical shifts and the quadrupole parameters, as well as a quantitative measure of the intensities of various boron resonances. The MAS NMR of v-B 2O 3 produced overlapping anisotropic lineshapes corresponding to the − 1/2 ↔ 1/2 transition in two distinct types of BO 3 units with 3(±0.08):1 intensity ratio. A combination of MAS and the multiple-quantum method resulted in a better resolved, isotropic 11B spectrum of v-B 2O 3. A remarkable enhancement of resolution of the MQMAS NMR proved instrumental in finding and identifying various impurities present in v-B 2S 3 and c-B 2S 3. In addition to the resonances from boron in two types of BS 3 groups, four other structural units, BOS 2 BO 2S, BO 3 and BS 4, were elucidated from the spectra of vitreous and crystalline samples. The effects of various experimental parameters, such as the magnitude of the B 0 and B 1 fields, on the resolution of the MAS and MQMAS techniques are also shown.