Quantitative study of the short range order in B2O3 and B2S3 by MAS and two-dimensional triple-quantum MAS B-11 NMR

Two-dimensional multiple-quantum magic angle spinning (MQMAS) NMR and MAS NMR of B-11 at various magnetic fields, were applied to elucidate the structure of vitreous (glassy) boron trioxide (v-B2O3), vitreous boron trisulfide (v-B2S3) and crystalline boron trisulfide (c-B2S3). 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-B2O3 produced overlapping anisotropic lineshapes corresponding to the -1/2 <-> 1/2 transition in two distinct types of BO3 units with 3(+/-0.08):1 intensity ratio. A combination of MAS and the multiple-quantum method resulted in a better resolved, isotropic B-11 spectrum of v-B2O3. A remarkable enhancement of resolution of the MQMAS NMR proved instrumental in finding and identifying various impurities present in V-B2S3 and c-B2S3. In addition to the resonances from boron in two types of BS, groups, four other structural units, BOS,, BO2S, BO3 and BS4, 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. (C) 1997 Elsevier Science B.V.