H2O/OH ratio determination in hydrous aluminosilicate glasses by static proton NMR and the effect of chemical shift anisotropy

Static H-1 NMR spectra of hydrous NaAlSi3O8 glasses have been acquired at l ow temperature (140 K) in order to quantitatively determine OH and H2O conc entrations. Since both components overlap in the spectra, an unambiguons de termination of the line shapes is required. The structurally bonded hydroxy l groups are well described by a Gaussian line and the water molecules exhi bit a Fake doublet-like line shape due to the strong proton-proton dipolar interaction. However, at proton resonance frequencies used in this study (3 60 MHz), the Fake doublet has an asymmetric line shape due to chemical shif t anisotropy (CSA), which is significant and must be included in any simula tion in order to reproduce the experimental line shape successfully. The si mulations for rigid water molecules dissolved in our hydrous aluminosilicat e glasses result in a CSA of 30 +/- 5 ppm and a dipolar interaction constan t of 63.8 +/- 2.5 kHz (i.e., dipolar coupling constant (DCC) of 42.6 +/- 1. 7 kHz), corresponding to a proton-proton distance of r(i j) = 154 +/- 2 pm. In contrast to earlier work, water speciation obtained from the simulation s of our H-1 NMR spectra are in excellent agreement with those obtained fro m infrared (IR) spectroscopy. (C) 2000 Elsevier Science B.V. All rights res erved.