Quantitative NMR studies of water in silicate glasses

The state of water in volcanic and synthetic silicate glasses is characterized by a number of different solid state NMR techniques. ^1H solid echo NMR is used to quantitate total water contents reliably down to levels of 0.1 wt%. High-speed ^1H magic-angle spinning NMR can be used to obtain relative species concentrations of OH and H_2O groups in glasses free from paramagnetic impurities. Similar results are obtained from deuterium quadrupolar echo NMR difference spectroscopy, utilizing the different spin-lattice relaxation times of OD and D_2O species. The results demonstrate that the percentage of water present in the form of molecular H_2O species increases with increasing total water content. The H_2O groups are anisotropically constrained, but undergo 180° flips about the bisector axis at rates greater than 10^5 s^(−1). The hydrous species are not clustered and are subjected to a distribution of hydrogen bonding strengths. A distribution function of O-D···O distances obtained from infrared results, shows good compatibility with the experimental ^2H NMR lineshapes. The relative merits of the different experimental approaches are discussed in detail.