2J Si−O−Si Scalar Spin−Spin Coupling in the Solid State: Crystalline and Glassy Wollastonite CaSiO3

Abstract

We have investigated the 29Si−O−29Si 2J scalar spin−spin coupling constant in two crystalline polymorphs and in a glass of 29Si isotopically enriched wollastonite CaSiO3 composition. In the crystalline samples, line widths of less than 1 Hz are observed in the indirect dimension of a simple J-resolved NMR experiment leading to 2JSiOSi values of 1.5, 3.6, and 8.0 Hz determined with a high accuracy of ±0.1 Hz. The very high resolution obtained with those two compositions enabled us to evidence strong coupling effects in the J-resolved two-dimensional spectra. In particular, additional lines in J-spectra are seen here for the first time in solid-state NMR experiments of inorganic samples. Cluster ab initio calculations led us to propose a stereochemical analysis of 2J, leading to a close-to-linear relationship between 2JSiOSi and the Si−O−Si bonding angle Ω ≃ 3.41J + 127 which takes into account the predominant influence of the Ca atoms. In the glass, this relationship allowed the analysis of the distribution of angle within each Q(n) species. The classical relationship between the 29Si isotropic chemical shift and the average Si−O−Si bond angle is experimentally confirmed and quantified. The most probable set of Ω’s are 128.0° for Q(1), (131.8°,135.4°) for Q(2), and (132.5°,137.1°,144.3°) for Q(3), and the presence of three-membered rings is evidenced. An unexpectedly strong correlation between the bonding at each corner of a given Q(n) unit and the bonding at adjacent corners is found.