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

We have investigated the 29 Si−O− 29 Si 2 J scalar spin−spin coupling constant in two crystalline polymorphs and in a glass of 29 Si isotopically enriched wollastonite CaSiO 3 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 2 J SiOSi 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 2 J, leading to a close-to-linear relationship between 2 J SiOSi and the Si−O−Si bonding angle Ω ≃ 3.41 J + 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 29 Si 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.