¹⁷O Multiple-Quantum MAS NMR Study of Pyroxenes

Abstract

Two-dimensional triple-quantum 17O (I = 5/2) MAS NMR powder spectra were obtained for orthoenstatite, clinoenstatite, and protoenstatite (MgSiO3 polymorphs) and diopside (CaMgSi2O6). The spectra were analyzed to yield the 17O isotropic chemical shifts (δCS) and quadrupolar coupling parameters (P Q = C Q(1 + η2/3)1/2) of the oxygen sites in each sample. High-resolution isotropic projections exhibit resolved 17O peaks equal in number to the number of crystallographically inequivalent oxygen sites. Bridging and nonbridging oxygens were distinguished by their characteristic P Q values. Further assignments were possible by comparing the isotropic 17O NMR spectra for this series of similar compounds with that reported previously for forsterite (Mg2SiO4). Full assignments of the three oxygen sites in protoenstatite and diopside and of the six sites in orthoenstatite and clinoenstatite are suggested.

Effects of the degree of polymerization on the structure of sodium silicate and aluminosilicate glasses and melts: An 17O NMR study

“…studies (Xue et al, 1994). This trend is similar to that of binary calcium silicate glasses (Lee and Stebbins, 2006) and is mostly likely due to an increase in the Si-O bond length (Ashbrook et al, 2002;Lee, 2004) and a decrease in the number of Na cations around BOs and NBOs. The effect of the former was supported by the recent quantum chemical calculations (Lee, 2004).…”

Disorder and the extent of polymerization in calcium silicate and aluminosilicate glasses: O-17 NMR results and quantum chemical molecular orbital calculations

“…This could lead to an increase in Si-O bond length. Recent NMR studies of crystalline chain silicates reported that O-17 isotropic chemical shifts do tend to increase with Si-NBO bond length (Ashbrook et al, 2002), which could account for the trend observed here for NBO.…”

Structural information from quadrupolar nuclei in solid state NMR

“…This can be seen in Fig. 2.2(b), a 17 O triple-quantum MAS spectrum (54.3 MHz) of orthoenstatite (MgSiO 3 ), a chain silicate (8). The fЈ 1 projection shows six sharp resonances corresponding to the six distinct O species in this material, while cross-sections parallel to f 2 yield anisotropically broadened MAS lineshapes.…”

“…See text for further details. (b) 17 O (54.3 MHz) NMR of orthoenstatite (MgSiO 3 ), including a conventional MAS spectrum, a two-dimensional triplequantum MAS spectrum, recorded using a split-t 1 sequence (15), and corresponding isotropic projection (8 ). At the end of t 1 the multiple-quantum coherence is then transferred to single-quantum coherence associated with the central transition ϩ1/2 3 Ϫ1/2.…”