The acidity and unique porous structures of zeolites play an important role in controlling the activity and selectivity of many zeolite-based catalysts. Although (27)Al, (29)Si and (1)H NMR spectroscopy represent standard analytical tools with which to study these materials, (17)O-NMR investigations are much less routine, owing to the very low natural abundance of (17)O (0.037%), its relatively low resonant frequency and its large quadrupole moment. (17)O-NMR resonances from framework oxygen sites in a variety of zeolites have been detected, but the (17)O-NMR resonance from oxygen directly bound to the Brønsted acid site (Si-O(H)-Al) has remained elusive. Here we report the direct observation of this resonance in dehydrated zeolite HY, by using high magnetic-field strengths. (17)O-(1)H double-resonance NMR experiments are used to prove unambiguously that the (17)O signal arises from O nearby H atoms. A large quadrupolar coupling constant, the measure of the local distortion of this site, of 6.6 MHz is determined, which is similar to that obtained in ab initio calculations of zeolite HY-like clusters; this value drops to 5 MHz on acetone binding. The results presented in this paper open up methods for characterizing zeolite acidity and investigating H(+)-sorbent interactions.

The 29Si and 17O NMR parameters of six polymorphs of MgSiO3 were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO3 is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the 17O high-resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO3 (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths >660 km. We present the first 17O NMR data for these materials and confirm previous 29Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested. The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For example, we find that, although there is a general correlation of increasing 17O chemical shift with increasing Si-O bond length, the shift observed also depends upon the exact coordination environment.