Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of 29Si–O–29Si scalar couplings

Abstract

Subtle structural details of siliceous zeolites are probed by using two-bond scalar (J) coupling constants to characterize covalently bonded ²⁹Si–O–²⁹Si site pairs and local framework order. Solid-state two-dimensional (2D) ²⁹Si{²⁹Si} NMR measurements and first-principles calculations of ²J(²⁹Si–O–²⁹Si) couplings shed insights on both the local structures of siliceous zeolites Sigma-2 and ZSM-12, as well as the sensitivity of J couplings for detailed characterization analyses. DFT calculations on a model linear silicate dimer show that ²J(Si–O–Si) couplings have complicated multiple angular dependencies that make semi-empirical treatments impractical, but which are amenable to cluster approaches for accurate J-coupling calculations in zeolites. DFT calculations of ²J(²⁹Si–O–²⁹Si) couplings of the siliceous zeolite Sigma-2, whose framework structure is known to high accuracy from single-crystal X-ray diffraction studies, yield excellent agreement between calculated and experimentally measured ²J(Si–O–Si) couplings. For the siliceous zeolite ZSM-12, calculated ²J(²⁹Si–O–²⁹Si) couplings based on less-certain powder X-ray diffraction analyses deviate significantly from experimental values, while a refined structure based on ²⁹Si chemical-shift-tensor analyses shows substantially improved agreement. ²⁹SiJ-coupling interactions can be used as sensitive probes of local structures of zeolitic frameworks and offer new opportunities for refining and solving complicated structures, in combination with complementary scattering, modeling, and other nuclear spin interactions.