Calculation of the ²⁹Si NMR Chemical Shifts of Aqueous Silicate Species

A DFT methodology for calculating ²⁹Si NMR chemical shifts of silicate species typically present prior to nucleation in zeolite synthesis solutions, incorporating solvent effects through an implicit representation is presented. We demonstrate how our methodology can reproduce the experimentally observed spectra and, by comparison to well characterized peaks in two different experimental studies, demonstrate the transferability and robustness of the methodology. We discuss certain cases in which caution must be exercised when implicit solvent representations are used for calculating silicate cluster geometries: those cases in which intramolecular hydrogen bonding can play a significant role in the geometry. A number of reassignments of previous tentative experimental assignments are proposed, and we also make assignments for the challenging substituted four-ring species. We present all of our computed chemical shift for previously observed species together with a number of other viable silicate clusters to serve as a reference point for future experimental studies