Distinct 29Si MAS NMR Peaks from Si–Al Permutation on Neighboring T Sites of Unequal Si–O–T Angles: Direct Evidence from J-Resolved Experiment on K-Cymrite (KAlSi3O8·H2O)

Abstract

29Si MAS NMR is a well-established tool for quantitative characterization of Si–Al distribution in aluminosilicates. In framework aluminosilicates, all tetrahedrally coordinated Si/Al (T) are linked via bridging oxygens to four other Si/Al (Q4). Each crystallographically unique T site is commonly assumed to give at most five 29Si NMR peaks with chemical shift spacing around 5 ppm, corresponding to five Si units each linked to nSi and (4 – n)Al (n = 0–4), abbreviated as Si(nSi) hereafter. Here we report a detailed one- and two-dimensional (2D) 29Si NMR study on K-cymrite (KAlSi3O8·H2O), which possesses a double-layered structure with all Si/Al distributed in one crystallographically unique T site (Q4). Contrary to general belief, more than five 29Si MAS NMR peaks were resolved. These peaks are attributable, via 2D J-coupling mediated experiments, to Si(nSi)(n = 0–4), with two peaks each (∼2 ppm spacing) for n = 1–3. The latter corresponds to different permutations of the nSi(4 – n)Al over neighboring T sit...