We have investigated the Al/Si ordering in the pseudoisolated pairs of tetrahedral sites of the structure of crystalline gehlenite Ca 2Al 2SiO 7 by means of 29Si and 27Al NMR and first-principles quantum mechanical calculations. 29Si NMR spectra of isotopically enriched samples enables the precise determination of the population of the two silicon sites Si-(OAl) 3-n(OSi) n (n = 0, 1) and hence the amount of Al-O-Al linkages. This leads to a reliable and model-free quantification of the departure from the Loewenstein rule and to an experimental Al/Si ordering enthalpy of 50.4 ± 1.6 kJ/mol fully reproduced by the quantum mechanical calculations. The seven aluminum sites arising from the Al/Si substitutions Al-(OAl) 4-p(OSi) p (0 ≥ p ≥ 4) and Al-(OAl) 3-p(OSi) p (p = 0, 1) are identified by 27Al MAS, MQMAS, and { 29Si} 27Al HMQC experiments, with their quantification being consistent with a fully disordered arrangement of the tetrahedral pairs in the a-b plane of the structure. Assignments of those strongly overlapping lines are further confirmed by density functional theory (DFT) calculations performed on a series of 2 × 25 supercells. An experimental and computational variation of -3 ppm of the 27Al isotropic chemical shift is obtained for the substitution of one Al by one Si in the second coordination sphere of a central Al atom. 29Si and 27Al isotropic chemical shifts are seen to be sensitive primarily to short-range structural variations whereas a more complex behavior related to the nearby presence of Loewenstein-violating pairs is observed for the 27Al quadrupolar coupling constant. Decomposition of the calculated EFG tensors into a sum of local, nonlocal, and ionic components demonstrates that it is almost entirely determined by the local electronic structure near the T 1 nucleus. The width of the distribution of NMR parameters is seen to strongly correlate to the degree of ordering present in the material. Scalar coupling constants 2J(T-O-T) (with T = Al, Si) are found to be linearly related to the TOT bond angle. © 2012 American Chemical Society.
