1D and 2D solid state NMR investigations of the framework structure of As-synthesized AlPO4-14

The framework structure of As-synthesized ALPO(4)-14 has been investigated with a combination of different one-dimensional Al-27 and P-31 solid State NMR techniques and Al-27/P-31 double resonance methods. The results are found to be fully consistent with the assumed structural model. Al-27 MAS and DOR experiments at three different magnetic field strengths together with simulations show the presence of two tetrahedral sites, one pentacoordinated and one octahedral aluminum site. The Al-27 quadrupolar coupling constants and the P-31 isotropic chemical shifts of the tetrahedral sites correlate well with tetrahedral shear-strain parameters and mean P-O-Al bond angles, respectively. These correlations allow one to assign all of the NMR resonances to specific T-sites in the proposed framework structure. The assignments are then further confirmed by the application of three different two-dimensional heteronuclear correlation methods (i.e., Al-27 --> P-31 TEDOR, CP, and INEPT) which reveal the connectivities between AlOx and PO4 polyhedra. The two-dimensional INEPT experiment is applied here for the first time in the solid state. (C) 1997 Published by Elsevier Science B.V.