Theoretical investigations of NMR chemical shieldings on the AlPON catalyst system

Using molecular cluster models we present a theoretical ab initio study of the Al-27 and P-31 nuclear magnetic resonance (NMR) chemical shieldings aim ed at obtaining short-range structural information on the aluminophosphate oxynitride (AlPON) catalyst system. We have employed orthophosphate-like mo lecular models with P/Al ratio equal to one and varying N/O ratios to simul ate the experimentally obtained compositions. The computed NMR chemical shi eldings reproduce quantitatively the observed features in the P-31 magic an gle spinning NMR (MAS-NMR) spectra and permit the explanation of these modi fications in terms of a progressive and preferential nitridation on the fir st coordination shell of the phosphorus atom leading to a mixture of [POxN4 -x] units in the solid. The comparison of Al-27 isotropic chemical shieldin gs and the experimental spectra allow us to conclude that N/O substitution does not happen in the first coordination shell of aluminum atoms. The expe rimentally observed broadening of the tetrahedral aluminum band can be expl ained by a nitridation of aluminum environment, but not by a nitridation of its first coordination shell. (C) 2000 Elsevier Science B.V. All rights re served.