Theoretical investigations of NMR chemical shieldings on the AlPON catalyst system Antonio M arquez a, * , Javier Fern andez Sanz a , Jos  e Antonio Odriozola b a Departamento de Qu  õmica F  õsica, Facultad de Qu  õmica, Universidad de Sevilla, E-41012 Sevilla, Spain b Departamento de Qu  õmica Inorg anica e Instituto de Ciencia de Materiales, de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, P.O. Box 874, E-41080 Sevilla, Spain Abstract Using molecular cluster models we present a theoretical ab initio study of the 27 Al and 31 P nuclear magnetic reso- nance (NMR) chemical shieldings aimed at obtaining short-range structural information on the aluminophosphate oxynitride (AlPON) catalyst system. We have employed orthophosphate-like molecular models with P/Al ratio equal to one and varying N/O ratios to simulate the experimentally obtained compositions. The computed NMR chemical shieldings reproduce quantitatively the observed features in the 31 P magic angle spinning NMR (MAS-NMR) spectra and permit the explanation of these modi®cations in terms of a progressive and preferential nitridation on the ®rst coordination shell of the phosphorus atom leading to a mixture of PO x N 4x  units in the solid. The comparison of 27 Al isotropic chemical shieldings and the experimental spectra allow us to conclude that N/O substitution does not happen in the ®rst coordination shell of aluminum atoms. The experimentally observed broadening of the tetrahedral aluminum band can be explained by a nitridation of aluminum environment, but not by a nitridation of its ®rst coordination shell. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Molecular models of solids are an adequate means of applying ab initio theoretical methods to understand solid-state problems [1]. It would be most valuable if predictions of local structures and properties of active sites of solids could be made with the same reliability that these methods have achieved for molecules in gas phase, where they are becoming in many cases an alternative to ex- periment to accurately determine structures, elec- tronic properties as well as molecular reactivities. Nevertheless, the reliability of computational chemistry studies of solids is limited mainly by the reduced size of the models compared to the basi- cally in®nite extent of the solid framework, and by inherent approximations on the theoretical meth- ods used. In spite of these limitations, an increas- ing number of ab initio quantum chemical studies of solid systems has been reported in the literature in recent years (see for example [1]). Amorphous aluminophosphates oxynitrides (AlPON), solids with a variable nitrogen content are new catalysts obtained by nitridation of AlPO 4 having high surface area (up to s > 500 m 2 g 1 ) and increased surface basicity [2,3]. These catalysts are amorphous and despite the studies done on their composition, texture and surface basicity, less attention, to our knowledge, has been paid to their Journal of Non-Crystalline Solids 263&264 (2000) 189±194 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +34-95 455 7177; fax: +34-95 455 7174. E-mail address: marquez@cica.es (A. Ma Ârquez). 0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 3 4 - 1