Characterization of intra-framework and guest/host interactions in the AlPO4-15 molecular sieve by charge-density analysis.

The electron-density distribution of AlPO4-15 has been determined using high-resolution single-crystal X-ray diffraction, and the topological properties of the charge density have been calculated using the 'atoms in molecules' (AIM) theory. Analysis of the topological properties at the bond critical points has been used to characterize the interactions within the framework, and between the framework and the extra-framework species (ammonium ions and water molecules), and to define atomic properties, such as volume and net charges, uniquely. A comparison between procrystal and multipolar representations of the density was performed in order to explore to what extent the former representation is likely to reflect the interactions in the solid. Correlation with geometrical properties (P-O and Al-O bond lengths, and Al-O-P angle) is found for topological charges obtained from the multipolar model, but not for the results from the procrystal representation.