Article révisé par les pairs
Résumé : The Fukui function, f{hook}+, was studied for a series of compounds which proved to be adequate model structures for zeolite systems containing bridging hydroxyls. The structures were derived from the basic structure H3SiOAlH3 in which the hydrogen atoms on Si and Al are gradually replaced by fluorine atoms, thus mimicking changes in the average framework electronegativity upon variation of the Si/Al ratio. A basis-set study of the condensed Fukui function for a series of small molecules (H2CO, NH3 and H2O) indicated that, at least for the larger systems, the 3-21G basis may be expected to yield an optimal quality/cost ratio. Calculations on NH3 show that highly diffuse functions should not be introduced in the evaluation of f{hook}+ in order to cope with the anionic metastability problem. The condensed Fukui function, f{hook}H+, correlates well with the experimentally observed increase in acidity upon increasing framework electronegativity. Moreover, the function is found to be a more sensitive probe for the acidity of the OH group than is the previously calculated isolated molecule OH group properties, including the dipole-moment derivative. The trends in f{hook}+H are duplicated in the contour plots of f{hook}+ in the OH region indicating that the "probing" capacity of f{hook}+ is not lost by the highly approximate integration performed when condensing f{hook}+ via a Mulliken population analysis. A study of f{hook}+ along the OH bond axis reveals that its sensitivity, being more difficult to extract than that of the condensed function, is of comparable magnitude to that of f{hook}H+. © 1990.