QUANTUM-CHEMICAL INVESTIGATION OF THE GEOMETRIC STRUCTURE OF THE LEWIS-ACID SITES OF THE ALPHA-AL2O3 FACES (0001), (11(2)OVER-BAR-0), (22(4)OVER-BAR-0)

The quantum-chemical investigation of the Lewis acid site (LAS) geometric structure for perfect dehydroxylated alpha-Al2O3 faces (0001), (11 $$($) over bar 20) and (22 $$($) over bar 40) produced by oxide chopping along the chosen crystallographic direction was performed by the semiempirical approximation SCF MO LCAO MINDO/3 in the frame of the cluster scheme. The relaxation the geometric position of the Al atoms turns out to be maximum for the (0001) face, the principal possibility of the existence was shown for the surface Al atoms being in the three-coordinated state. The characteristic features of the 'single' and 'paired' LAS geometric structure for the (22 $$($) over bar 40) face have been investigated, with the latter containing the aluminum atoms in the five-coordinated state. The influence of the mutual repulsion of the two incompletely coordinated atoms on the geometric characteristics of such sites is described. In addition, the process of the LAS-H2O molecule interaction according to the associative and dissociative adsorption mechanisms has been studied.