CHARGE SENSITIVITY AND BOND-ORDER ANALYSIS OF REACTIVITY TRENDS IN ALLYL-[MOO3] CHEMISORPTION SYSTEMS - 2-REACTANT APPROACH

The molecular charge responses and bond-order changes due to the chemi sorption are reported for model allyl-[MoO3] (010)-surface structures. Charge rearrangement patterns are determined from the charge sensitiv ity analysis (CSA) in the atomic resolution. The bond-order analysis i s based upon the valence indices calculated within the one-determinant al (Kohn-Sham, KS) difference approach, comparing bonding patterns in a molecule and in the separated reactants limit, respectively. Polariz ation (P) and charge transfer (CT) stages of adsorption are examined s eparately; the overall (CT+P) patterns are also reported to examine th e relative importance of these effects in the chemisorption systems co nsidered. These charge sensitivity criteria are calculated from the se miempirical hardness matrix for M = (adsorbate/substrate) reactive sys tems including a large two-layer surface cluster. Rough estimates of t he electrostatic (ES), P, and CT contributions to the interaction ener gy are also available in the CSA approach; they provide an approximate energetical hierarchy among the alternative chemisorption arrangement s. These predictions are compared with the corresponding KS results fo r small surface clusters. The set of collective charge displacements, which diagonalize the interreactant part of the hardness matrix, are u sed to interpret the isoelectronic Fukui function for the allyl --> [M oO3] CT. The mechanism of selective allyl oxidation to acrolein is exa mined in more detail. It is shown that the selective allyl oxidation c an be rationalized in a concerted bond-breaking-bond-forming mechanism , conjectured from a combination of the CSA charge rearrangements for large clusters and the KS bond multiplicity data for the smaller (acti ve site) chemisorption complexes.