CLUSTER-MODELS IN THE QUANTUM-CHEMICAL ANALYSIS OF COORDINATION OF NITROXIDE PROBES TO LEWIS-ACID SITES ON THE SURFACE OF AL2O3

The results of ESR-spectroscopic and quantum-chemical investigations o f the coordination of 2,2,6,6-tetramethylpiperidine N-oxyl, 2,2,3,4,5, 5-hexamethyl-3-imidazolidine N-oxyl, 2,2,4,5,5-pentamethyl-3-imidazoli ne N-oxyl, 2,4,5,5-tetramethyl-2-phenyl-3-imidazoline N-oxyl, and 2,4, 5,5-tetramethyl-2-octyl-3-imidazoline N-oxyl to Lewis acid sites (LAS) on alumina surface are described systematically and analyzed. The clu ster models of LAS accepted in radiospectroscopy and based on experime ntal data on g-factors and hyperfine coupling constants with N and Al nuclei in the corresponding donor-acceptor complexes are discussed. Wi thin the framework of the unrestricted Hartree-Fock (UHF) method using the STO-3G, STO-6G, 3-21G, and 6-31G basis sets and also in terms of semiempirical MNDO, AM1, and PM3 procedures, comparative quantum-chemi cal analysis of the structural, spin, electrostatic, energy, and radio spectroscopic characteristics of the coordination of the model cluster LAS to the simplest representative of nitroxides is performed. Three illustrative types of structures of the resulting surface complex are considered. A semiquantitative interpretation of the whole set of feat ures found experimentally for the coordination of nitroxide probes to the surface LAS on alumina is given.