QUANTUM-CHEMICAL ANALYSIS OF THE CLUSTER-MODELS COORDINATION OF IMIDAZOLINE NITROXIDES ON THE SILICA SURFACE

The results of ESR-spectroscopic and quantum-chemical studies of the c oordination of 2,2,4,5,5-pentamethyl-3-imidazoIine-N-oxyl and 2-phenyl -2,4,5,5-tetramethyl-3-imidazoline-N-oxyl with the surface acid sites (AS) of silica are analyzed. Cluster models of AS, corresponding to on e- and two-point coordination and accepted in radiospectroscopy on the basis of g-factors and constants of hyperfine splitting at the N nucl ei in the resulting donor-acceptor complexes, are discussed. Within th e framework of the unrestricted Hartree-Fock method using the MNDO app roach, a comparative quantum-chemical analysis of the structural, spin , electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster AS to the imidazoline nitroxide pro bes is performed. For the two-center adsorption, three types of AS str uctures are considered, together with the paramagnetic surface complex es they form. A consistent semi-quantitative interpretation of the who le set of features found experimentally for the coordination of imidaz oline nitroxides to the surface AS on silica is given.