ELECTRON-DENSITY AND ELECTROSTATIC POTENTIAL OF COESITE

Coesite is a framework silicate containing Si-O-Si groups similar to t he potential Bronsted acid sites of zeolite catalysts. In this paper i t is demonstrated that the details of the electrostatic potential and electron density near these bridging oxygens can be determined from si ngle-crystal X-ray diffraction intensities. The static deformation ele ctron density exhibits bond maxima near each O of about 0.6(1) e Angst rom(-3) in the Si-O bonds, which are strongly polarized toward Si. The total electron density (rho) reaches a minimum of, on average, 1.05(2 ) e Angstrom(-3) in the Si-O bonds, with del(2) rho at this point +20. 3(5) e Angstrom(-5), indicating bonds of an intermediate ionic/covalen t nature. On average, bond critical points are located 0.68 Angstrom f rom Si and 0.94 Angstrom from O. Maps of the deformation density, defo rmation electrostatic potential, and -del(2) rho show the influence of a local concentration of electron density in the nonbonding region of one bridging oxygen with an Si-O-Si angle of 137 degrees. Maps of the electrostatic potential near two representative bridging oxygens are quite different, supporting the commonly held conclusion that theoreti cal calculations on zeolite catalysts must properly account for long-r ange interactions which may significantly perturb local electron densi ty features.