ELECTRON-DENSITY AND ELECTROSTATIC POTENTIAL OF COESITE

Coesite is a framework silicate containing Si-O-Si groups similar to the potential Bronsted acid sites of zeolite catalysts. In this paper it is demonstrated that the details of the electrostatic potential and electron density near these bridging oxygens can be determined from single-crystal X-ray diffraction intensities. The static deformation electron density exhibits bond maxima near each O of about 0.6(1) e Angstrom(-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/covalent nature. On average, bond critical points are located 0.68 Angstrom from Si and 0.94 Angstrom from O. Maps of the deformation density, deformation 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 theoretical calculations on zeolite catalysts must properly account for long-range interactions which may significantly perturb local electron density features.