Quantum-chemical study of the geometric and electronic structure of the chromate anion CrO 4 2− and a chromate group on the surface of finely divided silica by the CNDO/2 method

A comparative study of the geometric and electronic structure of the chromate anion CrO ₄ ^2– and a chromate group on the surface of finely divided silica (Si-O)₂-CrO₂, which was simulated by a CrO₉Si₆H₁₂ cluster, has been carried out by the SCF-MO-LCAO method in the all-valence-electron CNDO/2 approximation. The data obtained on the equilibrium geometry of the chromate group attest to the formation of a double bond between the Cr atom and each O atom (which is not bonded to Si). It has been shown that the support has a significant stabilizing influence on the energy of the MO's of the chromate group. The chromate group on an SiO₂ surface is characterized by partial delocalization of the frontier MO's among the skeletal bonds; however, the dominant contribution to the HOMO is made by the 2p AO of the oxygen atoms in the coordination shell of the Cr atom (70%), and the dominant contribution to the LUMO is made by the 3d AO of the chromium atom (50%). The positions and composition of the lowest unoccupied molecular orbitals point out the possibility of the display of electron-acceptor properties by a chromate group on an SiO₂ surface.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 602–606, September–October, 1988.