Coordination, Structure, and Vibrational Spectra of Titanium in Silicates and Zeolites in Comparison with Related Molecules. An ab Initio Study

Ab initio quantum chemical calculations at the SCF level using a split valence plus polarization basis set have been performed on molecules containing Ti, Si, O, and H, which represent substructures of titanozeolites and ETS-10. Their purely siliceous analogues are also studied. Among them is octahydrooctasilasesquioxane. The calculations show that titanium prefers a 4-fold coordination in zeolites. The predicted relative energies and vibrational spectra make it very unlikely that TiO 4 tetrahedra share edges with SiO 4 tetrahedra. The typical 960-cm -1 vibrational band found for titanium-containing zeolites can be assigned to antisymmetric TiOSi stretching modes of corner-sharing tetrahedra. The frequency shift is due to the fact that the Ti−O bonds are about 20 pm longer than the Si−O bonds. The use of the crystal unit cell size as a measure for the titanium content of zeolites is supported.