Theoretical analysis of d-d transitions for the reduced Cr/silica system

Cluster models are constructed for mono- and dinuclear Cr(II) sites and mon onuclear Cr(III) sites on the Cr/SiO2 Phillips catalyst and used to compute d-d transition energies and intensities. Mononuclear pseudo-tetrahedral Cr (II) gives rise to two bands of electric-dipole-allowed d-d transitions, at 8,400 and 12,300 cm(-1). This doublet is lowered in energy and intensity a s the bond angle about chromium, angle OCrO, opens up. The dinuclear site g ives rise to bands at 5,200 and 10,300 cm(-1), consistent with calculations for a mononuclear cluster of comparable value for angle OCrO. A tri-coordi nated Cr(DI) cluster shows bands of comparable oscillator strengths at ener gies of 11,000, 16,000, 18,000-20,000 and 33,000 cm(-1). The predicted band s correspond well with d-d bands in experimental diffuse reflectance spectr a.