Abstract: ESR Spectra of molybdenum supported catalysts(Mo-SiO₂) were recorded with various contact time with propene.Two types of Mo⁵⁺-paramegnetic centers were formed during the course of reaction. The one having g_||=1.89,g_⊥=1.94 is due to the distorted octahadral coordination, while the other with g_||=1.86,g_⊥=1.95 is a square pyramidal distorted species.However,they are both approximately of C_4v symmetry. The anisotropic surperfine coupling contants A_||=90.3×10^-4cm^-1,A_⊥=44.8×10^-4cm^-1 of ^95,97Mo⁵⁺-were also detected. Meanwhile, the signals occurerd at ca. g=2.002 and g₁=2.018,g₂=2.011,g₃=2.005 were assigned to coking from propene and anionic radical 0₂^- respectively. LCAO-MO approximate calculations of data led to a set of molecular orbital coefficients. It was found that the plot of △g_||/△g_⊥ vs. △g is linear. Suggestion was made to employas △g_||/△g_⊥=4(△E(B₂→B₁))/(△E(B₂→E)·(β₁/ε)² a better measure of the extent of C_4v distortion for Mo⁵⁺.

Key words: Molybdenum, ESR, Coke, Radical