Resonance Raman spectra of dioxygen adducts of manganese(II) porphyrins in dioxygen matrices

The resonance Raman spectra of side‐on type dioxygen adducts of Mn(TMP), Mn(TPP) and Mn(OEP), where TMP, TPP and OEP denote the dianions of tetramesityl‐, tetraphenyl‐ and octaethyl‐porphines, respectively, were measured in O 2 ‐Ar(1 : 30) matrices at ca. 15 K. The bands near 990, 710 and 430 cm −1 were assigned to ν(O 2 ), ν s (MnO) and ν a (MnO), respectively, based on 16 O 2 / 18 O 2 isotopic shifts and normal coordinate calculations on an isosceles MnO 2 moiety. Vibrational coupling between the ν s (MnO) and porphyrin modes obscured the location of the former mode in the spectra of Mn(TPP)O 2 and Mn(OEP)O 2 . The bands near 825 cm −1 which appear in all three porphyrins are shifted 34–36 cm −1 on 16 O 2 / 18 O 2 substitution. Evidence is presented to show that this band is due to the ν(MnO) of oxomanganese(IV) porphyrin, which was produced by photo‐cleavage of the OO bond of the dioxygen adduct. These ν(MnO) are ca. 70 cm −1 higher than those observed in acetonitrile solution. This has been attributed to the difference in the spin state; the oxomanganese(IV) porphyrins in acetonitrile solution are of high spin whereas those produced in O 2 ‐Ar matrices are of low spin.