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Resonance Raman spectra of two isomeric dioxygen adducts of iron(II) porphyrins and π-cation radical and nonradical oxoferryl porphyrins produced in dioxygen matrices: Simultaneous observation of more than seven oxygen isotope sensitive bands
Proniewicz, Leonard M.,Paeng, Insook Rhee,Nakamoto, Kazuo
, p. 3294 - 3303 (2007/10/02)
The resonance Raman (RR) spectra of base-free dioxygen adducts of Fe(TPP), Fe(TPP-d8), Fe(TMP), Fe(OEP), and Fe(TPFPP) in 16O2, 18O2, and scrambled O2 (16O2:16O18O:18O 2=1:2:1) matrices at ～30 K have been measured with 406.7-nm excitation. The v(O-O), v(Fe-O2) and δ(FeOO) of end-on adducts were observed at 1223-1188, 516-486, and 349-337 cm-1, respectively, whereas the v(O-O) and va(Fe-O) of side-on adducts were located at 1105-1102 and 411-405 cm-1, respectively. In addition, some side-on adducts exhibit the va(Fe-O) coupled with porphyrin modes in the 700-500 cm-1 region. The band assignments of end-on isomers were confirmed by 56Fe/54Fe substitution as well as by normal coordinate calculations. During RR measurements, two oxoferryl species were formed as a result of homolytic cleavage of the O-O bond by laser irradiation. One exhibits the v(FeO) at 861-853 cm-1 (compound A), while the other (less stable species) shows it at 815-809 cm-1 (compound B). While the former originates in the well-established base-free, nonradical oxoferryl porphyrin, the latter is attributed to a new oxoferryl species in which the porphyrin ring assumes an a2u π-cation radical form. A mechanism has been proposed to account for the formation of these oxoferryl porphyrins in an O2 matrix.
