Effect of Protonation on Peroxo-Copper Bonding: Spectroscopic and Electronic Structure Study of
David E. Root1, Mohammed Mahroof-Tahir, Kenneth D. Karlin
1Departments of Chemistry, Stanford University, Stanford, California 94305, and The Johns Hopkins University, Baltimore, Maryland 21218.
Abstract:
Spectroscopic studies of a &mgr;-1,1-hydroperoxo-bridged copper dimer are combined with SCF-Xalpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu(2)OOH unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: nu(O-O) = 892 cm(-)(1), nu(as)(Cu-O) = 506 cm(-)(1), nu(s)(Cu-O) = 322 cm(-)(1), and nu(O-H) = 3495 cm(-)(1). The 892 cm(-)(1) O-O stretch of the &mgr;-1,1-hydroperoxo-bridged copper dimer is 89 cm(-)(1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-)(1) O-O stretch are used to assign an electronic absorption band at 25 200 cm(-)(1) (epsilon = 6700 M(-)(1) cm(-)(1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is approximately 5000 cm(-)(1) higher in energy than the corresponding transition in the unprotonated complex. The pi-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotonated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.
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