Modeling the bonding changes in chlorophyll cation radicals: Resonance Raman spectroscopy of nickel(II) methyl pyropheophorbide a

Electrochemistry, UV-Vis absorption and resonance Raman (RR) spectra are reported for nickel methyl pyropheophorbide a (NiMPPh) and its cation radical, in order to help understand the structural changes upon oxidation in chlorophyll cation radicals. RR band shifts, and also the oxidation potential, are consistent with the highest occupied molecular orbital (HOMO) of NiMPPh being an A(1u)-like orbital, which interacts with the keto-carbonyl group on the isocyclic ring. Of particular interest is the upshift of the keto-carbonyl stretching mode in the NiMPPh cation radical. The strengthening of the carbonyl bond upon oxidation is consistent with the expected anti-bonding character of the A(1u)-like orbital with respect to the keto C=O bond.