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 a re reported for nickel methyl pyropheophorbide a (NiMPPh) and its cati on radical, in order to help understand the structural changes upon ox idation in chlorophyll cation radicals. RR band shifts, and also the o xidation 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 int erest is the upshift of the keto-carbonyl stretching mode in the NiMPP h cation radical. The strengthening of the carbonyl bond upon oxidatio n is consistent with the expected anti-bonding character of the A(1u)- like orbital with respect to the keto C=O bond.