The effect of oxidation and reduction of chlorophyll a on its geometry, vibrational and spin density properties as revealed by hybrid density functional methods

The oxidation and reduction of a chlorophyll a model molecule is investigated using hybrid density functional, B3LYP, calculations. The geometry, vibrational modes, and spin density distributions of the free radical forms are investigated. Small bond length changes on oxidation or reduction are shown to lead to significant variations in certain vibrational mode frequency values. Bond length changes can be readily accounted for by the electron density distributions of the frontier HOMO and LUMO orbitals. Harmonic frequency shifts calculated for oxidation and reduction on the 13(1)CO stretching frequency show very good agreement with experimental determinations. The unpaired spin density distribution for the cation and anion free radicals is also computed, and calculated N-14 and Mg-25 isotropic hyperfine coupling constants are in good agreement with experiment.