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Spectral properties of protonated schiff base porphyrins and chlorins indo ci calculations and resonance raman studies


, : Spectral properties of protonated schiff base porphyrins and chlorins indo ci calculations and resonance raman studies. Journal of the American Chemical Society 106(14): 3950-3958

INDO-CI calculations successfully reproduce the striking changes in optical spectra that occur on protonation of mono- and disubstituted porphyrin, chlorin and bacteriochlorin Schiff base complexes. They ascribe the changes to Schiff base C.dbd.N .pi.* orbitals which drop in energy upon protonation and mix with and perturb the .pi.* orbitals of the macrocycle, a result consistent with resonance Raman data. The perturbation is predicted to affect not only transition energies and intensities but also dipole moment directions. The symmetry of the porphyrin and the substitution site of the chlorin are shown to play an important role, especially in governing whether the lowest energy transition will red shift or blue shift. Blue shifts are calculated for protonation of ketimine and enamine isomers of pyrochlorophyll a (PChl). Comparison with reported optical spectra suggests that PChl .alpha. Schiff base may undergo isomerization upon protonation. Resonance Raman data on CHO, CHNR, CHNHR+ .pi. system, intramolecular H bonding and selective enhancement of .nu.C.dbd.N for those species with a split Soret band. .nu.C.dbd.N is observed with 488.0-nm excitation into the lower-energy Soret and absent for 406.7-nm excitation into the higher-energy Soret, a result predicted by the calculations.

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