Spectroscopic properties of Mg-chlorin, Mg-porphin and chlorophylls a, b, c(1), c(2), c(3) and d studied by semi-empirical and ab initio MO/CI methods

The semi-empirical and ab initio molecular orbital/configuration interactio n (MO/CI) methods were used to study spectroscopic properties of chlorophyl ls a, b, c(1), c(2), c(3) and d and magnesium porphin and magnesium chlorin . Energy minimisation at the PM3 level of all chlorophylls put the magnesiu m atom away from the centre and above the porphyrin ring and the atomic cha rges on the nitrogen atoms became positive. At the ab initio HF/6-31G* leve l of calculation the magnesium is centrally located in the porphyrin plane and the atomic charge on the magnesium atom is positive and that on the sur rounding nitrogens negative. Three CI methods used, ZINDO/S CIS (15,15), PM 3 CISD (5,5) and ab initio CIS (5,5)/6-31G*, obeyed linear correlation betw een the experimentally observed and calculated spectroscopic transition ene rgies. The PM3 CISD (5,5) method gave best estimates of Q(y), Q(x) and the Soret transition energies, but predicted oscillator strengths poorly. The Z INDO/S CIS (15,15) method gave best results in the overall simulation of th e absorption spectra of chlorophylls, both intensities and wavelengths. The effect of solvent co-ordination on the excited states of chlorophyll a and chlorophyll b was also studied. Calculations predict solvent induced spect roscopic shifts of the Q(x) and Soret transitions but leave the Q(y) transi tion almost unshifted. This is a result of solvent-induced energy level shi fts and charge redistribution on the magnesium atom of chlorophylls in the excited states. The results are discussed with reference to spectroscopic p roperties of chlorophylls in solution, chlorophylls in aggregates and in ph otosynthetic light-harvesting antenna.