Spectroscopic properties of Mg-chlorin, Mg-porphin and chlorophyllsa, b, c1, c2, c3 and d studied by semi-empirical and ab initio MO/CI methods

Abstract

The semi-empirical and ab initio molecular orbital/configuration interaction (MO/CI) methods were used to study spectroscopic properties of chlorophylls a, b, c₁, c₂, c₃ and d and magnesium porphin and magnesium chlorin. Energy minimisation at the PM3 level of all chlorophylls put the magnesium atom away from the centre and above the porphyrin ring and the atomic charges on the nitrogen atoms became positive. At the ab initio HF/6-31G* level 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 surrounding nitrogens negative. Three CI methods used, ZINDO/S CIS (15,15), PM3 CISD (5,5) and ab initio CIS (5,5)/6-31G*, obeyed linear correlation between the experimentally observed and calculated spectroscopic transition energies. 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 ZINDO/S CIS (15,15) method gave best results in the overall simulation of the 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 spectroscopic shifts of the Q_x and Soret transitions but leave the Q_y transition almost unshifted. This is a result of solvent-induced energy level shifts and charge redistribution on the magnesium atom of chlorophylls in the excited states. The results are discussed with reference to spectroscopic properties of chlorophylls in solution, chlorophylls in aggregates and in photosynthetic light-harvesting antenna.