STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS. XI. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRA OF THE CHLOROPHYLLIDE a AND PHEOPHORBIDE a ANION RADICALS

Abstract

Abstract Ab initio configuration interaction wavefunctions and energies are reported for the ground and low‐lying excited doublet states of the anion radicals of ethyl chlorophyllide a (Et‐Chl a‐) and ethyl pheophorbide a (Et‐Pheo a‐), and are employed in a comparative analysis of their respective electronic absorption spectra.Strong similarities exist between the first five computed excited states of both molecules, providing no distinguishing features in the electronic absorption spectra below 20000 cm‐. Their ground state charge and spin density distributions are also very similar, and there is negligible spin density predicted on the magnesium atom in Et‐Chl a‐.The Soret bands of both molecules are predicted to arise from intense transitions to several closely‐spaced higher excited states, and the calculations indicate that there are significant differences in the number and composition of these states in the two molecules. It is suggested that these differences may provide a means of distinguishing between the two molecules using resonance Raman spectroscopy.