STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—IX. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRUM OF THE BACTERIOCHLOROPHYLLIDE a ANION RADICAL

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for 19 doublet states of the anion radical of ethyl bacteriochlorophyllide a (Et-BChl a˙), and are employed in a resolution of the electronic absorption spectrum, as well as in a comparison with a previously reported study of the electronic states and spectrum of the anion radical of ethyl bacteriopheophorbide a (Et-BPheo a˙). The lowest two excited doublet states, D₁ and D₂, of Et-BChl a˙ are found to be approximately degenerate and are predicted to contribute to the experimentally observed absorption at 10000 cm^?1. In contrast, the D₂←D₀ transition in Et-BPheo a˙ is predicted to contribute to the 11000 cm^?1 absorption, while the D₁←D₀ transition appears at approximately 8600 cm^?1 with a low oscillator strength (f= 0.002). The prominent visible absorption at ～15700 cm^?1 in both molecules is found to be due to the D₄← D₀ transition. Another difference between the predicted spectra of the two molecules appears in the low-energy shoulder of the Soret band. Here, two intense transitions, D₁₀←D₀ and D₁₁←D₀, are predicted for Et-BChl a˙, as opposed to three fairly intense transitions, D₇←D₀, D₈←D₀ and D₉←D₀, for Et-BPheo a˙, differences which may provide a means of distinguishing between the two molecules using resonance Raman spectroscopy. The remainder of the Soret band of Et-BChl a˙ above 26000 cm^?1 consists of a number of closely-spaced transitions to states D₁₂←D₁₈. The intense transitions D₁₂←D₀, D₁₃←D₀, D₁₄←D₀ are predicted to contribute to the Soret maximum near 30000 cm^?1. The ground state spin densities of the two molecules are similar, with the minor difference of somewhat less spin density located on the methine carbon atoms of Et-BChl a˙ compared with Et-BPheo a˙.