Metal-substituted bacteriochlorophylls. 2. Changes in redox potentials and electronic transition energies are dominated by intramolecular electrostatic interactions

Changes in the electronic transition energies and redox potentials because of metal substitution in bacteriochlorophyll a justify the recently suggested correlation between electronegativity chi(M), covalent radius, and an effective charge, Q(M), at the metal atom center. A simple electrostatic theory in which Q(M) modifies the energies of the frontier molecular orbitals by Coulombic interactions with the charge densities at the atomic pi centers is suggested. The relative change in electrostatic potential at a distance r(a) from the metal center is Delta Q(M)/r(a), where Delta Q(M), the change in the metal effective positive charge because of Mg being substituted by another metal, varies with the change in metal electronegativity (Mulliken's values) Delta chi(M) and covalent radius Delta r(M)(c). Delta Q(M) consists of two components: the major component, Delta Q(M)(0), characteristic of the central metal, is independent of the molecular environment and proportional to the electronegativity of the metal at a typical valence state. The second component, Delta q(M,N) reflects those perturbations induced by the molecular frame. It depends on the overlap between the metal and ligand orbitals hence changes both with the metal covalent radius (i.e., its typical "size") and the particular orbital environment. For the series of metals that we examined, we determined that Delta QM(0) = (0.12 +/- 0.02) Delta chi(M). Significant contributions of Delta q(M,N) to Delta Q(M,N) were found for the changes in the energies of the y-polarized electronic transitions B-y and Q(y) and to a lesser extent the first oxidation potential E-Ox(1). Minor contributions were found for the changes in the energies of the x-polarized electronic transitions B-x and Q(x) and the first reduction potential E-Red(1). The model agrees well with target testing factor analysis performed on the entire data space. Simulations of the experimental redox potentials and the four electronic transitions required mixing of single-electron promotions; however, the coefficients for the configuration interactions were assumed to be metal-independent within the examined series because the relative oscillator strengths of the various transitions did not show significant changes upon metal substitution. The reported observations and the accompanying calculations provide experimental support to the modern concepts of electronegativity and may help in better understanding biological redox centers consisting of porphyrins or chlorophylls.