Metal-Substituted Bacteriochlorophylls. 2. Changes in Redox Potentials and Electronic Transition Energies Are Dominated by Intramolecular Electrostatic Interactions

Citations Over Time

Abstract

Changes in the electronic transition energies and redox potentials because of metal substitution in bacteriochlorophyll a justify the recently suggested correlation between electronegativity χM, covalent radius, and an effective charge, QM, at the metal atom center. A simple electrostatic theory in which QM modifies the energies of the frontier molecular orbitals by Coulombic interactions with the charge densities at the atomic π centers is suggested. The relative change in electrostatic potential at a distance ra from the metal center is ΔQM/ra, where ΔQM, 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) ΔχM and covalent radius Δ . ΔQM consists of two components: the major component, Δ , 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, ΔqM,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 Δ = (0.12 ± 0.02) ΔχM. Significant contributions of ΔqM,N to ΔQM,N were found for the changes in the energies of the y-polarized electronic transitions By and Qy and to a lesser extent the first oxidation potential . Minor contributions were found for the changes in the energies of the x-polarized electronic transitions Bx and Qx and the first reduction potential . 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.

Related Papers

• → Theoretical studies of some sulphonamides as corrosion inhibitors for mild steel in acidic medium(2009)162 cited
• → Atomic orbitals in molecules: general electronegativity and improvement of Mulliken population analysis(2005)23 cited
• → “Unexpected” 29Si NMR Chemical Shifts in Heteroatom-Substituted Silyllithium Compounds: A Quantum-Chemical Analysis(2004)37 cited
• → Unusually Short Distances between the Carbonyl Oxygen and the Tin Atom in RCOSMR‘₃ (M = Ge, Sn, Pb): The Importance of Intramolecular n_O → σ*_MS Orbital Interactions(2001)29 cited
• → An examination of intermolecular and intramolecular hydrogen bonding in biomolecules by AM1 and MNDO/M semiempirical molecular orbital studies(1991)46 cited