Norman Sutin

Electron transfers in chemistry and biology

Electron-transfer reactions between ions and molecules in solution have been the subject of considerable experimental study during the past three decades. Experimental results have also been obtained on related phenomena, such as reactions between ions or molecules and electrodes, charge-transfer spectra, photoelectric emission spectra of ionic solutions, chemiluminescent electron transfers, electron transfer through frozen media, and electron transfer through thin hydrocarbon-like films on electrodes.

Metal—lingad and metal—metal coupling elements

Abstract The electronic matrix element coupling a ground and charge-transfer excited state can be calculated from the energy intensity of the appropriate charge-transfer transition. An expression for the electronic coupling element widely used for this purpose is based on equations derived by Mulliken and Hush for an effective two-state model and is frequently assumed to be valid only in the perturbation limit. This expression is shown to be exact within a two-state model. Provided that overlap can be neglected and that the spectroscopic transition is polarized along the donor—acceptor axis, it can be applied to system ranging from those which are very weakly coupled to those which are very strongly coupled. Application of the Mulliken—Hush expression to (NH3)5RuL2+ complexes, for which metal—lingand backbonding is important, yields metal—lingand coupling elements of 5000–6000 cm−1 with pyridyl lingands (donor—acceptor separation 3.5 A), in very good agreement with estimates obtained from a molecular orbital analysis of the band energies. With use of the superexchange formalism, the metal—lingand coupling elements were used to calculate metal—metal coupling elements for binuclear mixed-valence complexes. Comparison of these values with those obtained from the Mulliken—Hush expression applied directly to the metal-to-metal charge-transfer transition yields agreement within a factor of two or better.

Energy surfaces, reorganization energies, and coupling elements in electron transfer

Abstract The effects of changes in the shapes and intersections of the reactant and product free-energy surfaces on the vertical reorganization parameter and the free energy of activation for an electron self-exchange reaction are considered. Parabolic free energy surfaces provide a very good description of the inner-shell reorganization process even when the stretching force constants for the oxidized and reduced forms of the redox couple differ by a factor of two. The activation energy depends on the reorganization criterion and the contributions of the individual reactants to the inner-shell barrier are quite sensitive to the model used. Optical charge transfers and the consequences of reactant, product, and transition state stabilization in weakly interacting and very strongly interacting systems are also considered. Relationships between the equilibrium constant for the comproportionation reaction forming the mixed-valence complex and the optical charge transfer parameters are presented.

Light-induced electron transfer reactions

Abstract The reactions of the luminescent excited states of the polypyridine-ruthenium(II) complexes (*RuL32+) with electron acceptors and donors are discussed. These electron transfer reactions convert the excited state into RuL33+ and RuL3+, respectively. The former ruthenium complex is a more powerful oxidant and the latter is a more powerful reductant than the excited state itself. Some applications of these complexes in the conversion and storage of solar energy are presented. Theoretical models for electron transfer reactions are described and the implications of these models for the quenching and back electron transfer reactions are discussed. It is pointed out that the exploitation of the inverted region may provide a useful means of slowing down back electron transfer reactions.

The role of inner-sphere configuration changes in electron-exchange reactions of metal complexes

Metal-ligand distances were determined for various complexes of Ru, Co, Fe, and Cr. Changes in metal-oxygen or metal-nitrogen bond lengths which occur upon oxidation were alo determined. These changes correlate strongly with electron exchange rates. (DLC)

Photo-Induced Generation of Dihydrogen and Reduction of Carbon Dioxide Using Transition Metal Complexes

Abstract Homogeneous and microheterogeneous transition-metal-based systems that generate dihydrogen and/or reduce carbon dioxide upon irradiation with visible light are considered. Most of the systems involve polypyridine complexes of the d6 centers cobalt(III), rhodium(III), iridium(III), ruthenium(II) and rhenium(I). Complexes with diimine ligands serve as photosensitizers and/or catalyst precursors. The corresponding d8 metal centers and d6 hydrides are important intermediates: bimolecular reactions of the hydrides or their reactions with H2O/H3O+ are responsible for formation of dihydrogen. When carbon dioxide is also present, it may insert into the metal-hydride bond to yield formate. Mechanistic schemes for some dual-acting photoconversion systems that generate both dihydrogen and carbon monoxide or formate are considered.

Electroabsorption spectroscopy of charge transfer states of transition metal complexes

Abstract The use of electroabsorption spectroscopy to determine the dipole-moment changes that occur in the metal-to-ligand, ligand-to-metal, and metal-to-metal charge-transfer transitions in mononuclear and binuclear transition metal complexes is reviewed. The ground-excited state dipole-moment differences are much smaller than expected for the transfer of unit electronic charge between the donor and acceptor centers. The results are discussed in terms of a model in which two factors, electron delocalization and polarization of the acceptor, donor or bridging ligand electrons in response to the changed charge on the metal centers, are considered to be primarily responsible for the relatively small dipole-moment changes. The implications of the results for electronic coupling elements and reorganization energies are also discussed.

The oxidation of ascorbic acid by tris(2,2′-bipyridine) complexes of osmium(III), ruthenium(III) and nickel(III) in aqueous media: Applications of the Marcus cross-relation

Abstract The kinetics of the outer-sphere oxidation of L-ascorbic acid (H2A) by tris(2,2′-bypyridine) and tris-(4,4′-dimethyl-2,2′-bipyridine) complexes of osmium(III), ruthenium(III), and nickel(III) have been investigated in acidic aqueous media at 9–25 °C. The observed inverse acid dependence, -d[ML3+3]/dt = 2(k1 + k2K1/[H+])(ML3+3][H2A], suggest a mechanism involving H2A and HA− pathways, for which the specific rate and activation parameters have been determined. These and previously reported rate constants for cross-reactions involving ascorbate species are correlated in terms of the Marcus relationships. Estimates of the self-exchange parameters for the one-electron couples: H2A/H2A+·, k11 = 2 × 103 M− s−1; HA−/HA·, k11 = 1 × 105 M− s−1; A2−/A−·, k11 = 2 × 105 M− s−1], based on ascorbate radical reduction potentials calculated from proton and redox equilibrium constant, are presented.

The relation between the barriers for thermal and optical electron transfer reactions in solution

The relation between the energy of the intervalence (or metal-to-metal charge-transfer) transition in mixed-valence systems and the magnitude of the barrier to thermally activated electron transfer is described. We note that, contrary to what has frequently been assumed, the relation is in terms of free energies rather than internal energies or enthalpies. It is also shown, from plots of the enthalpies of the reactants and products versus the reaction coordinate, that negative activation enthalpies are a natural consequence when the standard enthalpy change for the reaction is sufficiently negative.

Reduction of carbon dioxide by tris(2,2′-bipyridine)cobalt(I)

Abstract Preliminary stoichiometric and kinetic results bearing on the mechanism of the reduction of HCO3− to CO by tris(2,2′-bipyridine)cobalt(I) in aqueous media are reported. The results indicate that CO (not formate) is the dominant carbon product and that it is scavenged by Co(bpy)3+ to give insoluble [Co(bpy)(CO)2]2. At pH ∼ 9, bicarbonate reduction occurs in competition with H2O reduction. Both processes are inhibited by bpy and promoted by H+, suggesting the common intermediate Co(bpy)2(H2O)H2+. The bicarbonate reaction itself branches to give H2 and CO in ∼ 3:1 ratio.