Jan Moens

Energy Levels and Redox Properties of Aqueous Mn2+/3+ from Photoemission Spectroscopy and Density Functional Molecular Dynamics Simulation

Energy-resolved photoemission spectroscopy and density functional molecular dynamics simulations are combined to construct an energy level diagram for the Mn(2+/3+) redox reaction in aqueous solution. Two peaks centered at 8.88 and 10.26 eV electron binding energies can be assigned to the Mn2+ hexa-aquo complex with a peak area ratio of 2:2.83. Using the notation of crystal field theory, the peak at lower energies can be interpreted as arising from ionization from the e(g) levels (highest occupied molecular orbital, HOMO), and the peak at higher energies are from ionization of the t(2g) levels. The difference corresponds to the average crystal field splitting, 1.38 eV. From the position of the HOMO level and the absolute redox potential, an experimental value for the reorganization free energy of the aqueous Mn3+ hexa-aquo complex is estimated to be 2.98 eV. Density functional molecular dynamics simulations can reproduce the experimental vertical ionization energy, redox free energy, and reorganization free energies fairly well, provided that the absolute potential shift in periodic boundary conditions, finite size effects, and inaccuracies of the exchange correlation functional are taken into account. Most strikingly, in the simulations, we observe spontaneous and reversible deprotonation of the aqueous Mn3+ hexa-aquo complex to form MnOH(H2O)5(2+) + H+, in line with the low experimental pKa value of this ion. The interconversion between protonation states leads to interesting redox phenomena for aqueous Mn3+, culminating in a bimodal thermal distribution of the electron affinity.

The Study of Redox Reactions on the Basis of Conceptual DFT Principles: EEM and Vertical Quantities

In this article, two new approaches are introduced which describe redox reactions through descriptors defined within the field of conceptual density functional theory (DFT). One approach starts with the grand canonical ensemble DFT from which a formula is derived for the chemical potential of the electrode in terms of intrinsic properties of oxidized and reduced states of the electroactive species. Second, starting from a Born-Haber scheme, the redox potential is solely expressed in terms of the vertical electron affinity and ionization potential of oxidized and reduced species, respectively. A large collection of 44 organic and inorganic systems are studied in different solvents including implicit and explicit solvation models. Both strategies seem well capable of reproducing experimental values of redox potentials.

Valence Photoemission Spectra of Aqueous Fe2+/3+ and [Fe(CN)(6)](4-/3-) and Their Interpretation by DFT Calculations

Aqueous solutions of ferrous and ferric iron (Fe(2+/3+)) and of the iron-hexacyano complexes [Fe(CN)(6)](4-/3-) are studied by photoelectron spectroscopy using a liquid microjet in conjunction with synchrotron soft X-rays for ionization. For Fe(2+)(aq) we observe two well-resolved peaks at 7.09 and 9.16 eV electron binding energy (BE) that can be assigned to the iron-hexaaquo complex. For Fe(3+)(aq) we observe only one peak above the highest valence band of liquid water, at 10.08 eV BE. Interpreting the spectra in terms of the one-electron levels of Kohn-Sham density functional theory, we find that the two peaks for Fe(2+)(aq) originate from the energy splitting between the highest occupied β (= minority) spin level (Fe d(t(2g))) and the five highest occupied α (= majority) spin levels (Fe d(t(2g)) and d(e(g))). The peak for Fe(3+)(aq) arises from d-levels that are strongly mixed with the solvent. The spectra of the aqueous hexacyano complexes show a single strong peak at 6.11 and 7.52 eV BE for [Fe(CN)(6)](4-) and [Fe(CN)(6)](3-), respectively, originating from the highest occupied Fe d(t(2g)) levels, and two further peaks at higher BE originating from the cyano ligands. The PE spectra of the reduced aquo and cyano ions are then used to obtain-solely on experimental grounds-values for the reorganization free energy of the oxidized ions. DFT/continuum calculations of this important parameter in the Marcus theory of oxidation reactions are in fairly good agreement with experiment.

Intrinsic nucleofugality scale within the framework of density functional reactivity theory.

Nucleofugality is a measure of the quality of a leaving group in substitution and elimination reactions. In a conceptual DFT context, the nucleofugality is calculated for an elaborate set of common organic leaving groups, both in the gas phase and in two organic solvents (dichloromethane and methanol). An intrinsic nucleofugality scale is constructed showing fair agreement with the classical trends in leaving group capacity in organic chemistry. The correlation of the results with acidities (tabulated pK(a) values) on one hand and experimental solvolysis reaction rate constants (kinetic parameters) on the other hand is discussed. Finally, a conceptual DFT based formula is derived, describing the influence of the solvation energy on the nucleofugality; excellent correlations were found with explicit calculations for the studied leaving groups.

Cooperativity in Al3+ Hydrolysis Reactions from Density Functional Theory Calculations

Aqua/hydroxo mononuclear Al(3+) species in aqueous solution are investigated using density functional theory (DFT B3LYP/6-311++G(d,p)) and the polarized continuum model (PCM). Optimized gas-phase geometries have been obtained for the species Al(OH)(n)(H(2)O)(m)((3-n)+) in which n = 0, 1, 2, 3, or 4 while (n + m) = 4, 5, or 6. For Al(OH)(2)(H(2)O)(4)(1+) the cis and trans geometries were considered. The structures were analyzed in terms of water and hydroxide M-O and O-H distances, which are shown to be strongly modulated by water hydrolysis. The atomic charges were computed and the electronic structure of these complexes is discussed. The conversion from one aqua-/hydroxo- species to another follows independent hydrolysis and dehydration reactions for which the aqueous Gibbs free energies have been estimated by means of constructing thermodynamic cycles. Results clearly demonstrate that the dehydration reaction is increasingly favorable as hydrolysis proceeds. Similarly, as the complex coordination number decreases the hydrolysis reaction proceeds increasingly more favorably. The aqueous Gibbs free energy of each species, relative to Al(H(2)O)(6)(3+), has been determined by combining the appropriate Gibbs free energies of the hydrolysis and dehydration reactions demonstrating that the additive effect is quite complex showing a gradual transition from preferring the 6-coordination to 5-coordination to 4-coordination as a function of ligand hydrolysis, in agreement with published experimental and theoretical work. We have also computed the equilibrium constants of each of the above reactions and, using [H(+)] as a parameter, estimated the mole fraction of each species as a function of pH. This offers a clear demonstration that the qualitative hydrolysis behavior, e.g., cooperativity, of aqueous Al(3)+ species is obtained at the B3LYP/IEF-PCM level of theory.

A new view on the spectrochemical and nephelauxetic series on the basis of spin-polarized conceptual DFT

The spectrochemical and nephelauxetic series are analyzed within the context of spin-polarized conceptual DFT. For a series of different [RuL(6)](x) complexes, the local spin-philicity omega(S,Metal)+ condensed on the metal ion shows a remarkable analogy with some semi-empirical scales of the spectrochemical series. The local f(SS,Metal)+ Fukui function in turn can be linked to the nephelauxetic effect. Herein, we present a non-empirical, unified approach for a quantitative discussion of both series.