Roberto Bianco

Theoretical Study of O–O Single Bond Formation in the Oxidation of Water by the Ruthenium Blue Dimer

The first key step in the oxidation of water to O(2) by the oxidized species [(bpy)(2)(O)Ru(V)ORu(V)(O)(bpy)(2)](4+) of the Ru blue dimer is studied using density functional theory (DFT) and an explicit solvent treatment. In the model reaction system [L(2)(O)Ru(V)ORu(V)(O)L(2)](4+)·(H(2)O)(4)·W(76), the surrounding water solvent molecules W are described classically while the inner core reaction system is described quantum mechanically using smaller model ligands (L). The reaction path found for the O--O single bond formation involves a proton relay chain: direct participation of two water molecules in two proton transfers to yield the product [L(2)(HOO)Ru(IV)ORu(IV)(OH)L(2)](4+)·(H(2)O)(3)·W(76). The calculated ∼3 kcal/mol reaction free energy and ∼15 kcal/mol activation free energy barrier at 298 K are consistent with experiment. Structural changes and charge flow along the intrinsic reaction coordinate, the solvents role in the reaction barrier, and their significance for water oxidation catalysis are examined in detail.

Depth-Dependent Dissociation of Nitric Acid at an Aqueous Surface: Car−Parrinello Molecular Dynamics

The acid dissociation of a nitric acid HNO(3) molecule located at various depths in a water slab is investigated via Car-Parrinello molecular dynamics simulations. HNO(3) is found to remain molecular when it is adsorbed on top of the surface with two hydrogen-bonds, and to dissociate--although not always--by transferring a proton to a water molecule within a few picoseconds when embedded at various depths within the aqueous layer. The acid dissociation events are analyzed and discussed in terms of the proton donor-acceptor O-O hydrogen bonding distance and the configurations of the nearest-neighbor solvent waters of an HNO(3).H(2)O pair. Four key structural features for the HNO(3) acid dissociation are identified and employed to analyze the trajectory results. Key solvent motions for the dissociation include the decrease of the proton donor-acceptor O-O hydrogen bonding distance and a 4 to 3 coordination number change for the proton-accepting water. The Eigen cation (H(3)O(+)), rather than the Zundel cation (H(5)O(2)(+)), is found to be predominant next to the NO(3)(-) ion in contact ion pairs in all cases.

VB resonance theory in solution. I. Multistate formulation

A theory for the description of electronic structure in solution for solution phase chemical reactions is formulated in the framework of a dielectric continuum solvent model which takes solute boundary effects into account. This latter feature represents a generalization of the Kim–Hynes theory, in which the solute boundary was treated in the dielectric image approximation. The electronic structure of the molecular solute, embedded in a cavity of the dielectric, is described by a manifold of orthogonalized diabatic—e.g., valence bond (VB)—states. The polarization of the dielectric solvent is partitioned into an electronic (fast) and an orientational (slow) component. The formulation encompasses both nonequilibrium and equilibrium regimes of the orientational polarization with respect to the solute charge distribution. The analysis is carried out in the general case of quantized solvent electronic polarization, but with reference to two limits in terms of which the general results can be most readily compr...

Bihalide ion combination reactions in solution: electronic structure and solvation aspects

Abstract The Kim—Hynes theory for electronic structure for reaction systems in solution is applied to the reaction class of the title, focusing on the I - +I → I 2 - reaction in acetonitrile solvent from a valence bond perspective. The transition between a delocalized electronic structure at small internuclear separations r to localized structures at large r is described in terms of a two-dimensional nonequilibrium free energy surface; the second coordinate is a solvent coordinate gauging the state of the solvent orientational polarization. The reaction path on this surface is described, and is contrasted with an equilibrium solvation perspective. An important focus is the polarization force, defined as the force on the diatomic ion coordinate r due to the solvent, which arises from the charge shifting, or electronic structure change, in the solute system along the reaction coordinate. It is argued that this force should play a significant role in the vibrational relaxation of the I 2 - ion.

On the activation free energy of the Cl− + CH3Cl SN2 reaction in solution

Abstract The activation free energetics of the identity SN2 reaction Cl− + CH3Cl → ClCH3 + Cl− in solution are examined theoretically. Two diabatic valence bond states, ψ1[Cl−1/CH3Cl] and ψ2[ClCH3/Cl−], are employed within the framework of the Kim-Hynes theory of solvation [H. J. Kim and J. T. Hynes, J. Chem. Phys. 96, 5088 (1992)] to calculate the free energies of the reactant and transition states in five solvents of different polarity. An electronically adiabatic vacuum potential energy surface for the reaction is presented, in terms of the energies associated with the two diabatic states ψ1 and ψ2 and the electronic coupling between them. Emphasis is placed on exposing the differences between two limiting solvent electronic polarization response regimes: the Born-Oppenheimer (BO) limit, where the solvent electronic polarization time scale is much smaller than that of the solute electronic motion, and the self-consistent (SC) limit, where the solute electronic motion time scale is much smaller than that of the solvent electronic polarization. It is found that the activation free energies calculated within the two limiting regimes can differ by as much as 7.7 kcal/mol, which is extremely significant, given the exponential sensitivity of the reaction rate constant. For the solvents studied, the BO activation free energies are always lower than the experimental estimates, whereas the SC results are invariably higher. The activation free energies calculated by the full Kim-Hynes theory, which properly treats the solvent electronic polarization, gives a result in between the BO and SC limits.

VB resonance theory in solution. II. I2−■I+I− in acetonitrile

The electronic structure in solution theory developed in the preceding article is applied to the molecular ion I2−■I+I− reaction system in the dipolar, aprotic solvent acetonitrile, which illustrates in detail the implementation of the general theory. A two‐dimensional, nonequilibrium free energy surface in the nuclear separation and a difference solvent coordinate is constructed via solution of a nonequilibrium solvation, nonlinear Schrodinger equation. The reduction to a single important solvent coordinate—from a manifold of three solvent coordinates—is motivated by an examination of the equilibrium solvation path and an analysis of the harmonic nonequilibrium fluctuations around this path. The evolving solute electronic structure over the basis of two orthogonal valence bond diabatic states—approximately corresponding to −II and II−—is discussed. Comparisons with the limiting Born–Oppenheimer and self‐consistent approximations for the solvent electronic polarization are made, with the former proving to...

ON THE PHOTODISSOCIATION OF ALKALI-METAL HALIDES IN SOLUTION

Gas-phase alkali-metal halide dissociation is influenced by the crossing of the covalent and ionic potential-energy surfaces at a certain internuclear separation, leading to interesting dynamical effects. The dissociation fragments for e.g. NaI may be trapped in a well formed by the avoided crossing of the covalent and ionic surfaces, and then undergo a non-adiabatic curve crossing transition to form atomic products. On the other hand, ionic products are stabilized by a polar environment and might be energetically accessible in solution. More generally, the photodissociation dynamics could be influenced by the solvent. A theoretical study of NaI photodissociation in a weakly polar solvent is presented here to explore the mechanism and timescale by which the ions are produced subsequent to photoexcitation. A solution-phase valence-bond resonance theory predicts that the diabatic ionic and covalent solution Gibbs free energy curves do not cross in the equilibrium solvation regime, such that atomic products would result. When considering non-equilibrium solvation and dynamical effects, the theory indicates the short-time dissociation products in solution to be atoms, but that on the ms timescale they could convert to ions by activated inverted regime electron transfer (ET). However, the radiative lifetime is estimated to be much shorter (≈ns) than this timescale, so that in fact no excited state ET is expected. Instead, the formation of ions proceeds by radiative deactivation of the photoexcited NaI and is followed by ionic recombination on the ground-state surface. Nevertheless it is estimated that the photodissociation of NaI in small clusters may proceed via activated ET and lead to some ionic dissociation products.

Proton relay and electron flow in the O–O single bond formation in water oxidation by the ruthenium blue dimer

The first, key step of water oxidation catalysis by the ruthenium blue dimer transition metal complex has been studied via density functional methods and with extensive explicit solvation, starting from the oxidized catalytically active form of the dimer. This step is the rate-limiting O–O single bond formation. This reaction is found to involve several proton transfers through a proton relay chain, synergetically coupled to electron flow through the μ-oxo bridge of the dimer. The barrier for the O–O formation step is found to arise primarily from the surrounding aqueous solvent, suggesting that it might be substantially lowered in suitable environments. Some remarks are given concerning the following, penultimate step prior to the formation of dioxygen and the stable form of the dimer, in which it is suggested that another proton relay chain is at play.

Theoretical studies of heterogeneous reaction mechanisms relevant for stratospheric ozone depletion

Aspects of the heterogeneous reaction HCI + ClONO{sub 2} {r{underscore}arrow} Cl{sub 2} + HNO{sub 3} on ice and N{sub 2}O{sub 5} + H{sub 2}O {r{underscore}arrow} 2 HNO{sub 3} on supercooled water are modeled via electronic-structure calculations involving reactant-water cluster systems. Reaction-path calculations for a HCI {sm{underscore}bullet} (H{sub 2}O){sub 9} system indicate a coupled proton-transfer (PT)/nucleophilic attack (S{sub N}2) process, in which the ice lattice is an active participant. Analysis of the N{sub 2}O{sub 5} {sm{underscore}bullet} (H{sub 2}O){sub 4} reaction system geometry and charge distribution suggests that a similar coupled PT/S{sub N}2 mechanism could occur in a larger system.

Theoretical study of water oxidation by the ruthenium blue dimer. II. Proton relay chain mechanism for the step [bpy2(HOO)Ru(IV)ORu(IV)(OH)bpy2]4+ → [bpy2(O2(–))Ru(IV)ORu(III)(OH2)bpy2]4+.

The oxidation of water to O2 by the oxidized species [L2(O)Ru(V)ORu(V)(O)L2]4+ of the Ru blue dimer catalyst (L = bpy, bipyridine) is examined using density functional theory with model ligands and explicit solvent approaches. Following our earlier study of the initial O–O formation by addition of water (step I) ( J. Phys. Chem. A 2011, 115, 8003), we report calculations on the subsequent, penultimate step in the superoxide production (denoted step II), involving proton transfer from the reactant [L2(HOO)Ru(IV)ORu(IV)(OH)L2]4+ to form [L(O2(–))Ru(IV)ORu(III)(H2O)L2]4+. The reaction profile of step II commences with a rearrangement of the HOO and OH groups and associated solvent relaxation in the complex, accompanied by a barrier of ~9 kcal/mol and a free-energy change of +3 kcal/mol. Subsequently, a water molecule connecting these two groups mediates a double proton transfer in a proton relay chain that proceeds spontaneously with a free-energy decrease of 8 kcal/mol to form step II’s product. Comparison with other calculations is made, and the implications for the overall water oxidation to O2 are discussed.