Reevaluation of Arrhenius parameters for hydrogen atom + hydroxide .fwdarw. (e-)aq + water and the enthalpy and entropy of hydrated electrons

Simulating the Competitive Ion Pairing of Hydrated Electrons with Chaotropic Cations

Experiments show that the absorption spectrum of the hydrated electron (ehyd-) blue-shifts in electrolyte solutions compared with what is seen in pure water. This shift has been assigned to the ehyd-'s competitive ion-pairing interactions with the salt cation relative to the salt anion based on the ions' positions on the Hofmeister series. Remarkably, little work has been done investigating the ehyd-'s behavior when the salts have chaotropic cations, which should greatly change the ion-pairing interactions given that the ehyd- is a champion chaotrope. In this work, we remedy this by using mixed quantum/classical simulations to analyze the behavior of two different models of the ehyd- in aqueous RbF and RbI electrolyte solutions as a function of salt concentration. We find that the magnitude of the salt-induced spectral blue-shift is determined by a combination of the number of chaotropic Rb+ cations near the ehyd- and the number of salt anions near those cations so that the spectrum of the ehyd- directly reflects its local environment. We also find that the use of a soft-cavity ehyd- model predicts stronger competitive interactions with Rb+ relative to I- than a more traditional hard cavity model, leading to different predicted spectral shifts that should provide a way to distinguish between the two models experimentally. Our simulations predict that at the same concentration, salts with chaotropic cations should produce larger spectral blue-shifts than salts with kosmotropic cations. We also found that at high salt concentrations with chaotropic cations, the predicted blue-shift is greater when the salt anion is kosmotropic instead of chaotropic. Our goal is for this work to inspire experimentalists to make such measurements, which will help provide a spectroscopic means to distinguish between simulations models that predict different hydration structures for the ehyd-.

Ab Initio Studies of Hydrated Electron/Cation Contact Pairs: Hydrated Electrons Simulated with Density Functional Theory Are Too Kosmotropic

We have performed the first DFT-based ab initio MD simulations of a hydrated electron (e_aq^-) in the presence of Na⁺, a system chosen because ion-pairing behavior in water depends sensitively on the local hydration structure. Experiments show that e_aq^-'s interact weakly with Na⁺; the e_aq^-'s spectrum blue shifts by only a few tens of meV upon ion pairing without changing shape. We find that the spectrum of the DFT-simulated e_aq^- red shifts and changes shape upon interaction with Na⁺, in contrast with experiment. We show that this is because the hydration structure of the DFT-simulated e_aq^- is too ordered or kosmotropic. Conversely, simulations that produce e_aq^-'s with a less ordered or chaotropic hydration structure form weaker ion pairs with Na⁺, yielding predicted spectral blue shifts in better agreement with experiment. Thus, ab initio simulations based on hybrid GGA DFT functionals fail to produce the correct solvation structure for the hydrated electron.

Structure of the aqueous electron

A cavity or excluded-volume structure best explains the experimental properties of the aqueous or “hydrated” electron.

A Simple ab Initio Model for the Hydrated Electron That Matches Experiment

Since its discovery over 50 years ago, the "structure" and properties of the hydrated electron have been a subject for wonderment and also fierce debate. In the present work we seriously explore a minimal model for the aqueous electron, consisting of a small water anion cluster embedded in a polarized continuum, using several levels of ab initio calculation and basis set. The minimum energy "zero Kelvin" structure found for any 4-water (or larger) anion cluster, at any post-Hartree–Fock theory level, is very similar to a recently reported embedded-DFT-in-classical-water-MD simulation (Uhlig, Marsalek, and Jungwirth, J. Phys. Chem. Lett. 2012, 3, 3071−3075), with four OH bonds oriented toward the maximum charge density in a small central "void". The minimum calculation with just four water molecules does a remarkably good job of reproducing the resonance Raman properties, the radius of gyration derived from the optical spectrum, the vertical detachment energy, and the hydration free energy. For the first time we also successfully calculate the EPR g-factor and (low temperature ice) hyperfine couplings. The simple tetrahedral anion cluster model conforms very well to experiment, suggesting it does in fact represent the dominant structural motif of the hydrated electron.

On the generation of the hydrated electron during the sonolysis of aqueous solutions

The formation of the hydrated electron through the secondary reaction, H + OH(–) → H(2)O + e(aq), has been examined in the sonolysis of argon-saturated aqueous solutions at an ultrasound frequency of 355 kHz. The detection of the hydrated electron was achieved by measuring its reaction with the one-electron acceptors Fe(CN)(6)(3–) and methyl viologen. The results obtained indicate that hydrated electrons are produced predominately at the bubble/aqueous solution interface at comparatively high local concentrations, estimated to be >>1.5 × 10(–3) M. The half life of the hydrated electron under such conditions is estimated to be <<60 ns.

Carbonate radical formation in radiolysis of sodium carbonate and bicarbonate solutions up to 250 degrees C and the mechanism of its second order decay

Pulse radiolysis experiments published several years ago (J. Phys. Chem. A, 2002, 106, 2430) raised the possibility that the carbonate radical formed from reaction of *OH radicals with either HCO(3)(-) or CO(3)(2-) might actually exist predominantly as a dimer form, for example, *(CO(3))(2)(3-). In this work we re-examine the data upon which this suggestion was based and find that the original data analysis is flawed. A major omission of the original analysis is the recombination reaction *OH + *CO(3)(-) --> HOOCO(2)(-). Upon reanalysis of the published data for sodium bicarbonate solutions and analysis of new transient absorption data we are able to establish the rate constant for this reaction up to 250 degrees C. The mechanism for the second-order self-recombination of the carbonate radical has never been convincingly demonstrated. From a combination of literature data and new transient absorption experiments in the 1-400 ms regime, we are able to show that the mechanism involves pre-equilibrium formation of a C(2)O(6)(2-) dimer, which dissociates to CO(2) and peroxymonocarbonate anion: *CO3(-)+*CO3(-)<-->C2O6(2-)-->CO2+O2COO(2-) *CO3(-) reacts with the product peroxymonocarbonate anion, producing a peroxymonocarbonate radical *O2COO(-), which can also recombine with the carbonate radical: *CO3(-)+CO4(2-)-->*CO4(-)+CO3(2-) *CO3(-)+CO4(-)-->C2O7(2-).

Nanocalorimetry in mass spectrometry: a route to understanding ion and electron solvation

A gaseous nanocalorimetry approach is used to investigate effects of hydration and ion identity on the energy resulting from ion-electron recombination. Capture of a thermally generated electron by a hydrated multivalent ion results in either loss of a H atom accompanied by water loss or exclusively loss of water. The energy resulting from electron capture by the precursor is obtained from the extent of water loss. Results for large-size-selected clusters of Co(NH(3))(6)(H(2)O)(n3)(+) and Cu(H(2)O)(n2)(+) indicate that the ion in the cluster is reduced on electron capture. The trend in the data for Co(NH(3))(6)(H(2)O)(n3)(+) over the largest sizes (n >/= 50) can be fit to that predicted by the Born solvation model. This agreement indicates that the decrease in water loss for these larger clusters is predominantly due to ion solvation that can be accounted for by using a model with bulk properties. In contrast, results for Ca(H(2)O)(n2)(+) indicate that an ion-electron pair is formed when clusters with more than approximately 20 water molecules are reduced. For clusters with n = approximately 20-47, these results suggest that the electron is located near the surface, but a structural transition to a more highly solvated electron is indicated for n = 47-62 by the constant recombination energy. These results suggest that an estimate of the adiabatic electron affinity of water could be obtained from measurements of even larger clusters in which an electron is fully solvated.

Reactivity and Equilibrium Thermodynamic Studies of Rhodium Tetrakis(3,5-disulfonatomesityl)porphyrin Species with H2, CO, and Olefins in Water

Aqueous (D2O) solutions of tetrakis(3,5-disulfonatomesityl)porphyrin rhodium(III) aquo/hydroxo complexes ([(TMPS)Rh(III)(D2O)2]-7 (1), [(TMPS)Rh(III)(OD)(D2O)]-8 (2), and [(TMPS)Rh(III)(OD)2]-9 (3)) react with hydrogen (D2) to form an equilibrium distribution with a rhodium hydride ([(TMPS)Rh-D(D2O)]-8 (4)) and a rhodium(I) complex ([(TMPS)Rh(I)(D2O)]-9 (5)). Equilibrium constants (298 K) are measured that define the distribution for all five of these (TMPS)Rh species in this system as a function of the dihydrogen (D2) and hydrogen ion (D+) concentrations. The hydride complex [(TMPS)Rh-D(D2O)]-8 is a weak acid in D2O (Ka(298 K) = 4.3 x 10(-8)). Steric demands of the TMPS porphyrin ligand prohibit formation of a Rh(II)-Rh(II)-bonded complex, related rhodium(I)-rhodium(III) adducts, and intermolecular association of alkyl complexes which are prominent features of the rhodium tetra(p-sulfonatophenyl)porphyrin ((TSPP)Rh) system. The rhodium(II) complex ([(TMPS)Rh(II)(D2O)]-8) reacts with water to form hydride and hydroxide complexes and is not observed in D2O. The (TMPS)Rh-OD and (TMPS)Rh-D bond dissociation free energies (BDFE) are virtually equal and have a value of approximately 60 kcal mol(-1). Reactions of [(TMPS)Rh-D(D2O)]-8 in water with CO and olefins produce rhodium formyl and alkyl complexes which have equilibrium thermodynamic values comparable to the values for the corresponding substrate reactions of [(TSPP)Rh-D(D2O)]-4.

Full Configuration Interaction Computer Simulation Study of the Thermodynamic and Kinetic Stability of Hydrated Dielectrons

The hydrated electron is a unique solvent-supported state comprised of an excess electron that is confined to a cavity by the surrounding water. Theoretical studies have suggested that two-electron solvent-supported states also can be formed; in particular, simulations indicate that two excess electrons could pair up and occupy a single cavity, forming a so-called hydrated dielectron. Although hydrated dielectrons have not been observed directly by experiment, their existence has been posited to explain the lack of an ionic strength effect in hydrated electron bimolecular annihilation [Schmidt, K. H.; Bartels, D. M. Chem. Phys. 1995, 190, 145]. To determine whether dielectrons may be created in the laboratory, we use thermodynamic integration (TI), combined with mixed quantum/classical molecular dynamics simulation, to examine the thermodynamic stability of hydrated electrons and dielectrons. For the dielectron calculations, we solve the two-electron quantum problem using full configuration interaction. Our results suggest that hydrated dielectrons are thermodynamically unstable relative to separated (single) hydrated electrons, although we also show that increasing the pressure could drive the equilibrium toward the formation of dielectrons. Because the simulations suggest that hydrated dielectrons are kinetically stable, we also examine a scenario for creating metstable, nonequilibrium populations of dielectrons, which involves the capture of a newly injected electron by a preexisting, equilibrated hydrated electron. These calculations, which allow for the full nonadiabatic relaxation of the injected electron, show that hydrated electrons may indeed act as trapping sites for unequilibrated electrons, so that capture may be a viable mechanism for creating dielectrons. We suggest possible experimental procedures to create such nonequilibrium hydrated dielectrons using either pulse radiolysis or ultrafast spectroscopic techniques.

Rate Constant and Activation Energy Measurement for the Reaction of Atomic Hydrogen with Thiocyanate and Azide in Aqueous Solution

Arrhenius parameters for the reaction of hydrogen atoms with azide and thiocyanate in aqueous solution have been determined using electron pulse radiolysis and electron paramagnetic resonance free induction decay attenuation measurements. Absolute values for SCN-, N3(-), and HN3 were well-described over the temperature range of 9-81 degrees C by the equations log k5 = (12.03 +/- 0.12) - [(21.05 +/- 0.66 kJ mol(-1))/2.303RT], log k10 = (12.75 +/- 0.21) - [(18.43 +/- 1.22 kJ mol(-1))/2.303RT], and log k15 = (11.59 +/- 0.12) - [(21.44 +/- 0.69 kJ mol(-1))/2.303RT], corresponding to room temperature (22 degrees C) rate constants of (2.07 +/- 0.03) x 10(8), (3.15 +/- 0.08) x 10(9), and (6.31 +/- 0.05) x 10(7) M(-1) s(-1) and activation energies for these chemicals of 21.05 +/- 0.66, 18.4 +/- 1.2, and 21.44 +/- 0.69 kJ mol(-1), respectively. The similarity of these three measured activation energies, taken together with the available information on reaction products, suggests a similar reaction mechanism, which is proposed to be an initial hydrogen atom adduct formation in these molecules, followed by single bond breakage.

Free radical destruction of N-nitrosodimethylamine in water

Absolute rate constants for the reactions of the hydroxyl radical, hydrated electron, and hydrogen atom with N-nitrosodimethylamine (NDMA) in water at room temperature have been determined using electron pulse radiolysis and transient absorption spectroscopy (*OH and e- aq) and EPR free induction decay attenuation (*H) measurements. Specific values of (4.30 +/- 0.12) x 10(8), (1.41 +/- 0.02) x 10(10), and (2.01 +/- 0.03) x 10(8) M(-1) s(-1) were measured, respectively. DMPO spin-trapping experiments demonstrated that the hydroxyl radical reaction with NDMA occurs by hydrogen atom abstraction from a methyl group, and the rate constant for the subsequent reaction of this radical transient with dissolved oxygen was measured as (5.3 +/- 0.6) x 10(6) M(-1) s(-1). This relatively slow rate constant implies that regeneration of the parent nitrosoamine from the oxidized transient could occur in natural waters containing dissolved organic compounds. The reaction of the hydrated electron with NDMA was to form a transient adduct anion, which could subsequently transfer this excess electron to regenerate the parent chemical. Such regeneration reactions would significantly reduce the effectiveness of any applied advanced oxidation technology remediation effort on NDMA-contaminated natural waters.

Equilibrium Thermodynamic Studies in Water: Reactions of Dihydrogen with Rhodium(III) Porphyrins Relevant to Rh−Rh, Rh−H, and Rh−OH Bond Energetics

Aqueous solutions of rhodium(III) tetra p-sulfonatophenyl porphyrin ((TSPP)Rh(III)) complexes react with dihydrogen to produce equilibrium distributions between six rhodium species including rhodium hydride, rhodium(I), and rhodium(II) dimer complexes. Equilibrium thermodynamic studies (298 K) for this system establish the quantitative relationships that define the distribution of species in aqueous solution as a function of the dihydrogen and hydrogen ion concentrations through direct measurement of five equilibrium constants along with dissociation energies of D(2)O and dihydrogen in water. The hydride complex ([(TSPP)Rh-D(D(2)O)](-4)) is a weak acid (K(a)(298 K) = (8.0 +/- 0.5) x 10(-8)). Equilibrium constants and free energy changes for a series of reactions that could not be directly determined including homolysis reactions of the Rh(II)-Rh(II) dimer with water (D(2)O) and dihydrogen (D(2)) are derived from the directly measured equilibria. The rhodium hydride (Rh-D)(aq) and rhodium hydroxide (Rh-OD)(aq) bond dissociation free energies for [(TSPP)Rh-D(D(2)O)](-4) and [(TSPP)Rh-OD(D(2)O)](-4) in water are nearly equal (Rh-D = 60 +/- 3 kcal mol(-1), Rh-OD = 62 +/- 3 kcal mol(-1)). Free energy changes in aqueous media are reported for reactions that substitute hydroxide (OD(-)) (-11.9 +/- 0.1 kcal mol(-1)), hydride (D(-)) (-54.9 kcal mol(-1)), and (TSPP)Rh(I): (-7.3 +/- 0.1 kcal mol(-1)) for a water in [(TSPP)Rh(III)(D(2)O)(2)](-3) and for the rhodium hydride [(TSPP)Rh-D(D(2)O)](-4) to dissociate to produce a proton (9.7 +/- 0.1 kcal mol(-1)), a hydrogen atom (approximately 60 +/- 3 kcal mol(-1)), and a hydride (D(-)) (54.9 kcal mol(-1)) in water.

Structure of the aqueous solvated electron from resonance Raman spectroscopy: lessons from isotopic mixtures

The structure and thermodynamics of the hydrated electron are probed with resonance Raman spectroscopy of isotopic mixtures of H(2)O and D(2)O. The strongly enhanced intramolecular bends of e(-)(H(2)O) and e(-)(D(2)O) produce single downshifted bands, whereas the e(-)(HOD) bend consists of two components: one slightly upshifted from the 1,446 cm(-1) bulk frequency to 1,457 cm(-1) and the other strongly downshifted to approximately 1,396 cm(-1). This 60 cm(-1) split and the 200 (120) cm(-1) downshifts of the OH (OD) stretch frequencies relative to bulk water reveal that the water molecules that are Franck-Condon coupled to the electron are in an asymmetric environment, with one proton forming a strong hydrogen bond to the electron. The downshifted bend and librational frequencies also indicate significantly weakened torsional restoring forces on the water molecules of e(-)(aq), which suggests that the outlying proton is a poor hydrogen bond donor to the surrounding solvent. A 1.6-fold thermodynamic preference of the electron for H(2)O is observed based on the relative intensities of the e(-)(H(2)O) and e(-)(D(2)O) bands in a 50:50 isotopic mixture. This equilibrium isotope effect is consistent with the downshifted vibrational frequencies and a relative reduction of the zero-point energy of H(2)O bound to the electron. Our results enhance the cavity model of the solvated electron and support only those models that contain water monomers as opposed to other molecular species.

La radiolyse de l'eau et des solutions aqueuses : historique et actualité

Experiments showing that water is decomposed by the action of high-energy radiations date back to the first days of the discovery of radioactivity, a century ago. On the occasion of this anniversary, we have attempted to give a comprehensive account of the radiation chemistry of water and its solutions since its origin, with special emphasis on the various physical and chemical stages that led to the present state of this science. To this aim, we describe the effect of different intervening factors on the molecular and radical yields, including dissolved solute concentration, pH, radiation intensity (or dose rate), type and energy of the radiation, presence of oxygen, temperature, phase, and pressure. We also discuss briefly the chemical behavior of the free radicals produced in radiolyzed aqueous solutions. A good, albeit incomplete, description of the phenomena is obtained that leads to various perspectives concerning, on the one hand, the development of this science and, on the other hand, its potential for applications.Key words : radical chemistry, dilution curve, water, hydrated electron, hydroxyl and superoxide radicals, free radicals, radiolysis, chain reactions, molecular and radical yields, cell survival, linear energy transfer.