A Computational Study of the Hydration of the OH Radical

The Mechanism of Ozone Oxidation of Coal and the Revelation of Coal Macromolecular Structure by Oxidation Products

Ozone was injected into a coal-water suspension, and an HRTEM test was carried out on the separated oxidation products. The results show that from the perspective of visualization the macromolecular network structure of coal contains a large number of graphite-like structures. However, the chemical reaction mechanism between the coal surface and O3 is not clear, and the microscopic formation mechanism of oxygen-containing functional groups in carbon quantum dots has not been explained. As a result, the reaction process between O3 and methylene on the coal surface was studied by the DFT method. We found that OH• generated by O3 in water can oxidize two adjacent carbon atoms in methylene into double bonds (C=C), and finally, aldehydes and carboxylic acids were generated. By calculation of thermodynamic parameters ΔG and ΔH, it is found that all reactions are spontaneous exothermic processes. The above chemical reaction is based on the physical adsorption of OH• with Ar-(CH2)6-Ar and O3 with Ar-CH2-CH=CH-(CH2)3-Ar. The calculated adsorption energies of the two systems are -9.41 and -12.55 kcal/mol, respectively. Then, the charge transfer and atomic orbital interaction before and after adsorption are analyzed from the perspectives of Mulliken charge, density of states, deformation density, and total charge density. The results show that the electrostatic attraction is the main driving force of adsorption. The ether bond (C-O-C) in coal is finally oxidized to an ester group (RCOOR'), the hydroxyl group (CH2-CH-OH) on the aliphatic chain is oxidized to a carbonyl group (CH2-C=O), and the benzene with two OH• forms phenol hydroxyl and one molecule of water. Finally, the coal and the corresponding coal-based carbon quantum dots were investigated by infrared spectroscopy; the difference in functional groups before and after oxidation was clarified, and the result was in good agreement with the simulation.

Role of Hydroxyl Radical in Degradation of NTO: DFT Study

Hydroxyl radicals are important reactive oxygen species produced in the aquatic environment under sunlight irradiation. Many organic pollutants may be decomposed as they encounter hydroxyl radicals, due to their high oxidative ability. NTO (5-nitro-1,2,4-triazol-3-one), an energetic material used in military applications, may be released to the environment and dissolved in surface water and groundwater due to its good water solubility. A detailed investigation of the possible mechanism for NTO decomposition in water induced by hydroxyl radical as one of the pathways for NTO environmental degradation was performed by computational study at the PCM/M06-2X/6-311++G(d,p) level. Decomposition of NTO was found to be a multistep process that may begin with an addition of hydroxyl radical to the carbon atom of C═N double bond and consequent release of a nitrite radical. The formed intermediate undergoes a series of chemical transformations that include the attachments of hydroxyl radical to carbon atoms, the transfer of hydrogen to hydroxyl radical, isomerization, and bond cleavage, leading to low-weight inorganic compounds, such as ammonia, nitrogen gas, nitrous acid, nitric acid, and carbon(IV) oxide. The anionic form of NTO is more reactive toward interaction with the hydroxyl radical as compared with its neutral form. Calculated activation energies and high exergonicity of the studied process support the significant contribution of the hydroxyl radical to NTO mineralization in environment.

Resonant Inelastic X-Ray Scattering Reveals Hidden Local Transitions of the Aqueous OH Radical

Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities to interrogate ultrafast dynamics in liquids. Here we use RIXS to study the fundamentally and practically important hydroxyl radical in liquid water, OH(aq). Impulsive ionization of pure liquid water produced a short-lived population of OH(aq), which was probed using femtosecond x-rays from an x-ray free-electron laser. We find that RIXS reveals localized electronic transitions that are masked in the ultraviolet absorption spectrum by strong charge-transfer transitions-thus providing a means to investigate the evolving electronic structure and reactivity of the hydroxyl radical in aqueous and heterogeneous environments. First-principles calculations provide interpretation of the main spectral features.

The water trimer reaction OH + (H2O)3→ (H2O)2OH + H2O

All important stationary points on the potential energy surface (PES) for the reaction OH + (H₂O)₃→ (H₂O)₂OH + H₂O have been fully optimized using the "gold standard" CCSD(T) method with the large Dunning correlation-consistent cc-pVQZ basis sets. Three types of pathways were found. For the pathway without hydrogen abstraction, the barrier height of the transition state (TS1) is predicted to lie 5.9 kcal mol^-1 below the reactants. The two major complexes (H₂O)₃OH (CP1 and CP2a) are found to lie 6.3 and 11.0 kcal mol^-1, respectively, below the reactants [OH + (H₂O)₃]. For one of the H-abstraction pathways the lowest classical barrier height is predicted to be much higher, 6.1 kcal mol^-1 (TS2a) above the reactants. For the other H-abstraction pathway the barrier height is even higher, 15.0 (TS3) kcal mol^-1. Vibrational frequencies and the zero-point vibrational energies connected to the PES are also reported. The energy barriers for the H-abstraction pathways are compared with those for the OH + (H₂O)₂ and OH + H₂O reactions, and the effects of the third water on the energetics are usually minor (0.2 kcal mol^-1).

Structure, stability, infrared spectra, and bonding of OHm(H2O)7 (m = 0, ±1) clusters: ab initio study combining the particle swarm optimization algorithm

The various structural candidates of anionic, neutral, and cationic water clusters OHm(H2O)7 (m = 0, ±1) have been globally predicted by combining the particle swarm optimization method and quantum chemical calculations. Geometry optimization and vibrational analysis for the optimal structures were performed with the MP2/aug-cc-pVDZ method, and the energy profile was further refined at the CCSD(T)/CBS level. Special attention was paid to the relationships between configurations and energies, particularly the first solvation shell coordination number of OH- and OH. For OH-(H2O)7, OH(H2O)7, and OH+(H2O)7 clusters, the most stable species at room temperature are predicted to be the tetra-solvated multi-ring structure A6, the tri-solvated hemibond cage structure N1, and the single five-membered ring structure C2, respectively. The temperature effects on the stability of these three systems were also explored via Gibbs free energies. Furthermore, for the OH-(H2O)7 clusters, the assignments of vibrational transitions in the OH stretching region are in good agreement with the studies of small hydroxide ion-water clusters, and the IR spectra of two isomers (tetra-solvated multi-ring A6 and penta-solvated cage A3) may match future experimental observation well. By topological analysis and reduced density gradient analysis, the structural characteristics and bonding strengths of the studied clusters were investigated. This work indicates the excellent performance of the PSO search algorithm and CALYPSO on water clusters, and may further provide extensive insights into the chemical behavior such as the transport mechanism of OH- ions and OH radicals in the aqueous phase.

Tracking the energy flow in the hydrogen exchange reaction OH + H2O → H2O + OH

The prototypical hydrogen exchange reaction OH + H2O → H2O + OH has attracted considerable interest due to its importance in a wide range of chemically active environments. In this work, an accurate global potential energy surface (PES) for the ground electronic state was developed based on ∼44 000 ab initio points at the level of UCCSD(T)-F12a/aug-cc-pVTZ. The PES was fitted using the fundamental invariant-neural network method with a root mean squared error of 4.37 meV. The mode specific dynamics was then studied by the quasi-classical trajectory method on the PES. Furthermore, the normal mode analysis approach was employed to calculate the final vibrational state distribution of the product H2O, in which a new scheme to acquire the Cartesian coordinates and momenta of each atom in the product molecule from the trajectories was proposed. It was found that, on one hand, excitation of either the symmetric stretching mode or the asymmetric stretching mode of the reactant H2O promotes the reaction more than the translational energy, which can be rationalized by the sudden vector projection model. On the other hand, the relatively higher efficacy of exciting the symmetric stretching mode than that of the asymmetric stretching mode is caused by the prevalence of the indirect mechanism at low collision energies and the stripping mechanism at high collision energies. In addition, the initial collision energy turns ineffectively into the vibrational energy of the products H2O and OH while a fraction of the energy transforms into the rotational energy of the product H2O. Fundamental excitation of the stretching modes of H2O results in the product H2O having the highest population in the fundamental state of the asymmetric stretching mode, followed by the ground state and the fundamental state of the symmetric stretching mode.

One- or Two-Electron Water Oxidation, Hydroxyl Radical, or H2O2 Evolution

Electrochemical or photoelectrochemcial oxidation of water to form hydrogen peroxide (H₂O₂) or hydroxyl radicals (^•OH) offers a very attractive route to water disinfection, and the first process could be the basis for a clean way to produce hydrogen peroxide. A major obstacle in the development of effective catalysts for these reactions is that the electrocatalyst must suppress the thermodynamically favored four-electron pathway leading to O₂ evolution. We develop a thermochemical picture of the catalyst properties that determine selectivity toward the one, two, and four electron processes leading to ^•OH, H₂O₂, and O₂.

The hemibond as an alternative condensed phase structure for the hydroxyl radical

Despite the critical importance of the hydroxyl radical in major scientific fields, there are still open questions on the behavior of this species in the aqueous phase. In particular, there has been much debate on the existence of a hemibonded interaction between the hydroxyl radical and water molecules. While some reports indicate that the hemibonded radical might explain some experimental data, others have claimed that this interaction is simply a density functional theory (DFT) artifact. Here, we provide results from high level (basis set limit of coupled-cluster levels up to single, double, triple excitations (CCSD(T)) and beyond) ab initio calculations of different OH•(H2O)n clusters in the gas phase to accurately explore the existence of the hemibonded interaction and its energy difference with respect to other well-defined hydrogen bond interactions. Additional comparisons with second order perturbation theory (MP2) and DFT are also presented. Constrained molecular dynamics was applied to determine the free energy for the formation/disruption and ice systems. Overall, our findings confirm that the hemibond can be an alternative structure for the hydroxyl radical in the condensed phase when the formation of hydrogen bonds is impeded. These results will aid the understanding of theoretical and experimental data and help future experimental designs for the detection of this important species.

Mobility mechanism of hydroxyl radicals in aqueous solution via hydrogen transfer

The hydroxyl radical (OH*) is a highly reactive oxygen species that plays a salient role in aqueous solution. The influence of water molecules upon the mobility and reactivity of the OH* constitutes a crucial knowledge gap in our current understanding of many critical reactions that impact a broad range of scientific fields. Specifically, the relevant molecular mechanisms associated with OH* mobility and the possibility of diffusion in water via a H-transfer reaction remain open questions. Here we report insights into the local hydration and electronic structure of the OH* in aqueous solution from Car-Parrinello molecular dynamics and explore the mechanism of H-transfer between OH* and a water molecule. The relatively small free energy barrier observed (~4 kcal/mol) supports a conjecture that the H-transfer can be a very rapid process in water, in accord with very recent experimental results, and that this reaction can contribute significantly to OH* mobility in aqueous solution. Our findings reveal a novel H-transfer mechanism of hydrated OH*, resembling that of hydrated OH(-) and presenting hybrid characteristics of hydrogen-atom and electron-proton transfer processes, where local structural fluctuations play a pivotal role.

Study of electronic structure and dynamics of interacting free radicals influenced by water

We present a study of electronic structure, stability, and dynamics of interaction and recombination of free radicals such as HO(2) and OH influenced by water. As simple model calculations, we performed ab initio and density functional calculations for the interaction of HO(2) and OH in the presence of water cluster. Results indicate that a significant interaction, overcoming the repulsive Columbic barrier, occurs at a separation distance between the radicals of 5.7 A. This confirms early predictions of the minimum size of molecular dianions stable in the gas phase. It is well known that atomic dianions are unstable in the gas phase but molecular dianions are stable when the size of the molecule is larger than 5.7 A. Ab initio molecular dynamics calculations with Car-Parrinello scheme show that the reaction is very fast and occurs on a time scale of about 1.5 ps. The difference in stability and dynamics of the interacting free radicals on singlet and triplet potential energy surfaces is also discussed.

The structures of ozone and HOx radicals in aqueous solution from combined quantum/classical molecular dynamics simulations

Ozone in aqueous solution decomposes through a complex mechanism that involves initial reaction with a hydroxide ion followed by formation of a variety of oxidizing species such as HO, HO(2), and HO(3) radicals. Though a number of hydrogen-bonded complexes have been described in the gas phase, both theoretically and experimentally, the structures of ozone and HO(x) in liquid water remain uncertain. In this work, combined quantum/classical computer simulations of aqueous solutions of these species have been reported. The results show that ozone undergoes noticeable electron polarization but it does not participate in hydrogen bonds with liquid water. The main contribution of the solvation energy comes from dispersion forces. In contrast, HO(x) radicals form strong hydrogen bonds. They are better proton donors but weaker proton acceptors than water. Their electronic and geometrical structures are significantly modified by the solvent, especially in the case of HO(3). In all cases, fluctuations in amplitudes of electronic properties are considerable, suggesting that solvent effects might play a crucial role on oxidation mechanisms initiated by ozone in liquid water. These mechanisms are important in a broad range of domains, such as atmospheric processes, plant response to ambient ozone, and medical and industrial applications.

A molecular dynamics study of structural relaxation in tetrahedrally coordinated nanocrystals

The reorganisation of nanocrystals in order to reduce their surface energies has been examined in computer simulations. The relaxation takes a qualitatively different path for sphalerite- and wurtzite-structured particles. The surfaces of the sphalerite particles reconstruct into hexagonal nets, but the interior remains identifiable as sphalerite-like, whereas wurtzite particles form facetted, hexagonal nanorods by virtue of a reorganisation of the whole particle which involves the creation of a low energy internal interface between oppositely oriented domains. Despite the reorganisation, the diffraction patterns remain compatible with a wurtzite structure with some internal strain. The dipole moments of thermalized wurtzite particles are compared with experimental results for CdSe.

Do Hydroxyl Radical−Water Clusters, OH(H2O)n, n = 1−5, Exist in the Atmosphere?

It has been speculated that the presence of OH(H2O)n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.

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.

The OH radical-H2O molecular interaction potential

The OH radical is one of the most important oxidants in the atmosphere due to its high reactivity. The study of hydrogen-bonded complexes of OH with the water molecules is a topic of significant current interest. In this work, we present the development of a new analytical functional form for the interaction potential between the rigid OH radical and H(2)O molecules. To do this we fit a selected functional form to a set of high level ab initio data. Since there is a low-lying excited state for the H(2)O.OH complex, the impact of the excited state on the chemical behavior of the OH radical can be very important. We perform a potential energy surface scan using the CCSD(T)/aug-cc-pVTZ level of electronic structure theory for both excited and ground states. To model the physics of the unpaired electron in the OH radical, we develop a tensor polarizability generalization of the Thole-type all-atom polarizable rigid potential for the OH radical, which effectively describes the interaction of OH with H(2)O for both ground and excited states. The stationary points of (H(2)O)(n)OH clusters were identified as a benchmark of the potential.

Enhanced Activity and Selectivity in Cyclohexane Autoxidation by Inert H-Bond Acceptor Catalysts

Herein, we demonstrate that the chain-initiating dissociation of cyclohexyl hydroperoxide, CyOOH, is substantially accelerated by H-bond acceptors (e.g. Teflon), which assist O-O bond breaking by stabilising the leaving *OH radical. This is a completely new approach to boost the chain-propagating radical concentration. Indeed, up to now, literature has remained focussed on transition metal catalysis. In addition to this initiation effect, we demonstrate how inert perfluorinated compounds are also able to steer the selectivity at the molecular level, by promoting the conversion of the intermediate cyclohexyl hydroperoxide to the most desired end-product, cyclohexanone. This effect is explained by an enhanced, H-bond-assisted, hydroperoxide propagation. This hitherto overlooked hydroperoxide propagation was recently presented by us as the dominant cyclohexanone and cyclohexanol source. We herein thus confirm our previously reported autoxidation scheme, and illustrate its usefulness as a solid basis for designing new catalytic systems.

Predicting Aqueous Free Energies of Solvation as Functions of Temperature

This work introduces a model, solvation model 6 with temperature dependence (SM6T), to predict the temperature dependence of aqueous free energies of solvation for compounds containing H, C, and O in the range 273-373 K. In particular, we extend solvation model 6 (SM6), which was previously developed (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Chem. Theory Comput. 2005, 1, 1133) for predicting aqueous free energies of solvation at 298 K, to predict the variation of the free energy of solvation relative to 298 K. Also, we describe the database of experimental aqueous free energies of solvation for compounds containing H, C, and O that was used to parametrize and test the new model. SM6T partitions the temperature dependence of the free energy of solvation into two components: the temperature dependence of the bulk electrostatic contribution to the free energy of solvation, which is computed using the generalized Born equation, and the temperature dependence of first-solvation-shell effects which is modeled using a parametrized solvent-exposed surface-area-dependent term. We found that SM6T predicts the temperature dependence of aqueous free energies of solvation with a mean unsigned error of 0.08 kcal/mol over our entire database, whereas using the experimental value at 298 K produces a mean unsigned error of 0.53 kcal/mol.

Hydroxyl Radical and Hydroxide Ion in Liquid Water: A Comparative Electron Density Functional Theory Study

Ab initio density functional theory molecular dynamics simulations of the solvated states of the hydroxyl radical and hydroxide ion are performed using the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional (Becke, A. D. Phys. Rev. A 1988, 38, 3098. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785). The structures of the solvation shells of the two species are examined. It is found that the OH radical forms a relatively well-defined solvation complex with four neighboring water molecules. Three of these molecules are hydrogen bonded to the OH, while the fourth is hemibonded via a three-electron two-centered bond between the oxygen atoms of the OH and water. The activity and the diffusion mechanism of the OH radical in water is discussed in comparison with the OH- ion. Although the results are partially influenced by the tendency of the BLYP density functional to overestimate hemibonded structure, the present simulations suggest that the widely accepted picture of rapid diffusion of OH radical in water through hydrogen exchange reaction may need to be reconsidered.

Classical molecular-dynamics simulation of the hydroxyl radical in water

We have studied the hydration and diffusion of the hydroxyl radical OH0 in water using classical molecular dynamics. We report the atomic radial distribution functions, hydrogen-bond distributions, angular distribution functions, and lifetimes of the hydration structures. The most frequent hydration structure in the OH0 has one water molecule bound to the OH0 oxygen (57% of the time), and one water molecule bound to the OH0 hydrogen (88% of the time). In the hydrogen bonds between the OH0 and the water that surrounds it the OH0 acts mainly as proton donor. These hydrogen bonds take place in a low percentage, indicating little adaptability of the molecule to the structure of the solvent. All hydration structures of the OH0 have shorter lifetimes than those corresponding to the hydration structures of the water molecule. The value of the diffusion coefficient of the OH0 obtained from the simulation was 7.1x10(-9) m2 s(-1), which is higher than those of the water and the OH-.

Molecular Dynamics Simulations of Atmospheric Oxidants at the Air−Water Interface: Solvation and Accommodation of OH and O3

A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass accommodation (alpha) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated toward the liquid surface when they are approximately 5 angstroms above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 to 140 ps, and the values of alpha for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atmospheric chemistry are discussed.

Hydroxyl radical at the air-water interface

Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air-water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (DeltaGa) being about 1 kcal/mol more negative than the hydration free energy (DeltaGs). The propensity of the hydroxyl radical for the air-water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.

Rotational Spectrum and Hydrogen Bonding of the H2O−HO Radical Complex

The first spectroscopic identification of the H2O-HO radical complex in the gas phase has been conducted by utilizing pulsed-discharge nozzle Fourier transform microwave spectroscopy. R-branch transitions in the Ka = 0 manifold appeared as in Hund's case (b), but extraordinarily large spin doubling implies a strong spin-orbit coupling between the electronic ground and low-lying excited states that correlate to the degenerate 2Pi state in free OH. The geometry of the complex is of C2v symmetry as a zero-point vibrational average, in which the OH radical acts as a proton donor to water. Precisely determined hyperfine coupling constants associated with hydrogen nuclei indicate a substantial rearrangement in unpaired electron distribution: there exists small but nonzero spin density on the H atoms in water.

Solvation Structure of Hydroxyl Radical by Car−Parrinello Molecular Dynamics

Car-Parrinello molecular dynamics simulations of a hydroxyl radical in liquid water have been performed. Structural and dynamical properties of the solvated structure have been studied in details. The partial atom-atom radial distribution functions for the hydrated hydroxyl do not show drastic differences with the radial distribution functions for liquid water. The OH is found to be a more active hydrogen bond donor and acceptor than the water molecule, but the accepted hydrogen bonds are much weaker than for the hydroxide OH- ion. The first solvation shell of the OH is less structured than the water's one and contains a considerable fraction of water molecules that are not hydrogen bonded to the hydroxyl. Part of them are found to come closer to the solvated radical than the hydrogen bonded molecules do. The lifetime of the hydrogen bonds accepted by the hydroxyl is found to be shorter than the hydrogen bond lifetime in water. A hydrogen transfer between a water molecule and the OH radical has been observed, though it is a much rarer event than a proton transfer between water and an OH- ion. The velocity autocorrelation power spectrum of the hydroxyl hydrogen shows the properties both of the OH radical in clusters and of the OH- ion in liquid.

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.

Infrared measurements and calculations on H2O.HO

We have measured the infrared spectrum of H2O.HO in argon matrices at 11.5 +/- 0.5 K. We have also calculated the vibrational frequencies and intensities of the H2O.HO complex. As a result of these measurements and calculations, we have assigned a previously unassigned absorption band at 3442.1 cm-1 to the OH stretch in the radical complexed to the water molecule. This absorption originates from a complex that is situated in a different site within the argon matrix to those absorptions already assigned to this vibration at 3452.2 and 3428.0 cm-1. We observe a decrease in intensity of the OH radical stretching vibration of the H2O.HO complex upon isotopic substitution of D for H that agrees well with our calculations.