David M. Bartels

Moment analysis of hydrated electron cluster spectra: Surface or internal states?

Moment analysis is applied to the absorption spectra of size-selected water anion clusters measured by Ayotte and Johnson [J. Chem. Phys. 106, 811 (1997)], to extract the radius of gyration and the kinetic energy for the unpaired electron. Implications for the surface versus interior binding model controversy are discussed.

Ultrafast dynamics for electron photodetachment from aqueous hydroxide

Charge-transfer-to-solvent reactions of hydroxide induced by 200 nm monophotonic or 337 and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, e(aq) (-), have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component ( approximately 13 ps) and a slower power tail due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% versus 17.5% at 25 degrees C, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is <4 cm TW(-1) M(-1) (assuming unity quantum efficiency for the photodetachment). For [OH(-)] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10(-3) K(-1) and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.

Diffusion and CIDEP of H and D atoms in solid H2O, D2O and isotopic mixtures☆

Abstract Hydrogen and deuterium atoms have studied by pulsed EPR spectroscopy in polycrystalline samples of H 2 O ice, D 2 O ice, and various isotopic mixtures. At high temperature (−10°C) the pattern and the time-dependence of the EPR line intensities are similar to previous results for H and D in liquid water. Chemically induced dynamic electron polarization (CIDEP) is generated in second-order atom combination reactions. The CIDEP behavior was found to change over the temperature range studied (−5°C to 130°C), consistent with additional contributions from spur reactions, and at lower temperatures, geminate recombination of (D…OD) radical pairs. Transverse spin relaxation times were measured by the spin-echo technique, and interpreted in terms of translational motion of free atoms diffusing through the ice lattice. One surprising result is that D atoms diffuse faster than H atoms below 200 K. This is explained as a vibrational zero point energy effect, by applying transition state theory to a model in which the diffusing atom must pass through a tight “bottleneck” in the electronic potential surface, as it passes from one minimum energy site in the lattice to the next. The H and D spin relaxation rates were successfully simulated by means of a semiclassical potential which was constructed by pairwise addition of atom-atom contributions represented by modified Buckingham potential functions. Extension of the model to include tunneling resulted in little change to the fit of the H and D data. Although predictions of muonium diffusion rates using the same potential do not give quantitative agreement with published results from spin relaxation measurements, they do serve to illustrate the dominant effect of tunneling over a wide temperature range for that light atom.

Isotope and temperature effects on the hyperfine interaction of atomic hydrogen in liquid water and in ice

The Fermi contact hyperfine interaction of hydrogen isotopes in liquid and solid water is below the vacuum value, shows a mass dependence, and has a negative temperature coefficient in the liquid. Furthermore, it shows a pronounced solvent isotope effect in H2O/D2O mixtures. This behavior is analyzed in terms of Dalgarno–Lewis perturbation theory for the atom in a spherical parabolic solvent potential and with a phenomenological model for spin delocalization onto solvent molecules. The results support previous suggestions that hydrogen atoms induce clathrate‐like cages in liquid water. These may resemble the static structures of noble gas clathrates except that they are of transient dynamic nature in liquid water.

Free radical reactions of monochloramine and hydroxylamine in aqueous solution

Abstract The use of Advanced Oxidation Technologies to destroy organic contaminants in drinking water may be impacted by the presence of disinfection chemicals such as monochloramine (NH2Cl). To allow a quantitative evaluation of the effect of NH2Cl on the destruction of organics in water rate constants for its reaction with the hydrated electron, the hydroxyl radical and the hydrogen atom were determined in this study. The corresponding values of (2.2±0.2)×1010, (2.8±0.2)×109, and (1.2±0.1)×109xa0M−1xa0s−1, respectively, were incorporated into a kinetic computer model whose predictions were in good agreement with experimental chloramine removal under large scale, steady-state electron-beam irradiation conditions. Rate constants were also determined for the reaction of the hydroxyl radical and hydrogen atom with the chloramine hydration product hydroxylamine to supplement established literature data. Hydroxyl radical rate constants for the basic (NH2OH) and acidic (NH3OH+) forms were determined as (8.5±0.4)×109 and ⩽5×107xa0M−1xa0s−1, respectively, while for hydrogen atom reaction, corresponding rate constants of (4.5±0.1)×107 and (3.6±1.5)×105xa0M−1xa0s−1 were found.

Lack of ionic strength effect in the recombination of hydrated electrons: (e−)aq + (e−)aq → 2(OH−) + H2

Abstract We report measurements of the rate constant for bimolecular reaction between hydrated electrons in the presence of added inert salt. At 23°C, the rate constant is 6.0 × 10 9 M −1 s −1 , virtually independent of added LiClO 4 up to 0.06 M concentration, where the Bronsted-Bjerrum equation would predict a 50% rate enhancement. However, the diffusion rate of ions is reduced by the additional friction from the ionic atmosphere. We demonstrate that for this peculiar diffusion-limited reaction the two effects counterbalance, producing almost no change in the reaction rate. Based on the large reaction distance and activation energy, it appears that a solvent-separated singlet pair of electrons is stable relative to kT .

Design of an optical cell for pulse radiolysis of supercritical water

The design of a flow cell that is applicable to pulse radiolysis/transient absorption experiments on supercritical water is described. The cell is designed to minimize dead volume and prevent the accumulation of radiolytic products. It is also necessary to minimize emission and absorption of sapphire windows from high energy electron beam irradiation. To obtain an optical throughput of f/4, the inner diameter is 6 mm, and distance between windows is 25 mm. The effective optical path length is 20 mm for irradiation from the side through a thin Hastelloy wall. Belleville spring washers were used to keep a constant force on the 3 mm sapphire windows, which were sealed to the Hastelloy body with copper gaskets. An application of this cell to measurements of solvated electrons in supercritical water is demonstrated.

Reaction of OH* radicals with H2 in sub-critical water

Abstract The rate constant for the reaction of hydroxyl radicals (OH ) with hydrogen in aqueous solutions has been measured from 200 to 350 °C by competition kinetics using nitrobenzene as a competing OH * scavenger. Measurements below 250 °C agree with previous results in other laboratories. At higher temperatures, the rate constant undershoots an extrapolation of the Arrhenius plot and actually decreases in value above 275 °C. At 350 °C, the measured rate constant is more than a factor of 5 below the Arrhenius extrapolation. We propose an explanation based largely on the hydrophobic solvation properties of the H 2 molecule.

Observations of heisenberg spin exchange between reactive free radicals

Abstract We report measurements of the ratio of Heisenberg spin exchange and self-recombination rates for a series of small free radicals in aqueous solution. In all cases, the exchange rate is greater than the reaction rate. The critical exchange distance is between one and three times the hard-sphere encounter distance, in agreement with recent theoretical predictions.

The free radical chemistry of tert-butyl formate: rate constants for hydroxyl radical, hydrated electron and hydrogen atom reaction in aqueous solution

Abstract Transients generated in situ by advanced oxidation technologies (AOTs) to destroy organic contaminants in ground and drinking water often give large concentrations of chemical by-products. These by-products may have adverse health effects, and can also interfere with the desired chemical removal by competing for the generated transients, thus lowering the overall efficiency of the remediation process. To allow for a quantitative evaluation of the influence of tert -butyl formate (TBF), a major by-product formed in the AOT destruction of methyl tert- butyl ether, rate constants for TBF reaction with the hydroxyl radical, the hydrated electron and the hydrogen atom in aqueous solution were measured in this study. Absolute values of (5.23±0.07)×10 8 , (5.48±0.09)×10 8 and (3.58±0.07)×10 6 xa0M −1 xa0s −1 , were determined at 22°C, respectively.