Recombination of the hydrated electron at high temperature and pressure in hydrogenated alkaline water.

Pulse radiolysis experiments were performed on hydrogenated, alkaline water at high temperatures and pressures to obtain rate constants for the reaction of hydrated electrons with hydrogen atoms (H* + e-(aq) --> H(2) + OH-, reaction 1) and the bimolecular reaction of two hydrated electrons (e-(aq) + e-(aq) --> H(2) + 2 OH-, reaction 2). Values for the reaction 1 rate constant, k(1), were obtained from 100 - 325 degrees C, and those for the reaction 2 rate constant, k(2), were obtained from 100 - 250 degrees C, both in increments of 25 degrees C. Both k(1) and k(2) show non-Arrhenius behavior over the entire temperature range studied. k(1) shows a rapid increase with increasing temperature, where k(1) = 9.3 x 10(10) M(-1) s(-1) at 100 degrees C and 1.2 x 10(12) M(-1) s(-1) at 325 degrees C. This behavior is interpreted in terms of a long-range electron-transfer model, and we conclude that e-aq diffusion has a very high activation energy above 150 degrees C. The behavior of k(2) is similar to that previously reported, reaching a maximum value of 5.9 x 10(10) M(-1) s(-1) at 150 degrees C in the presence of 1.5 x 10(-3) m hydroxide. At higher temperatures, the value of k(2) decreases rapidly and above 250 degrees C is too small to measure reliably. We suggest that reaction 2 is a two-step reaction, where the first step is a proton transfer stimulated by the proximity of two hydrated electrons, followed immediately by reaction 1.

Hydroxyl radical self-recombination reaction and absorption spectrum in water up to 350 degrees C.

The rate constant for the self-recombination of hydroxyl radicals (*OH) in aqueous solution giving H2O2 product has been measured from 150 to 350 degrees C by direct measurement of the *OH radical transient optical absorption at 250 nm. The values of the rate constant are smaller than previously predicted by extrapolation to the 200-350 degrees C range and show virtually no change in this range. In combining these measurements with previous results, the non-Arrhenius behavior can be well described in terms of the Noyes equation kobs-1 = kact-1+ kdiff-1, using the diffusion-limited rate constant kdiff estimated from the Smoluchowski equation and an activated barrier rate kact nearly equal to the gas-phase high-pressure limiting rate constant for this reaction. The aqueous *OH radical spectrum between 230 and 320 nm is reported up to 350 degrees C. A weak band at 310 nm grows in at higher temperature, while the stronger band at 230 nm decreases. An isosbestic point appears near 305 nm. We assign the 230 nm band to hydrogen-bonded *OH radical, and the 310 nm band is assigned to "free" *OH. On the basis of the spectrum change relative to room temperature, over half of the *OH radicals are in the latter form at 350 degrees C.

Reaction of O2 with the hydrogen atom in water up to 350 degrees C.

The reaction of the H* atom with O2, giving the hydroperoxyl HO2* radical, has been investigated in pressurized water up to 350 degrees C using pulse radiolysis and deep-UV transient absorption spectroscopy. The reaction rate behavior is highly non-Arrhenius, with near diffusion-limited behavior at room temperature, increasing to a near constant limiting value of approximately 5 x 10(10) M(-1) s(-1) above 250 degrees C. The high-temperature rate constant is in near-perfect agreement with experimental extrapolations and ab initio calculations of the gas-phase high-pressure limiting rate. As part of the study, reaction of the OH* radical with H2 has been reevaluated at 350 degrees C, giving a rate constant of (6.0 +/- 0.5) x 10(8) M(-1) s(-1). The mechanism of the H* atom reaction with the HO2* radical is also investigated and discussed.

Pulse radiolysis of supercritical water. 3. Spectrum and thermodynamics of the hydrated electron.

Spectra of the hydrated electron in pressurized light and heavy water at temperatures up to and beyond the critical temperature are reported, for wavelengths between 0.4 and 1.7 microm. In agreement with previous work, spectra can be approximately represented by a Gaussian function on the low-energy side, and a Lorentzian function on the high-energy side in subcritical water, but deviations from this form are very clear above 200 degrees C. The spectrum shifts strongly to the red as temperature rises. At supercritical temperatures, the spectrum shifts slightly to the red as density decreases, and the Gaussian-Lorentzian form is a very poor description. Application of spectral moment theory allows one to make an estimate of the average size of the electron wave function and of its kinetic energy. It appears that for water densities below about 0.6 g/cc, and down to below 0.1 g/cc, the average radius of gyration for the electron remains constant at around 3.4 angstroms, and its absorption maximum is near 0.9 eV. For higher densities, the electron is squeezed into a smaller cavity and the spectrum is shifted to the blue. The enthalpy and free energy of electron hydration are derived as a function of temperature on the basis of existing equilibrium data and absolute proton hydration energies derived from the cluster-based common point method. In a discussion, we compare the effective "size" of the hydrated electron derived from both methods.

Reaction of hydrogen atoms with hydroxide ions in high-temperature and high-pressure water.

The rate constant for the reaction of hydrogen atoms (H(*)) with hydroxide ions (OH(-)) in aqueous solution has been measured from 100 to 300 degrees C by direct measurement of the hydrated electron ((e(-))(aq)) product growth rate. In combining these measurements with previous results, the reaction is observed to display Arrhenius behavior in two separate temperature regions, 3-100 and 100-330 degrees C, where the data above 100 degrees C show an obvious decrease in activation energy from 38.2 +/- 0.6 to 25.4 +/- 0.8 kJ mol(-1). The value of the rate constant is smaller than that estimated previously in the 200-300 degrees C range. The very unusual activation energy behavior of the forward and backward reactions is discussed in the context of transition state theory.