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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-).

No abstract

SF(6) has been used as a specific scavenger to investigate the beta/gamma radiolysis yield of hydrated electrons in pressurized high temperature sub- and supercritical water. SF(6) is thermally stable in supercritical water, and each scavenging reaction is known to produce six fluoride ions, which can be readily measured using a fluoride-selective electrode. Problems in the application of this method are described, including buildup of acid product and chain reduction of SF(6) in the presence of organic (*)H and (*)OH radical scavengers. Ultimately we find that the combination of SF(6) and phenol in neutral solution gives reliable results in supercritical water, because the protons and fluoride ions remain associated as HF molecules. The beta/gamma yields in supercritical water are similar to previous measurements using N(2)O scavenger.

Cross-check ExperimentsThe HD yield at room temperature (Table 1) is very low compared to earlier measurements of H • atom yield, and we were not certain that a 0.02 m concentration of ethanol-d6 gives enough scavenging power (5.4×10 4 s -1 at room temperature) to compete efficiently for H • atoms with other second order reactions or impurities. As a test we performed a series of experiments with 0.01 m (normal) ethanol aqueous solutions with 2.5×10 -3 m N 2 O. Figure S1 shows the radiation yields of H 2 in 0.01 m ethanol compared to the HD+H 2 yield in 0.02 m ethanol-d6. Both yields compare well from 100 o C up to 300 o C, justifying the small concentration of ethanol-d6 used for the H • atom yield determination.