Transient Solvated Electron, Hydroxyl, and Hydroperoxy Radicals in Pulse-Irradiated Crystalline Ice

Abstract

Neutral, crystalline ice samples were pulse irradiated in order to study phase and temperature effects on the properties of transient intermediates produced in radiolyzed water. These studies demonstrate that optical absorption bands with peaks near 670, at 280 ± 5, and at 230 ± 8 nm are detectable. The transient visible absorption is attributed to a solvated electron es−. Its spectrum, though unaffected by phase change, is influenced by temperature, dEmax / dT being −1.2 × 10−3 eV/deg. Ges− also depends on temperature, decreasing markedly from −5° to −40°C, but only slowly thereafter. The decay of es−, which is second order at −14°C (k = 1.5 × 1011 M−1·sec−1) and partly first order from −40° to −100°C [k(−60°C) = 1.1 × 104sec−1; ΔEact = 9 ± 2 kcal/mole], becomes slower with decreasing temperature. Over-all spectral, yield and kinetic considerations indicate that es− is structurally similar to eaq−, forms via pre-existing traps, and though immobile as a unit, decays both by reaction with H3O+ and by means of an equilibrial, mobile partner em−. These findings are viewed in terms of the polaron theory and other models for the solvated electron. The transient 280-nm absorption is assigned to a hydrogen-bonded hydroxyl radical OHt·. ESR data showing chemical and kinetic characteristics similar to the optical results confirm this assignment. Its molar extinction coefficient at −196°C is estimated as ε280 = 560 ± 50 M−1·cm−1 [using GOHt· (stable) = 0.8]. The over-all OHt· decay is complex. After prolonged irradiation, pseudo-first-order kinetics representing reaction with H2 and/or H2O2 is primarily observed. For low doses and at temperatures below −100°C, separate fast and slow decaying portions can be distinguished, the former attributable to H· reacting with OHt·, the latter to reaction involving only OHt·. Based on an empirical 32-order kinetic treatment [k(−59°C) ≤ 1 × 104 M−1 / 2·sec−1], ΔEact for the slow decay is determined to be 5.7 ± 0.7 kcal/mole. Qualitatively, this decay and the reaction with products are reconcilable with a mechanism involving OHt· in equilibrium with a mobile species OHm·. Second-order kinetic behavior observed at −14°C (k appears to be ∼108 M−1·sec−1) may also be consistent with this scheme. The full transient yield at −131°C is estimated to be 1.2. These findings imply that OH· is structurally different in both phases, but chemically similar. The relatively stable absorption at 230 nm is ascribed to HO2·. Spectral, chemical, and possibly, ESR evidence support this identification. Its yield is low, and it decays only very slowly at −14°C.