TEMPERATURE-DEPENDENCE OF ELECTRON SCAVENGING IN ETHANOL AND ETHANOL-O-D

Inefficient scavenging of solvated electrons (es-) may provide understanding of details which are difficult to explore for nearly diffusion-controlled processes. Alcohols as solvents permit use of organic reagents, exhibit interesting dynamic solvation effects, and are particularly appropriate for dry-electron scavenging. Sixteen scavengers, with keaq- in the range 107-109 M-1 s-1, have been investigated in ethanol from 150 to 298 K. Values of the activation energy were higher for two efficient scavengers, ∼4.5 kcal/mol for CCl4 and HClO4, than ∼3.8 kcal/mol for inefficient scavengers such as toluene, with no evident pattern. Activation contributes little to differences in kes-. Many of the inefficient scavengers were moderately efficient as dry-electron (e-) scavengers at 298 K. For toluene 63% of dry electrons were scavenged at 298 K by C37 = 1.8 M, but at 123 K by only C37 = 0.43 M. Temperature-dependent ionization C6H5CH3- → C6H5CH3 + es- is indicated. At 298 K and large combined concentrations of toluene (kes- ≃ 107 M-1 s-1) and biphenyl (kes- ≃ 109 M-1 s-1), toluene converts e- to es-, measured as biphenylide ion. Localization of e- by solvent, which is less efficient in C2H5OD, competes with scavenging and C37 decreases with decreasing temperature to a constant limiting value at ∼150 or ∼125 K which is ∼25∼50% of the value at 298 K. Localization and scavenging are activationless at low temperature and the isotope effect disappears. It is proposed that e- scattering in the liquid maintains a large zero-point kinetic energy which is available for resonances, e.g., benzene, to account for activationless scavenging. Analogously, localization is attributed to C2H5OH-. © 1978 American Chemical Society.