Temperature dependence of electron scavenging in ethanol and ethanol-O-d

Abstract

Inefficient scavenging of solvated electrons (e/sup -//sub s/) 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 k/sub e/sub aq/sup -/// in the range 10/sup 7/ to 10/sup 9/ M/sup -1/ s/sup -1/, have been investigated in ethanol from 150 to 298 K. Values of the activation energy were higher for two efficient scavengers, approximately 4.5 kcal/mol for CCl/sub 4/ and HClO/sub 4/, than approximately 3.8 kcal/mol for inefficient scavengers such as toluene, with no evident pattern. Activation contributes little to differences in k/sub e/sup -//sub s//. Many of the inefficient scavengers were moderately efficient as dry-electron (e/sup -/) scavengers at 298 K. For toluene 63% of dry electrons were scavenged at 298 K by C/sub 37/ = 1.8 M, but at 123 K by only C/sub 37/ = 0.43 M. Temperature-dependent ionization C/sub 6/H/sub 5/CH/sub 3//sup -/ ..-->.. C/sub 6/H/sub 5/CH/sub 3/ + e/sup -//sub s/ is indicated. At 298 K and large combined concentrations of toluene (k/sub e/sup -//sub s// approximately equal to 10/sup 7/ M/sup -1/ s/sup -1/) andmore » biphenyl (k/sub e/sup -//sub s// approximately equal to 10/sup 9/ M/sup -1/ s/sup -1/), toluene converts e/sup -/ to e/sup -//sub s/, measured as biphenylide ion. Localization of e/sup -/ by solvent, which is less efficient in C/sub 2/H/sub 5/OD, competes with scavenging and C/sub 37/ decreases with decreasing temperature to a constant limiting value at approximately 150 or approximately 125 K which is approximately 25 to 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/sup -/ 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 C/sub 2/H/sub 5/OH/sup -/. 1 table; 9 figures.« less