Scavenging of electrons prior to solvation in liquid alcohols

Abstract

A solvent effect similar to that reported earlier (J. Am. Chem. Soc. 98, 6825 (1976)) has been found for the rates of reaction of solvated electrons with benzene and toluene in a series of alcohols. The value of the rate constant for benzene at 299 K increases abruptly from 1.0 (106 M−l s−1) in methanol to 6.8 in ethanol, then increases only slightly to 7.0 in 1-propanol and 8.4 in 1-butanol. A further large increase occurs on going to a secondary alcohol solvent: 42 (106 M−1 s−1) in 2-propanol and 69 in 2-butanol. The activation energies are ∼ 30 kJ/mol in all cases, so the rate constants reflect different entropies of activation. The entropies are affected by the solvation energies of the species.The reactions of benzene and toluene with electrons prior to solvation show relatively little solvent effect, due to their higher energy state. In 2-butanol at 163 K, reactions of quasifree, localized (absorbing in the infrared), and equilibrium solvated electrons have been distinguished. Reactions of eqf− obey the relation (Aio/Ai)qf ≈ 1 + [P/(1 − P)]fqfNs, where Ai/Aio is the fraction of electrons that have not been scavenged up to time i, and Ni is the mol fraction of scavenger. See text for remaining symbols. Reactions of eloc− obey the relation (Ai/Aio)loc ≈ exp(−nflocNs). The total probability that electrons reach the equilibrium solvated state is Ai/Aio ≈ exp (−nflocNs)/{1 + [P/(l − P)]fqfNs}. Both diffusion and tunneling may contribute to scavenging of eloc−. The contribution of these processes, compared to the amount of scavenging eqf− increases with increasing scavenger concentration and decreasing temperature.The parameters fqfP/(l − P) and nfloc each vary approximately as T−4, which is equivalent to an Arrhenius temperature coefficient of about −8 kJ/mol. The negative temperature coefficients could imply either that the scavenging step has a negative temperature coefficient, or that the lowering of the electron energy state has a positive temperature coefficient.