Effect of solvent structure on electron reactivity: 2-propanol/water mixtures

Abstract

The reactivity of solvated electrons [Formula: see text] with efficient (nitrobenzene, acetone) and inefficient (phenol, toluene) scavengers is affected greatly by the solvent composition in 2-propanol/water mixed solvents. 2-Propanol is the only secondary alcohol that is completely miscible with water. The variation of the nitrobenzene rate constant k2 with solvent composition displays four viscosity zones, as in primary and tertiary alcohol/water mixtures. In zone (c), where the Stokes–Smoluchowski equation applies, the nitrobenzene k2 values in the secondary alcohol/water mixtures are situated between those in the primary and tertiary alcohols, due to the relative values of the dielectric permittivity ε. The charge–dipole attraction energy varies as ε−1.The two water-rich zones (c) and (d) are characterized by a large change of viscosity η and a small change in [Formula: see text] solvation energy (trap depth) Er; here k2 for all the scavengers correlates with the inverse of the viscosity. In the two alcohol-rich zones (a) and (b) the change of η is small but that of Er is large; here k2 of inefficient scavengers correlates with the inverse of Er, due to the difficulty of electron transfer out of deeper traps. Activation energies E2 and entropies [Formula: see text] also show composition zone behaviour. The value of [Formula: see text] is more negative for less efficient scavengers; E2 varies less and does not correlate with reactivity or Er. Electron transfer from solvent to inefficient scavenger is driven by solvent rearrangement around the reaction center, reflected in [Formula: see text].