Electron behavior in mixed solvents: optical spectra and reactivities in water/alkane diols

Abstract

Investigation of the effect of solvent structure on the optical absorption spectrum and reactivity of solvated electrons has been extended to diol/water mixed solvents, using 1,2-ethanediol (12ED) and 1,4-butanediol (14BD). The large effects that had been found in mono-ol/water mixed solvents did not occur in diol/water. Although addition of 3 mol% of a diol to water increased the optical absorption energies of e−s by 0.06 eV, similar to the shift caused by addition of 3 mol% of a mono-ol, the variation of the spectrum over the rest of the composition range was nearly ideal in diol/water, in contrast to the very non-ideal variation in mono-ol/water. Reaction rate constants kS at 298 K in the diol/water mixed solvents vary approximately as the inverse viscosity,η−1.0, in the diffusion-controlled limit. However, when reactions are two or three orders of magnitude slower than diffusion controlled, kS at 298 K is independent of η. Toluene reacts with e−s at a 10−3-fold smaller rate than does nitrobenzene; the difference is nearly completely due to a ~50 J/mol K lower entropy of activation of the former reaction.