Optical absorption of solvated electrons in water and tetrahydrofuran/water mixtures

In the past Jou and Dorfman (J. Chem. Phys. 1973, 58, 4715) have determined the optical absorption spectra of solvated electrons (e(solv)(-)) in binary mixtures of tetrahydrofuran (THF) and water over the entire concentration range. From these experiments they concluded that the position of the absorption maximum and the width at half-height of the band are dominated by water. By additional experiments we show that at least for some mixtures with a mole fraction X(THF) less than or equal to 0.50, we obtain spectra at 298 K which agree almost completely with those of e(solv)(-) in pure water at elevated temperatures obtained by Jou and Freeman (J, Phys. Chem. 1979, 83, 2383). It follows that for this range of composition, excess electrons are obviously hydrated and the large amount of THF of up to X(THF) = 0.49 changes the water structure of the hydrated electron only slightly. The same effect can be produced by a temperature increase of T = 80 K in pure water. From the spectra we have determined equilibrium ground-state properties of the hydrated electron as a function of temperature and composition of the mixture. Results of quantum-statistical simulations on hydrated electrons show very poor agreement with the experimental results. Therefore, it seems to be presumptuous to take these simulations as a basis to explain the behavior of excess electrons in water on a femtosecond time scale.