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) ≤ 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.