Pressure effects on electron reactions and mobility in nonpolar liquids

Energetics and Volume Changes in Electron Attachment to Pyrazine in Supercritical Xenon

The attachment of electrons to pyrazine occurs reversibly over a wide range of pressures at and above room temperature in supercritical xenon. The rate constant for attachment increases with pressure at low pressures, passes through a maximum, and levels off at values of 1-3x10(12) m(-1) s(-1) at high pressure. The activation volumes for attachment (DeltaVa*) are quite small but show maxima near the compressibility maxima. In contrast, DeltaVa* is always negative for this reaction in sc-ethane and exhibits minima near the compressibility maxima. The rate constants for electron detachment change little with pressure but increase with temperature. Activation volumes for detachment are small. To explain the small volume change observed for this reaction, it is proposed that at the higher pressures clustering around the neutral pyrazine is comparable to that around the ion; i.e., the partial molar volumes are comparable. The free energy change (DeltaGr) of this reaction decreases between 40 and 60 bar and then is fairly constant at higher pressures. The dependence of DeltaGr on pressure is consistent with clustering around the neutral pyrazine at higher pressure. Also, the electron affinity of the clusters, pyrazineXen, increases with n to a few tenths of an eV.

Toward Electron Encapsulation: Polynitrile Approach

This study seeks an answer to the following question: Is it possible to design a supramolecular cage that would "solvate" the excess electron in the same fashion in which several solvent molecules do that cooperatively in polar liquids? Two general strategies are outlined for this "electron encapsulation", viz. electron localization using polar groups arranged on the (i) inside of the cage or (ii) outside of the cage. The second approach is more convenient from the synthetic standpoint, but it is limited to polynitriles. We demonstrate, experimentally and theoretically, that this second approach faces a problem: the electron attaches to the nitrile groups, forming molecular anions with bent C-C-N fragments. Because the energy cost of this bending is high, for dinitrile anions in n-hexane, the binding energies for the electron are low and, for mononitriles, these binding energies are lower still, and the entropy of electron attachment is anomalously small. Density functional theory modeling of electron trapping by mononitriles in n-hexane suggests that the solute molecules substitute for the solvent molecules at the electron cavity, "solvating" the electron by their methyl groups. We argue that such species would be more correctly viewed as multimer radical anions in which the electron density is shared (mainly) between C 2p orbitals in the solute/solvent molecules, rather than cavity electrons. The way in which the excess electron density is shared by such molecules is similar to the way in which this sharing occurs in large di- and polynitrile anions, such as 1,2,4,5,7,8,10,11-octacyanocyclododecane(-). Only in this sense is the electron encapsulation possible. The work thus reveals limitations of the concept of "solvated electron" for organic liquids: it is impossible to draw a clear line between such species and a certain class of radical anions.

Photostimulated electron detrapping and the two-state model for electron transport in nonpolar liquids

In common nonpolar liquids, such as saturated hydrocarbons, there is a dynamic equilibrium between trapped (localized) and quasifree (extended) states of the excess electron (the two-state model). Using time-resolved dc conductivity, the effect of 1064 nm laser photoexcitation of trapped electrons on the charge transport has been observed in liquid n-hexane and methylcyclohexane. The light promotes the electron from the trap into the conduction band of the liquid. From the analysis of the two-pulse, two-color photoconductivity data, the residence time of the electrons in traps has been estimated as ca. 8.3 ps for n-hexane and ca. 13 ps for methylcyclohexane (at 295 K). The rate of detrapping decreases at lower temperature with an activation energy of ca. 200 meV (280-320 K); the lifetime-mobility product for quasifree electrons scales linearly with the temperature. We suggest that the properties of trapped electrons in hydrocarbon liquids can be well accounted for using the simple spherical cavity model. The estimated localization time of the quasifree electron is 20-50 fs; both time estimates are in agreement with the "quasiballistic" model. This localization time is significantly lower than the value of 310+/-100 fs obtained using time-domain terahertz (THz) spectroscopy for the same system [E. Knoesel, M. Bonn, J. Shan, F. Wang, and T. F. Heinz, J. Chem. Phys. 121, 394 (2004)]. We suggest that the THz signal originates from the oscillations of electron bubbles rather than the free-electron plasma; vibrations of these bubbles may be responsible for the deviations from the Drude behavior observed below 0.4 THz. Various implications of these results are discussed.

Properties and Reactions of Charged Species in Nonpolar Supercritical Fluids

This review focuses on the properties and reactions of charged species in supercritical fluids. The techniques of pulse conductivity and transient absorption are used to follow the behavior of charged species. We begin with a discussion of the mobilities, yields, and energy levels of electrons. Studies of the pressure dependence of electron attachment reactions lead to information on the magnitude of activation volumes. This as well as diffusion and energetics are factors that influence the rates of electron attachment. The free energy changes in electron attachment-detachment equilibrium reactions decrease rapidly at pressures where the compressibility maximizes. The transport properties of ions in supercritical fluids are also discussed, as these studies provide a straightforward indication of the degree of interaction between ions and the medium. We conclude with a review of electron transfer reactions in supercritical fluids.

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.