Dry excess electrons in room-temperature ionic liquids

In this article, we describe general trends to be expected at short times when an excess electron is generated or injected in different room-temperature ionic liquids (RTILs). Perhaps surprisingly, the excess electron does not localize systematically on the positively charged cations. Rather, the excess charge localization pattern is determined by the cation and anion HOMO/LUMO gaps and, more importantly, by their relative LUMO alignments. As revealed by experiments, the short-time (ps/ns) transient UV spectrum of excess electrons in RTILs is often characterized by two bands, a broad band at low energies (above 1000 nm) and another weaker band at higher energies (around 400 nm). Our calculations show that the dry or presolvated electron spectrum (fs) also has two similar features. The broad band at low energies is due to transitions between electronic states with similar character on ions of the same class but in different locations of the liquid. The lower-intensity band at higher energies is due to transitions in which the electron is promoted to electronic states of different character, in some cases on counterions. Depending on the chemical nature of the RTIL, and especially on the anions, excess electrons can localize on cations or anions. Our findings hint at possible design strategies for controlling electron localization, where electron transfer or transport across species can be facilitated or blocked depending on the alignment of the electronic levels of the individual species.     

Discrete models for describing photophysical and thermodynamic properties of polar liquids

A number of properties of liquids with polar molecules are considered using water and ammonia as examples in the discrete approach, i.e., placing the centers of molecules on nodes of a particular lattice while assuming different orientations of the molecules. Different experimental properties are explained by different assumptions concerning the average values of pair orientations of adjacent molecules. Rather than using a uniform thermodynamic distribution over orientations to study fast processes related to the formation and migration of solvated electrons, a quasi-chemical approximation of the lattice gas model is used to calculate the thermodynamic properties of a liquid.     

A method for the generation of solvated electrons in polar solvents

A simple method for generating solvated electrons in a polar liquid by a rapid discharge of a capacitor bank is suggested. New theoretical concepts of excess electrons in polar liquids are used.     

Aspects on the photophysical processes in riboflavin

Two aspects on the photophysics of riboflavin have been analyzed:first the donnor-acceptor complexes formation in order to decide if the complexes formation is from the ground or from the first excited state of Riboflavin; second the variation of lifetime fluorescence of Riboflavin with the pH of medium have been studied. Single photocounting system and deconvolution procedure were used.     

Does Marcus theory apply to redox processes in ionic liquids? A simulation study

Simulation studies on a model system of a spherical ion with various charges in two imidazolium ionic liquids and in acetonitrile are compared. The average vertical ionisation potentials as a function of the charge on the ion are similar for all three systems. The Landau free energies of each system as a function of the vertical ionisation potential are computed and are close to being parabolic. Results are shown for the solvent reorganisation energies and for the activation free energies. The similarities of all these quantities are interpreted in terms of continuum models. However, the dynamics are likely to be very different in a polar liquid and in an ionic liquid as in the former case screening occurs by reorientation of molecules and in the latter case it occurs by translation of ions.     

The spectroscopy of OClO in polar liquids

The near-UV A²A₂←X²B₁, transition of OClO is examined in polar solutions. Both the λ_max of the ²A₂ electronic band and the inhomogeneous vibronic widths vary significantly with solvent. The dependence of the absorption maximum on dielectric properties of the solvent can be qualitatively described by continuum solvation models. Changes in the inhomogeneous widths are discussed in terms of inhomogeneity resulting from different solvation geometries. Picosecond time resolved absorption studies following the UV photolysis of OClO at 355 nm indicate a solvent dependence on the excited state reactivity. In many solvents. there is competition between bond breakage (to form ClO and O) and isomerization (to form ClOO). The possible involvement of this OClO heterogeneous photochemistry in stratospheric ozone depletion is described.     

On the molecular theory of relaxation processes and the viscoelastic properties of magnetic liquids

The kinetic equations for one-and two-particle distribution functions including the contributions of spatial correlation of density and correlation of velocities were used to obtain equations of generalized hydrodynamics of magnetic liquids, whose transfer coefficients microscopically depended on the spatial and time scales. (The relaxing flows in these equations comprise kinetic and potential parts, which ensures the inclusion of translational and structural relaxation processes.) The system of generalized hydrodynamics equations obtained can be used to study transfer phenomena in magnetic liquids. Smoluchowski equations for binary density n ₂(q ₁, q ₂, t) and binary flow of particles I ₂(q ₁, q ₂, t) were obtained and their general solutions found to construct a closure of the initial kinetic equation for the one-particle distribution function. The asymptotic behavior of these solutions at low frequencies (ω → 0) was analyzed and found to coincide with the long-time asymptotes of autocorrelation functions. The viscoelastic properties of magnetic liquids were studied over a wide range of frequencies in the presence of an external magnetic field H. Microscopic equations for viscosity coefficients and elastic moduli were found and their asymptotic behavior in slow and fast processes considered.     

Relative permittivity of polar liquids. Comparison of theory, experiment, and simulation

A molecular-based second-order perturbation theory is applied to calculate the relative permittivity of polar liquids. Our basic model is the dipolar hard sphere fluid. The main purpose of this work is to propose various approaches to take into account the molecular polarizability. In the continuum approach, we apply the Kirkwood-Fr?hlich equation and use the high-frequency relative permittivity. The Kirkwood g-factor representing molecular correlations is calculated by a perturbation theory. In the molecular approach, the molecular polarizability is built into the model on the molecular level (the polarizable dipolar hard sphere fluid). To calculate the relative permittivity of this system, an equation obtained from a renormalization procedure is used. In both approaches, we apply a series expansion for the relative permittivity and show that these series expansions give results in better agreement with simulation data than the original equations. After testing our theoretical equations against our own Monte Carlo simulation results, we compare the results obtained from our theoretical equations and simulations to experimental data for amines, ethers, and halogenated, sulfur, and hydroxy compounds. We propose a procedure to calculate potential parameters (hard sphere diameter, reduced polarizability, and reduced dipole moment) from experimental data such as the permanent dipole moment, refractive index, density, and temperature. We show that for compounds of low relative permittivity the polarizable dipolar hard sphere (PDHS) model and the continuum approach give reasonable results. For nonassociative liquids of higher relative permittivity, the PDHS model overestimates experimental data due to unsatisfactory representation of the shape of the molecules. In the case of associative liquids, the PDHS model works well, and in some cases it underestimates the experimental values due to the unsatisfactory treatment of electrostatic interactions.     

Photoelectron spectroscopy of sodium chloride anions with two excess electrons

Photoelectron spectroscopy measurements of (NaCl)_nNa^? (n=1–13, except n=10) are reported. The observed electron energy spectra fall into three distinct types, reflecting different correlations between the two excess electrons and ions. Depending on the host clusters' structures, the experimental evidence indicates that the two excess electrons in these sodium chloride clusters could be spin paired, forming a bipolaron or a Na^? anion. The two excess electrons could also be spin parallel, forming a double F-center state.     

The absorption coefficient of a polar liquid with solvated electrons

An equation for the absorption coefficient of a polar liquid with excess (solvated) electrons is derived. It is taken into account that (1) each excess electron can for a certain time reside on only one liquid molecule (during this time, the molecule is in the anion-resonance state), and (2) a polar liquid is electrostatically nonuniform because it has different local potentials, which can be calculated for each molecule. The probabilities of quantum movements of excess electrons in a liquid from one molecule to another caused by the absorption of photons are considered.     

Molecular mobility and the structure of polar liquids

Self-diffusion coefficients of different classes of polar solvents are measured by the proton spin echo method in the temperature range of 288–318 K. In the same temperature range viscosities and dielectric relaxation times of the liquids under study are measured or taken from the literature and the activation energies of self-diffusion processes, viscous flow, and dielectric relaxation are calculated. The results obtained are compared with the literature data on structural relaxation times in the studied solvents and the conclusion is drawn about the role of the spatial hydrogen bond network in the mobility of molecules forming this network.     

Mobilities of thermal electrons in gases and liquids

Mobilities, μ_G, of thermal electrons in a number of gases are reported, compared with those, μ_L, in the corresponding liquids, and discusse     

A well-tempered density functional theory of electrons in molecules

This Invited Article reports extensions of a recently developed approach to density functional theory with correct long-range behavior (R. Baer and D. Neuhauser, Phys. Rev. Lett., 2005, 94, 043002). The central quantities are a splitting functional gamma[n] and a complementary exchange-correlation functional E[n]. We give a practical method for determining the value of gamma in molecules, assuming an approximation for E is given. The resulting theory shows good ability to reproduce the ionization potentials for various molecules. However it is not of sufficient accuracy for forming a satisfactory framework for studying molecular properties. A somewhat different approach is then adopted, which depends on a density-independent gamma and an additional parameter w eliminating part of the local exchange functional. The values of these two parameters are obtained by best-fitting to experimental atomization energies and bond lengths of the molecules in the G2(1) database. The optimized values are gamma = 0.5 a and w = 0.1. We then examine the performance of this slightly semi-empirical functional for a variety of molecular properties, comparing to related works and experiment. We show that this approach can be used for describing in a satisfactory manner a broad range of molecular properties, be they static or dynamic. Most satisfactory is the ability to describe valence, Rydberg and inter-molecular charge-transfer excitations.     

A theory of metastable liquids and gases

A model of the metastable state of matter is suggested. According to this model, excess potential energy stored at the initial time is gradually transferred to macroscopic fluctuations. As a result, the potential energy of a system continuously decreases, whereas the amplitude of fluctuations increases. This eventually causes spontaneous transition from the metastable to thermodynamically equilibrium state. The suggested model was statistically grounded, and a system of equations describing the homogeneous formation of a new phase was obtained.