Localizing solvents

In a microscopic equilibrium description the pressure-dependent local solvent shell structure enters tlirough... 

As is inversely proportional to solvent viscosity, in sufficiently viscous solvents the rate constant k becomes equal to k y. This concerns, for example, reactions such as isomerizations involving significant rotation around single or double bonds, or dissociations requiring separation of fragments, altiiough it may be difficult to experimentally distinguish between effects due to local solvent structure and solvent friction. 

The relation between the microscopic friction acting on a molecule during its motion in a solvent enviromnent and macroscopic bulk solvent viscosity is a key problem affecting the rates of many reactions in condensed phase. The sequence of steps leading from friction to diflfiision coefficient to viscosity is based on the general validity of the Stokes-Einstein relation and the concept of describing friction by hydrodynamic as opposed to microscopic models involving local solvent structure. In the hydrodynamic limit the effect of solvent friction on, for example, rotational relaxation times of a solute molecule is [ ]... 

The simple difhision model of the cage effect again can be improved by taking effects of the local solvent structure, i.e. hydrodynamic repulsion, into account in the same way as discussed above for bimolecular reactions. The consequence is that the potential of mean force tends to favour escape at larger distances > 1,5R) more than it enliances caging at small distances, leading to larger overall photodissociation quantum yields [H6, 117]. 

The local solvent structural information inherent in deviations from Parsons-Zobel plots suggests that this effect deserves further experimental investigation.126,283 284 The reported accuracy of recent capacitance data (5%) for dilute solutions,285 however, must be improved before unambiguous conclusions about deviations can be drawn. 

The modification by method 2 is more acceptable. Although several types of modifications have been reported, Abraham and Liszi [15] proposed one of the simplest and well-known modifications. Figure 2(b) shows the proposed one-layer model. In this model, an ion of radius r and charge ze is surrounded by a local solvent layer of thickness b — r) and dielectric constant ej, immersed in the bulk solvent of dielectric constant ),. The thickness (b — r) of the solvent layer is taken as the solvent radius, and its dielectric constant ej is supposed to become considerably lower than that of the bulk solvent owing to dielectric saturation. The electrostatic term of the ion solvation energy is then given by... 

On the assumption that = 2, the theoretical values of the ion solvation energy were shown to agree well with the experimental values for univalent cations and anions in various solvents (e.g., 1,1- and 1,2-dichloroethane, tetrahydrofuran, 1,2-dimethoxyethane, ammonia, acetone, acetonitrile, nitromethane, 1-propanol, ethanol, methanol, and water). Abraham et al. [16,17] proposed an extended model in which the local solvent layer was further divided into two layers of different dielectric constants. The nonlocal electrostatic theory [9,11,12] was also presented, in which the permittivity of a medium was assumed to change continuously with the electric field around an ion. Combined with the above-mentioned Uhlig formula, it was successfully employed to elucidate the ion transfer energy at the nitrobenzene-water and 1,2-dichloroethane-water interfaces. 

Figure 4.9 illustrates time-gated imaging of rotational correlation time. Briefly, excitation by linearly polarized radiation will excite fluorophores with dipole components parallel to the excitation polarization axis and so the fluorescence emission will be anisotropically polarized immediately after excitation, with more emission polarized parallel than perpendicular to the polarization axis (r0). Subsequently, however, collisions with solvent molecules will tend to randomize the fluorophore orientations and the emission anistropy will decrease with time (r(t)). The characteristic timescale over which the fluorescence anisotropy decreases can be described (in the simplest case of a spherical molecule) by an exponential decay with a time constant, 6, which is the rotational correlation time and is approximately proportional to the local solvent viscosity and to the size of the fluorophore. Provided that... 

Consequently, we will follow the example of Mills et al. (29) who recently presented the first measurements of local solvent concentration using the Rutherford back-scattering technique. They analyzed the case of 1,1,1-trichloroethane (TCE) diffusing into PMMA films in terms of a simpler model developed by Peterlin 130-311, in which the propagating solvent front is preceded by a Fickian precursor. The Peterlin model describes the front end of the steady state SCP as ... 

These local density augmentations can affect reactions in several ways. The higher local solvent density may change local values of density-dependent properties such as the dielectric constant, thereby influencing reaction rates. It is also... 

Trasatti ° assumed that the value of at ct = 0 is constant (-0.31 V) and independent of the nature of the solvent. Therefore, if the contact potential difference at cr = 0 is known, the values of Sx for a given metal can be calculated. It should be noted that the idea that the potential shift due to the interaction of metal electrons with solvent is independent of the nature of the solvent is open to criticism. For example, the local solvent field can interfere with electron distribution in the metal in the vicinity of the interface. The data obtained for a mercury electrode and different solvents show that the contact potential difference is mainly determined by the orientation of solvent dipoles at the interface. The positive values of gjUdip)o are due to orientation of the solvent dipoles with their negative ends directed toward the mercury surface. 

Nelsen et al. (2007) have revealed one more aspect of solvent control over charge localization. Solvents with marked electron-donor properties contribute to charge localization in cation-radicals, whereas anion-radicals experience the same changes in better electron-accepting solvents. Thus, naked (non-ion-paired) anion-radicals of 4,4 -dinitrostilbene and 4,4 -dinitrotolane show the spectra of delocalized species in HMPA and THF, but essentially spectra of localized species in DMF, DMSO, and MeCN. 

Hydrogen transfer does occur during oxidation of DMS, but it is not the determining factor of the activation barrier. Hydrogen transfer can occur via multiple different pathways depending on the local solvent configuration. 

Sfudies on fhe properfies/behavior of ILs may be complicated because the latter easily absorb small amounts of water (up to 3-5% w/w). The absorbed water affects the bulk solvent properties (i.e., viscosity and density) and may therefore alter the local solvent microenvironment surrounding a solute. Water can quench the luminescence and the resulting luminescence properties of ions can be strongly influenced by water. The dissolved organic solvent and water in ILs can be removed by keeping the IL under vacuum (10 -10 bar) at 50-60°C for at least 7-10 h [38]. 

Some information about the molecular structure and relaxation of the solvent cage has been deduced from neutron scattering experiments [521]. The local solvent density can be observed and the equation of... 

Class IA consists of simple inorganic anions and cations that are so weakly solvated that electrostatic attraction between ionic charge and an oppositely charged electrode surface pulls Reaction 2.72 to the right. Examples of class 1A ions are CIO4, NOj, H2PC>4, PFg, Cs+, and R4N+. The hydrophobic character of some class 1A anions is attributed to a tendency to disrupt the local solvent structure when they are dissolved in water. 

To date, there have been only a handful of time-resolved studies in dense fluid media (33,34,69-72). Of these, the bulk have focused on understanding a particular chemical reaction by adjusting the solvent environment (69-71). Only over the past two years have there been experiments directed toward studying the peculiar effects of supercritical fluids on these solvation processes (33,34,72). The initial work (33,34) showed that 1) time-resolved fluorescence can be used to improve our understanding of solvation in supercritical fluids and 2) the local solvent composition, about a solute molecule, could change significantly on a subnanosecond time scale. 

Most recently, Robinson and co-workers (72) used time-resolved decay of anisotropy experiments to probe the AOT reverse micelle system in ethane and propane. These authors conclude there was no local solvent density augmentation about the reverse micelle. In addition, the rotational dynamics of their probe (perylene tetracarboxylate) was independent of fluid density. This observation was consistent with results from Johnston and co-workers and Smith and co-workers (61-65,72). 

Previous experimental and theoretical studies have found what appears to be clear evidence for cluster formation, or local density enhancement, in near critical solutions (7t12t42-45). These include experimental optical absorption, fluorescence and partial molar volume measurements as well as theoretical simulation studies. These offer compelling evidence for local solvent density enhancement in near critical binary SCF systems. Theoretical models suggest that local density enhancement should be strongly dependent on the relative size and attractive force interactions strengths of the solute and solvent species as well as on bulk density and temperature (7,44). 

Kim and Johnston (27), and Yonker and Smith (22) have used solute solvatochroism to determine the composition of the local solvent environment in binary supercritical fluids. In our laboratory we investigate solute-cosolvent interactions by using a fluorescent solute molecule (a probe) whose emission characteristics are sensitive to its local solvent environment. In this way, it is possible to monitor changes in the local solvent composition using the probe fluorescence. Moreover, by using picosecond time-resolved techniques, one can determine the kinetics of fluid compositional fluctuation in the cybotactic region. 

Steady-State Fluorescence. The fluorescence characteristics of PRODAN are extremely sensitive to the physicochemical properties of the solvent (38). As benchmarks, the steady-state emission spectra for PRODAN in several liquid solvents are presented in Figure 1. It is evident that the PRODAN emission spectrum red shifts with increasing solvent polarity. This red shift is a result of the dielectric properties of the surrounding solvent and the large excited-state dipole moment (ca. 20 Debye units) of PRODAN (38). It is the sensitivity of the PRODAN fluorescence that will be used here to investigate the local solvent composition in binary supercritical fluids. 

The time-resolved emission spectra were reconstructed from the fluorescence decay kinetics at a series of emission wavelengths, and the steady-state emission spectrum as described in the Theory section (37). Figure 4 shows a typical set of time-resolved emission spectra for PRODAN in a binary supercritical fluid composed of CO2 and 1.57 mol% CH3OH (T = 45 °C P = 81.4 bar). Clearly, the emission spectrum red shifts following excitation indicating that the local solvent environment is becoming more polar during the excited-state lifetime. We attribute this red shift to the reorientation of cosolvent molecules about excited-state PRODAN. 

We have utilized the static and dynamic fluorescence characteristics of an environmentally-sensitive solute molecule, PRODAN, to investigate the local solvent composition in binary supercritical fluids. In the two solvent systems studied (C02/1.57 mol% CH3OH and C02/1.44 mol% CH3CN), specific cosolvent-solute interactions are clearly evident. Time-resolved fluorescence emission spectra indicate that the cosolvent-solute interactions become more pronounced with time after excitation. Hence, the local composition of cosolvent around the excited-state solute becomes greater than that surrounding the ground-state solute. That is, the photon-induced increase in excited-state dipole moment drives picosecond cosolvent augmentation about PRODAN. 

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