Solvation clusters

Hydrogen-bonded clusters are an important class of molecular clusters, among which small water clusters have received a considerable amount of attention [148, 149]. Solvated cluster ions have also been produced and studied [150, 151]. These solvated clusters provide ideal model systems to obtain microscopic infonnation about solvation effect and its influence on chemical reactions. 

K. J. Klabunde, Method of Coating Substrates with Solvated Clusters of Metal Particles, U.S. Patent 4,877,647 (1989). 

Fig. 6 compares the nuclearity effect on the redox potentials [19,31,63] of hydrated Ag+ clusters E°(Ag /Ag )aq together with the effect on ionization potentials IPg (Ag ) of bare silver clusters in the gas phase [67,68]. The asymptotic value of the redox potential is reached at the nuclearity around n = 500 (diameter == 2 nm), which thus represents, for the system, the transition between the mesoscopic and the macroscopic phase of the bulk metal. The density of values available so far is not sufficient to prove the existence of odd-even oscillations as for IPg. However, it is obvious from this figure that the variation of E° and IPg do exhibit opposite trends vs. n, for the solution (Table 5) and the gas phase, respectively. The difference between ionization potentials of bare and solvated clusters decreases with increasing n as which corresponds fairly well to the solvation free energy of the cation deduced from the Born solvation model [45] (for the single atom, the difference of 5 eV represents the solvation energy of the silver cation) [31]. 

The sharing of imperfect cluster faces of the clathrate-like clusters can be viewed as a thermodynamic tendency to minimize the negative entropies of solution. The tendency for face- or edge-sharing of individual solvation clusters, as Stillinger (1980) pointed out, is the same as the tendency for clustering of pure supercooled water. 

An inversion temperature was found for the ring opening of meso-epoxides by amines in dichloromethane.23 These reactions, which were catalysed by samarium iodobinaphthoxide, gave amino alcohols in high enantiomeric excess. The inversion temperature was thought arise because the reaction can occur with two different solvation clusters. 

To conclude this first part of the analysis, all the results presented show that H-bond effects and long-range non-specific interactions can combine and give rise to a synergic (or cooperative) action and that the complete picture can only be obtained by taking into account both of them introducing solvated clusters. 

Kenney-Wallace GA, Jonah CD. (1982) Picosecond spectroscopy and solvation clusters. The dynamics of localizing electrons in polar fluids. J Phys Chem 86 2572-2586. 

By contrast at low TPAOH content the lower availability of OH favors large and ramified oligomers, surrounding the TPA solvated clusters. The dimension of TPA clusters seems independent on the TPAOH amount, indeed the pore diameter decreases when the amount of TPAOH increases. 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Studies on eleetron donor-acceptor (EDA) molecules in condensed phases have a long history. Since the EDA interaction was first proposed by Mulliken [64], an appreciable number of reviews on this subject have been published during the latter half of the 20th eentury [65]. Similar studies under isolated jet-cooled conditions started in the 1980s and have supplied detailed information on the static and dynamic characters of excited electronic states responsible for the electron transfer [66]. The present section does not intend to cover all the details of the EDA studies in jet-cooled eonditions but rather focuses on seleeted EDA moleeules consisting of anthracene derivatives. Although limited in scope, the subjeet provides the most essential aspects of the EDA phenomena observed in an isolated system or in solvated clusters. 

The effect of microscopic solvation on the electronically excited states of AA was then studied by using solvated clusters of AA produced in a free jet [85]. The magnitude of the red shift and the broadness in the dispersed fluorescence spectra depended on the solvent polarity and the clustering number. Both the low-lying partial CT state and the ordinary full CT state are involved as the fluorescing state, and their relative contributions depend on the polarity and the number of solvent molecules. 

It came out immediately clear that the supermolecule approach cannot represent the method to be used in extensive studies of solvent effects. The computational costs increase in the ab initio versions with more than the fourth power of the number of basis set functions, at a given nuclear geometry of the supermolecule. Even more important it has been the recognition that, when the size of the solvation cluster exceeds some very low limits, the number of different nuclear conformations at an equivalent energy increases exponentially computational costs increase in parallel, and the introduction of thermal averages on these conformations becomes necessary. These facts, and some attempts to overcome them, are well summarized in a dementi s monograph (Clementi, 1976). The problem of multiple equivalent minima still plagues some discrete solvation models. 

Monte Carlo calculations for structure 1 and 2, in order to obtain the two free energy curves. MM interaction potentials of the type shown in eq.(119) are used here. The solute parameters (atomic charges and van der Waals coefficients) are drawn from SCF calculation on the trimer 1 and 2, as well as on other small solvation clusters ... 

The effects of association in the ion-dipole model are more rich than in the RPM. In this model in addition to the association between ions it is also possible to consider the clusterization between solvent molecules as well as between ions and solvent molecules the later can describe the formation of chains or network due to the bonds between solvent molecules, and the formation of specific solvation clusters around the ions. For the sake of simplicity, we focus here only on the case of ion pairs that is characterized by the MAL (2) or in the form of Eq. (5) for the degree of dissociation, a. 

Experimental and theoretical results on microsolvation of benzene were summarized by Brutschy [105]. Infrared (IR) spectral investigations of benzene-(H20)g have clearly revealed the changes in the structural features of water clusters in the presence of benzene. Zwier has reported the detailed analysis on the IR spectra of large size solvated clusters [106]. 

In a recent publication Okamura et ah (12) describe similar results in a different system. It is believed that the unusual rate increase observed in these various systems which are chemically so different is caused by the physical state of the reaction medium at temperatures a few degrees above Tg. The high viscosity of this gel-like medium presumably favors chain propagation in its competition with termination. This effect, which is kinetically similar to the "gel-effect in free radical polymerizations, can only arise if the termination step (charge recombination) becomes diffusion controlled. The latter process would arise if both ionic species involved in the reaction were of macromolecular size. This is undoubtedly true for the growing chain, but the mobility of the counter ion should only be significantly reduced in such a medium if it is of a polymolecular structure, involving perhaps a voluminous solvation cluster. 

The difference between ionization potentials of bare and solvated clusters decreases with increasing n and corresponds fairly well to the solvation free energy deduced from the Bom model (for the single atom, the difference of 5 eV represents the solvation energy of the silver cation). ... 

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