Solvent stabilized species

Lightly solvent stabilized species [(CsHsjColsolv)] have been postulated C. E. Barnes, J. A. Orvis, Organometallics 1993, 12, 1016. 

The synthetic procedures applied utilize the photochemical generation of the solvent stabilized 16e -species [(ij5 -H3CC5H4)Mn(CO)2 x THF] 6 by photolysis of (7j5-H3CC5H4)Mn(CO)3 5. [(t/5-... 

To test the validity of this mechanism, chromium carbonyl (1.0 g) was photolyzed under Ar at ambient temperature in a solution of methanol and hexamethylphosphoramide in the apparatus shown in Figure 5. The lamp was turned off periodically to check for the disappearance of slightly soluble Cr(C0) . Several photolyzing cycles were necessary to effect nearly complete conversion to the solvent-stabilized coordinately unsaturated species (equivalent to in Figure 4),... 

Polar solvents stabilize and lower the enthalpies of charged reactants and charged transition states. The more diffuse the charge on the species, the less effective the stabilization by the polar solvent. 

Stable anchoring of an oxidation catalyst can be promoted by reaction conditions, including the choice of solvents, catalytic species, and bases. For example, Fe- or Cu-exchanged materials have improved stability if they are used under alkaline conditions (138,181). On the other hand, the presence of strong or complexating acids must be avoided since they induce metal leaching (162). 

It is well known that ACN reacts with active metals (Li, Ca) to form polymers [48], These polymers are products of condensation reactions in which ACIST radical anions are formed by the electron transfer from the active metal and attack, nucleophilically, more solvent molecules. Species such as CH3C=N(CH3)C=N are probably intermediates in this polymerization. ACN does not react on noble metal electrodes in the same way as with active metals. For instance, well-re-solved Li UPD peaks characterize the voltammograms of noble metal electrodes in ACN/Li salt solutions. This reflects a stability of the Li ad-layers that are formed at potentials above Li deposition potentials. Hence, the cathodic limit of noble metal electrodes in ACN solutions is the cation reduction process (either TAA or active metal cations). As discussed in the previous sections, with TAA-based solutions it is possible that the electrode surfaces remain bare. When the cations are metallic (e.g., Li+), it is expected that the electrode surfaces become covered with surface films originating from atmospheric contaminants reduction if the electrodes are polarized below 1.5 V (Li/Li+). As Mann found [13], in the presence of Na salts the polarization of metal electrodes in ACN solutions to sodium deposition potentials leads to solvent decomposition, with evolution of H2, CH4 and sodium cyanide (due to reaction with metallic sodium). 

V-Methylimidazole is a powerful donor solvent that can stabilize species which are difficult to isolate in other solvents. In 1994, Evans and coworkers reported the synthesis and crystal... 

Interactions between a polar solvent and a charged species are stabilizing. Protic solvents can stabilize both anionic and cationic species, whereas polar aprotic solvents stabilize only cationic species. Thus, protic solvents favor reactions in which charge separation occurs in the transition state, the high-energy point in the reaction pathway. In nucleophilic substitution reactions, the pathway where two charged species are formed (i.e., S l reaction) is favored in protic solvents, whereas the pathway with a less polar transition state (i.e., 5 2 reaction) is favored in nonprotic solvents. 

These effects arise from the different degrees of solvent stabilization of the ground and excited state molecules. The ground state molecules of this particular compound have a much greater dipole moment than the excited state species and are thus stabilized to a greater extent in the more polar media (37,38). More detailed discussions of solvent effects on CT spectra may be found elsewhere (35,36,39). 

Chapters 10 and 11 describe the special properties of liquid water. Because of its substantial dipole moment, water is especially effective as a solvent, stabilizing both polar and ionic solutes. Water is not only the solvent, but also participates in acid-base reactions as a reactant. Water plays an integral role in virtually all biochemical reactions essential to the survival of living organisms these reactions involve acids, bases, and ionic species. In view of the wide-ranging importance of these reactions, we devote the remainder of this chapter to acid-base behavior and related ionic reactions in aqueous solution. The Bronsted-Lowry definition of acids and bases is especially well suited to describe these reactions. 

Solvent polarity can restrict the possibilities for reaction paths. Whenever charged species are present in a reaction, the reaction barrier is highly dependent on the polarity of the solvent (Fig. 2.27). Solvent stabilization of the reactants more than the products decreases reactivity. Solvent stabilization of the products more than the reactants increases reactivity. In addition to stabilizing ions, polar protic solvents can also allow proton transfer and equilibration between the various ionic species in solution. 

We have attempted to account for these observations by proposing the following simplified reaction path (Figure 6). Protonation of the epoxide is followed by the rearrangement of the protonated species to a covalent intermediate or a solvent stabilized ion pair. Homopolymerization then proceeds through the ion-pair intermediate, but the covalent Intermediate leads to catalyst transformation and, in most cases, loss of catalytic activity. 

The highly polar nature of TFE and HFIP enables dissolution of ionic species and ion pairs. Moreover, the low or nonnucleophilic nature as well as low-lying HOMO of the solvents stabilizes cation and radical cation intermediates in the solvents. 

Extraction properties, including efficiency, selectivity, water coextraction, and temperature dependence, are strongly affected by the properties of the extracting medium. These depend on the amine concentration and on the diluent used. In a nonpolar solvent, charged species such as ion pairs tend to stabilize by aggregation. Polar and particularly protic solvents solubilize ion pairs, reduce their aggregation, and substantially enhance their extraction efficiency. The effects of diluents are in many cases so strong that they are frequently refered to as extractant modifiers or enhancers. 

One particular advantage of chemically anchored as opposed to physically adsorbed species such as Rh complexes on 7-AI2O3 is that no migration of Rh occurs over the surface of the support. At the same time the absence of solvent stabilization in gas-phase reactions gives rise to a greater potential for deactivation of catalyst due to thermal degradation or poisoning. 

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