Solvent Coordination Effects

In our early studies we observed with R. Jones and others, some significant solvent effects on coordination or complex formation as they influenced the reactivities of some organometallic compounds. They had been known for some time, and appeared rather striking when examined in connection with Michler s ketone and the color test. From our study it appeared that the order of decrease in relative reactivity and increase in tendency to form coordinate compounds with ketones was PhLi, PhMgBr, PhjGa. 

The use of ethylenediamine to stabilize highly sensitive organogold compounds, and of TMEDA to augment the metalating activity of some RLi compounds has been mentioned elsewhere. 

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Solvent coordination effects were demonstrated by addition of HMPA. Basicity increased when HMPA was added to THF Grignard solutions. This was rationalized as a complex formation between Grignard reagent and HMPA, increasing the reduction potential. A 2 5 pK -unit increase was observed w hen 1.5 2 mol HMPA was added per mole of Grignard reagent. 

In homopolymers all tire constituents (monomers) are identical, and hence tire interactions between tire monomers and between tire monomers and tire solvent have the same functional fonn. To describe tire shapes of a homopolymer (in the limit of large molecular weight) it is sufficient to model tire chain as a sequence of connected beads. Such a model can be used to describe tire shapes tliat a chain can adopt in various solvent conditions. A measure of shape is tire dimension of tire chain as a function of the degree of polymerization, N. If N is large tlien tire precise chemical details do not affect tire way tire size scales witli N [10]. In such a description a homopolymer is characterized in tenns of a single parameter tliat essentially characterizes tire effective interaction between tire beads, which is obtained by integrating over tire solvent coordinates. 

Solvation Thermodynamics and the Treatment of Equilibrium and Nonequilibrium Solvation Effects by Models Based on Collective Solvent Coordinates... 

There are two major approaches to including nonequilibrium effects in reaction rate calculations. The first approach treats the inability of the solvent to maintain its equilibrium solvation as the system moves along the reaction coordinate as a frictional drag on the reacting solute system.97, 100 The second approach adds one or more collective solvent coordinate to the nuclear coordinates of the solute.101 107 When these solvent coordinates are... 

Nonequilibrium solvent effects can indeed by significant at the kcal level-maybe even at a greater level, but so far there is no evidence for that when the reaction coordinate involves protonic or heavier motions. Our goal in this section has been to emphasize just how powerful and general the equilibrium model is. In addition, in both the previous section and the present section, we have emphasized the use of models based on collective solvent coordinates for calculating both equilibrium and nonequilibrium solvation properties. 

Finally, for the PT problem, dynamical friction effects have been examined for a model for a phenol-amine acid-base reaction in methyl chloride solvent [12]. With the quantization of the proton and the O-N vibration, the problem can be reduced to a one-dimensional solvent coordinate problem, similar to the ET case. Again, GH theory is found to agree with the MD results to within the error bars of the computer simulation. 

One of the more important recent developments in organometallic aluminum chemistry has been the formation and isolation of low-coordinate compounds, and, in particular, cations. These were first prepared in reactions of various aluminum reagents with crown ethers to form the inclusion compounds known as liquid clathrates. 71,72 Most of the evidence supports the presence of ion pairs as the basis of the solvent inclusion effect. Indeed, the compound [AlMe2-18-crown-6]+[AlMe2Cl2] was isolated from one such system (the cation is shown in Figure 6(a)).73 This was the first time the Me2Al+ unit had been structurally characterized. 

The reaction potentials displayed in Scheme 2.6 are those appropriate for the symmetric transfer of a proton in a vacuum, AH+ A <-> A HA+. However, when the system is placed in a polar solvent, the effect of the polar solvent upon the stability of the reactant and product state must be taken into account. The reactant and the proton state will have different solvent structures (Scheme 2.7). The effect of having different solvent structures associated with the reactant and product state is to break the symmetry of the potential energy surface associated with the proton-transfer coordinate. 

The half-wave potentials of K+, Tl+ and Ca2+ in water are slightly more negative and thosefor Zn2+, Cd2+, Mn2+, Ni2+ and Co2+ considerably more negative than is expected according to the donicity rule. It has been shown in the previous sections that water is a rather unique solvent. The effect in question may be interpreted by the so-called Katzin-effect according to which water forms a royal core of coordinated water molecules which are hooked together by hydrogen bonds 70,71>122,1231. 

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