Solvation contributions

The substituent stabilization effects calculated for the methyl cation and the methyl anion refer to the gas phase, where no solvation effects are present, and therefore are substantially larger, in terms of eneigy, than would be the case in solution, where solvation contributes to stabilization and attenuates the substituent effects. 

High-level quantum mechanical calculations have been used to explore the Horner-Wandsworth-Emmons reaction in the gas phase and also with a solvation contribution evaluated using the PCM/DIR method. Ring closure of the P—O bond (TS2), to form oxaphosphetane, is rate determining in the absence of solvation however, the oxyanion becomes a true intermediate, at an energy minimum on the reaction path, only in response to the effects of solvation, whereupon its formation by carbonyl addition (TSl) becomes rate limiting. Formation of F-product is always... 

The standard partial molal volume of a generic ion (or electrolyte) in solution can be expressed, along the hnes of the preceding section, through the summation of nonsolvation and solvation contributions—i.e., omitting the subscripts J and/or K) ... 

The activation volume for peroxodisulfate oxidation of [Fe(CN)5(pentylpz)] , 0 2cm moF suggests, as do the close-to-zero values for analogous reactions of [Fe(CN)6j" and of [Fe(diimine)(CN)4], compensation between intrinsic and solvational contributions." ... 

Temperature and pressure effects on rate constants for [Fe(phen)3] +/[Fe(phen)3] + electron transfer in water and in acetonitrile have yielded activation parameters AF was discussed in relation to possible nonadiabaticity and solvation contributions. Solvation effects on AF° for [Fe(diimine)3] " " " " half-cells, related diimine/cyanide ternary systems (diimine = phen, bipy), and also [Fe(CN)6] and Fe aq/Fe aq, have been assessed. Initial state-transition state analyses for base hydrolysis and for peroxodisulfate oxidation for [Fe(diimine)3] +, [Fe(tsb)2] ", [Fe(cage)] " " in DMSO-water mixtures suggest that base hydrolysis is generally controlled by hydroxide (de)hydration, but that in peroxodisulfate oxidation solvation changes for both reactants are significant in determining the overall reactivity pattern. ... 

A consideration of these relationships reveals8 that because E° is a thermodynamic parameter and represents an energy difference between two oxidation states and in many cases the spectroscopic or other parameter refers to only one half of the couple, then some special conditions must exist in order for these relationships to work. The special conditions under which these relationships work are that (a) steric effects are either unimportant or approximately the same in both halves of the redox couple and (b) changes in E° and the spectroscopic or other parameters arise mainly through electronic effects. The existence of many examples of these relationships for series of closely related complexes is perhaps not too unexpected as it is likely that, for such a series, the solvational contribution to the enthalpy change, and the total entropy change, for the redox reaction will remain constant, thus giving rise to the above necessary conditions. 

The solvation contributions to the free energy functional of Eq. (4) cannot be distinguished on the scale of Fig. 5 for the lower two curves, which are respectively, the total free energy and the covalent structure diabatic curve. Instead in the region of the minimum of the upper curve, the solvent lowers the free energy by about 15 kcal/mol, but this effect is not sufficient to product a particularly significant variation in the total wavefunction. 

It should be noted that the entropy component of the polychelate effect is largely controlled by the same contributions as for LMWM, i.e., translation, rotation, symmetry, isomerism, oscillation, internal rotation and solvation contributions [41]. Besides both the chelate and polychelate effects depend greatly on the size of the chelate cycle. In this case the PCMU obey the known Chugaev rule by... 

The slope of a plot of K versus, is expected to be 31.9 X 10 cm pm for self-exchange reactions (for which a=b and = 2a). Equation(31) is expected to give the general pattern for solvational contributions to the reorganizational free energy, but it does not take account of entropy, specific solvational effects, the effect... 

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