Influence of solvation

The degree of solvation of the reactants and activated complex affect the rate of reaction. When the activated complex is solvated to a greater extent in comparison to reactants, the rate of reaction will be greater than that in a non solvating solvent. This is because the activity coefficient of the complex is smaller than it is in a solvent that does not solvate it. This lowers the potential energy of activated complex or causes a decrease in the activation energy of the reaction. 

On the other hand, if one or both the reactants are solvated, while the activated complex is not, the necessary energy of activation is increased and consequently the rate of reaction is diminished. In case the reactant and activated complex are both solvated, the overall effect on both the activation energy and rate of reaction may be small. 

---

A statistical mechanical fonnulation of implicit solvent representations provides a robust theoretical framework for understanding the influence of solvation biomolecular systems. A decomposition of the free energy in tenns of nonpolar and electrostatic contributions, AVF = AVF " + AVF ° , is central to many approximate treatments. An attractive and widely used treatment consists in representing the nonpolar contribution AVF " by a SASA surface tension term with Eq. (15) and the electrostatic contribution by using the... 

Ab initio molecular orbital calculations are being used to study the reactions of anionic nucleophiles with carbonyl compounds in the gas phase. A rich variety of energy surfaces is found as shown here for reactions of hydroxide ion with methyl formate and formaldehyde, chloride ion with formyl and acetyl chloride, and fluoride ion with formyl fluoride. Extension of these investigations to determine the influence of solvation on the energy profiles is also underway the statistical mechanics approach is outlined and illustrated by results from Monte Carlo simulations for the addition of hydroxide ion to formaldehyde in water. 

The importance of solvation on reaction surfaces is evident in striking medium dependence of reaction rates, particularly for polar reactions, and in variations of product distributions as for methyl formate discussed above and of relative reactivities (18,26). Thus, in order to obtain a molecular level understanding of the influence of solvation on the energetics and courses of reactions, we have carried out statistical mechanics simulations that have yielded free energy of activation profiles (30) for several organic reactions in solution (11.18.19.31. ... 

A. Elucidating the Influence of Solvation on the Site of Energy Absorption and Ensuing Ion-Molecule Reactions via Ultrafast Laser Pump-Probe Techniques... 

Thermal reaction techniques enable a quantification of the influence of solvation on reactivities.1,2,19 One particular reaction which is a good example of how solvation can affect the nature of a core ion reaction site comes from a study118 of the interaction of OH with C02. The gas-phase reaction between the individual species is quite exothermic and can only take place by a three-body association mechanism. The reaction proceeds very slowly in the liquid phase and has been calculated119 to have a barrier of about 13 kcal mol-1. In biological systems, the reaction rate is enhanced by about 4 orders of magnitude through the enzyme carbonic anhydrase. Recent studies carried out in our laboratory provide detailed... 

B. Influence of Solvation on the Composition of Protonated Clusters Mixed Cluster Systems... 

---