Solvent-modified reaction coordinate

The model offers insights on several key points regarding the role of the solvent. But in the final analysis it cannot substitute for realistic dynamical simulations. In particular, the model overlooks any role played the internal modes of the solvent molecules or the internal modes of the solute. What the model provides is insight into the motion along the solvent-modified reaction coordinate. Specifically, the diffusion forward and backward across the barrier, that in the Kramers modef occurs along the reaction coordinate of the isolated solute, is shown in the model to be equivalent to a single crossing of the transition state provided that we use a collective reaction coordinate, one that involves both solute and solvent motion. 

Reactions in solution proceed in a similar manner, by elementary steps, to those in the gas phase. Many of the concepts, such as reaction coordinates and energy barriers, are the same. The two theories for elementary reactions have also been extended to liquid-phase reactions. The TST naturally extends to the liquid phase, since the transition state is treated as a thermodynamic entity. Features not present in gas-phase reactions, such as solvent effects and activity coefficients of ionic species in polar media, are treated as for stable species. Molecules in a liquid are in an almost constant state of collision so that the collision-based rate theories require modification to be used quantitatively. The energy distributions in the jostling motion in a liquid are similar to those in gas-phase collisions, but any reaction trajectory is modified by interaction with neighboring molecules. Furthermore, the frequency with which reaction partners approach each other is governed by diffusion rather than by random collisions, and, once together, multiple encounters between a reactant pair occur in this molecular traffic jam. This can modify the rate constants for individual reaction steps significantly. Thus, several aspects of reaction in a condensed phase differ from those in the gas phase ... 

As shown in Scheme 34, a rather profound solvent effect on dienolate alkylation diastereoselectivity has been noted for the steroidal enone (71). Such large solvent effects have not been documented for other systems. Possible explanations based upon the position of the transition state along the reaction coordinate and/or specific solvation of the dienolate have been advanced to account for preferential axial alkylation in benzene and equatorial alkylation in t-butyl alcohol.However, in view of the fact that the degree of aggregation of the dienolate as well as the structure of the aggregates may be modified considerably in going from one solvent to the other, rationalization of the results is difficult. 

The reactivity of diiodosamarium in solvents other than THF or in mixtures of solvents are discussed. The influence of additives able to coordinate to samarium are then considered (amides, amines or ethers). Proton donors sometimes drastically modify the selectivity of reactions induced by diiodosamarium some metal salts [such as Fe(III) or Ni(II)] in catalytic amounts may also strongly accelerate or modify reactions induced by diiodosamarium. Thanks to the above tuning of the diiodosamarium reactivity, rich and diversified organic transformations have been performed, some examples of which are presented. 

This analysis concerns for instance the formation of the twisted intramolecular charge transfer state, the photoisomerization processes, etc... in solution for which the intramolecular motion is related with the solvent motion. The unimolecular reaction -the passage from the reactant well to the product well in the Kramers treatment- is modeled by a sink term depending on the reaction coordinate. In the high-viscosity case the motion is governed by the modified Smoluchowski equation [6 ]... 

H. Electron transfer with strong coupling to the solvent. As in Problem F we need to choose the reaction coordinate to incorporate the role of the solvent [Warshel, 1991 I. Benjamin and E. Poliak, J. Chem. Phys. 105, 9093 (1996) Barzykin et al, 2002]. The purpose is to show that the solvent can modify the dynamics so that the diabatic picture used in Section 11.1.2, Figure 11.3 in particular, needs to be replaced by a diabatic picture [L. D. Zusman, Chem. Phys. 49, 250 (1980)]. 

The transition state in this picture is the point where both dipoles have the same length therefore the difference in bond length, denoted x, may be taken as the reaction coordinate, and the top of the barrier is located at x = 0. There is an intrinsic barrier frequency a>, but as we tried to make clear in Section 9.6, this frequency is modified by a potential of mean force due to the solvent fluctuations. 

The relative merits of two possible reaction schemes have been discussed (20). The schemes considered involve either an associative n-complex (Fig. 1) or a dissociative 7r-complex (Fig. 2). These figures are drawn essentially as given by Garnett (20) they have been modified to include solvent molecules to fill and vacate coordination sites as required, and to make all the catalytic cycles in this review as comparable as possible.1 It should be noted that these catalytic cycles for H—D exchange are symmetrical, the second half of the cycle being the reverse of the first half with deuterium instead of protium. 

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