Solvent Effects on Chemical Reactions

Before considering the analysis of solvent effects on chemical reactions, some of the shortcomings of potential energy hypersurfaces as a tool to describe chemical process are... 

Tapia, O. and Lluch, J. M. Solvent effects on chemical reaction profiles. Monte Carlo simulation of hydration effects on quantum chemically calculated stationary structures, J. Chem.Phys., 83 (1983, 3970-3982... 

The scope of this book goes beyond the proper field of solvent effects on chemical reactions. It actually goes deeper in the analysis of solvent effects as such and of chemical reactions. It also addresses the problem of mimicking chemical reactions in condensed phases and bioenvironments. The authors have gone through the problems raised by the limitations found in the theoretical representations. In order to understand, it is not sufficient to have agreement with experiments, the schemes should meet the requirements put forward by well founded physical theories. 

Contemporary computer-assisted molecular simulation methods and modern computer technology has contributed to the actual numerical calculation of solvent effects on chemical reactions and molecular equilibria. Classical statistical mechanics and quantum mechanics are basic pillars on which practical approaches are based. On top of these, numerical methods borrowed from different fields of physics and engineering and computer graphics techniques have been integrated into computer programs running in graphics workstations and modem supercomputers (Zhao et al., 2000). 

The classification of solvents has been dealt with in various books on non-aque-ous solvents [25, 26]. In the classification of solvents, it is usual to use some solvent properties as criteria. In order to discuss solvent effects on chemical reactions, it is convenient to use relative permittivities and acid-base properties as the criteria. 

The various factors that contribute to ion solvation were discussed in Section 2.2.1. In this section, we deal with the solvent effects on chemical reactions more quantitatively [5, 22]. To do this, we introduce two quantities, the Gibbs energy of transfer and the transfer activity coefficient. 

We can use the transfer activity coefficients to predict solvent effects on chemical reactions and equilibria [22]. Some examples are shown below. 

In the last few years, the polarizable continuum model for the study of solvation has been extended to consider multideterminantal wavefunctions. Such novel techniques allow the study of the most important solvent effects on chemical reactions. In this context, the valence bond theory provides a way to analyze such effects through the transcription of the, generally, complicated multiconfigurational wavefunctions into sums of few selected classical structures, which are, in fact, more useful to understand the electron distribution rearrangement along a reaction path. In this chapter, the valence bond analysis of CASSCF wavefunctions calculated for chemical reactions in solution is discussed in details. By way of example, the results for some basic chemical processes are also reported. 

It is assumed that all the explanatory variables are independent of each other and truly additive as well as relevant to the problem under study [144], MRA has been widely used to establish linear Gibbs energy (LGE) relationships [144, 149, 150], The Hammett equation is an example of the simplest form of MRA, namely bivariate statistical analysis. For applications of MRA to solvent effects on chemical reactions, see Chapter 7.7. 

Solvent effects on chemical reactions are presently being studied intensively by molecular computer simulation methods. Outer sphere ET (e. g. Ref. [217-228]), ion transfer [229-232], proton transfer [233], and bondbreaking [234, 235] are among the reactions studied near electrodes and liquid-Kquid interfaces. Much of this work has been reviewed recently [170, 236, 237] and will therefore not be discussed here. All studies involve the calculation of a free energy profile as a function of a spatial or a collective solvent coordinate. 

In Sect. 4, an overview of solvent effects evaluation with the techniques described in the preceding sections is presented. MC and MD studies on molecular properties are overviewed. Of particular interest are recent developments in the simulation of solvent effects on chemical reactions. The focus is in computer simulations. Analytical models for describing chemical reactions have been thoroughly discussed by Hynes [11] and will not be examined here. 

Progress is forseen in the study of solvent effects on chemical reactions in liquids, solids, miscelles, and enzymes. Brute force MD simulations of solvent effects on the dynamics properties of protein summarized in this work will serve as benchmark calculations to gauge model representations of solvent effects on biomacro-molecules. The use of realistic dielectric models can be also be seen as a complementary approach to represent solvent effects on biomolecules. In chemical dynamics, important advances have been made with analytical simple model approaches [11, 104, 105]. The conditions are now ripe for including more sophisticated ab initio studies into the description of time-dependent phenomena. 

Gao et al. worked out QM/MM-based methods to consider solvent effects on chemical reactions. The authors combine semiempirical and ab initio calculations applied to the QM region with the so-called polarizable MM approach for the MM region. The latter includes mutual solute-solvent polarization interaction which the authors show to be significant. [Note that strong polarization effects means that the second order perturbation QM-MM interaction is large which, in turn, indicates that the this interaction may be not small, see Section II]. 

NMR spectroscopy is routinely used in today s organic synthesis laboratories to identify and structurally characterize reaction products. Yet despite the enormous structural information content of NMR and the availability of a variety of high-pressure NMR techniques (3-13), it is little used for studying in situ chemical reactions and solvent effects on chemical reactions in supercritical media. 

Treatment of the solvent effect on chemical reaction rates by means of the phenomenological theory is greatly facilitated by the transition state theory, which postulates that the ini-... 

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