Solvation effects continuum

Mineva T, Russo N and Sicilia E 1998 Solvation effects on reaction profiles by the polarizable continuum model coupled with Gaussian density functional method J. Oomp. Ohem. 19 290-9... 

OPW (orthogonalized plane wave) a band-structure computation method P89 (Perdew 1986) a gradient corrected DFT method parallel computer a computer with more than one CPU Pariser-Parr-Pople (PPP) a simple semiempirical method PCM (polarized continuum method) method for including solvation effects in ah initio calculations... 

At this point we note the existence of several classic and recent reviews devoted to, or with considerable attention paid to, continuum models of solvation effects, and we direct the reader to these works [71-83] for other perspectives that we consider complementary to what is presented here. 

A tautomeric equilibrium is a unimolecular equilibrium in which the various contributors differ based upon bond connectivity. In the special case of a protomeric tautomeric equilibrium, they differ only in how many protons are attached to each heavy atom. In-text figures throughout this section illustrate molecules for which multiple tautomers exist. When the molecules of interest are heterocycles, different tautomers may exhibit very large differences in electronic properties [266], In particular, they may span a wide range of polarities. That being the case, tautomeric equilibria can be quite sensitive to solvation effects, and they have thus proven to be attractive testing grounds for continuum solvation models. 

In addition to heterocycles, other molecular systems have attracted theoretical attention with respect to prediction of tautomeric equilibria and solvation effects thereon. The most commonly studied example in this class is the equilibrium between formamide and formamidic acid, discussed in the next section. In addition, some continuum modeling of solvation effects on keto/enol equilibria have appeared these are presented in section 4.2.2.2. We note that the equilibrium... 

In this contribution, we describe and illustrate the latest generalizations and developments[1]-[3] of a theory of recent formulation[4]-[6] for the study of chemical reactions in solution. This theory combines the powerful interpretive framework of Valence Bond (VB) theory [7] — so well known to chemists — with a dielectric continuum description of the solvent. The latter includes the quantization of the solvent electronic polarization[5, 6] and also accounts for nonequilibrium solvation effects. Compared to earlier, related efforts[4]-[6], [8]-[10], the theory [l]-[3] includes the boundary conditions on the solute cavity in a fashion related to that of Tomasi[ll] for equilibrium problems, and can be applied to reaction systems which require more than two VB states for their description, namely bimolecular Sjy2 reactions ],[8](b),[12],[13] X + RY XR + Y, acid ionizations[8](a),[14] HA +B —> A + HB+, and Menschutkin reactions[7](b), among other reactions. Compared to the various reaction field theories in use[ll],[15]-[21] (some of which are discussed in the present volume), the theory is distinguished by its quantization of the solvent electronic polarization (which in general leads to deviations from a Self-consistent limiting behavior), the inclusion of nonequilibrium solvation — so important for chemical reactions, and the VB perspective. Further historical perspective and discussion of connections to other work may be found in Ref.[l],... 

TvaroSka, KoS r and Hricovini in this book). One way to account for the effect of solvent on conforxnation might be to represent the molecule without environmental influences, and then explicitly include the solvent or other environmental molecules in the calculation. While avoiding built-in influences of environment is a satisfying concept, it is difficult to obtain by experiment parameters that lack those influences. Several methods have been used to study solvation effects, including continuum descriptions (24) and the explicit treatment of solvent molecules in Monte Carlo and molecular dynamics simulation. 

Discrete and continuum models for the solvent involvement have been employed to steady equilibrium and non-equilibrium solvation effects on bromination of ethylene. Two mechanisms were identified that lead to transition states of different symmetry. One mechanism operates in the gas phase and non-polar solvents. The second one, that leads to the typical C2V transition state, holds in medium-to-very polar solvents. In water, the solvent molecules participate actively and non-equilibration solvations effects proved to be substantial and larger than those previously reported for the >SN2 reaction.23... 

In the previous chapter, we have seen how Born s simple and successful idea of a dielectric continuum approximation for the description of solvation effects has been developed to a considerable degree of perfection. Almost all workers in this area have been trying to obtain more efficient and more precise methods for the solution of dielectric boundary conditions combined with molecular electrostatics, but the question of the validity of Born s basic assumption has rarely been discussed. This will be done in the following sections, with a surprising result. 

Abstract For some purposes solution-phase computations are necessary, e.g. for understanding certain reactions, and for the prediction of p/ in solution. For introducing the effects of solvation there are two methodologies (and a hybrid of these two) explicit solvation and continuum solvation. 

Nevertheless, the concept of spatial dispersion provides a general background for a qualitative understanding of those solvation effects which are beyond the scope of local continuum models. The nonlocal theory creates a bridge between conventional and well developed local approaches and explicit molecular level treatments such as integral equation theory, MC or MD simulations. The future will reveal whether it can survive as a computational tool competitive with these popular and more familiar computational schemes. 

Overall, it can be concluded that there must be a close correspondence between the use of a QM-SCRF continuum model and the specific details of the underlying parameterization in order to obtain an accurate description of solvation effects. 

This section will focus on the application of QM-SCRF continuum methods to chemical processes in solution. For brevity, however, we will limit the discussion to two kinds of chemical processes. Firstly, we will examine selected examples of tautomeric equilibria, which are well known to be highly sensitive to solvation effects. Secondly, we will move on the analysis of selected chemical reactions involving formation and breaking of bonds, whose description constitutes a challenge for any QM-SCRF continuum model. 

I. V. Leontyev, M. V. Vener, I. V. Rostov, M. V. Basilevsky and M. D. Newton, Continuum level treatment of electronic polarization in the framework of molecular simulations of solvation effects,./. Chem. Phys. 119 (2003) 8024-8037. 

Of course, there is more to a chemical reaction than its rate constant the reaction path or mechanism is also of central interest. Once again, nonequilibrium solvation is crucial in describing this path. In an equilibrium solvation picture, the solvent polarization would remain equilibrated throughout the reaction course, but this assumption is rarely satisfied for an actual reaction path, because of the same considerations noted above for the rate constant. Indeed these nonequilibrium solvation effects can qualitatively change the character of the reaction path as compared with an equilibrium solvation image. Dielectric continuum dynamic descriptions thus have an important role to play here as well. Indeed, we will employ in this contribution the reaction path Hamiltonian formulation previously developed [48,49], which can be used to generate a reaction path which is the analog in solution of the well-known Fukui reaction path in the gas phase [50], The reaction path will be discussed for both reaction topics in this contribution. 

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