Theoretical methods solvent effect modeling

The initial attempts to account theoretically for solvent effects were made in the 1930s [1], but the suitability of these models for understanding chemical events in condensed phases was limited by their intrinsic simplicity. Thus, it was not until the 1970s, when continuum models were implemented within the quantum mechanical framework [2], that an accurate theoretical representation of solvent effects became possible. The last decade of this century has witnessed the spectacular growth of this new area of research [3]. It is expected that in the next century continuum methods will be the most used approach for the study of solvent effects in chemical systems. 

However in most of the cases, mainly in polar systems, the interactions with the surroundings are not negligible. There are a wide variety of theoretical methods for taking into account the effect of environment, ranging from various solvent effect models to pseudopotential-like approaches. One cannot overview all of them, but a few selected examples will be shown in the following. 

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

Tliis interpretation is based only upon the structural and electronic properties of the pyridinium cations. Tire calculation of relative activation Ijarri-ers for the competing substitution reactions will give more reliable results —especially if solvent effects are included in the calculations. In order to assess the reliability of actual theoretical methods as applied to model sys-... 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

The structure of this contribution is as follows. After a brief summary of the theory of optical activity, with particular emphasis on the computational challenges induced by the presence of the magnetic dipole operator, we will focus on theoretical studies of solvent effects on these properties, which to a large extent has been done using various polarizable dielectric continuum models. Our purpose is not to give an exhaustive review of all theoretical studies of solvent effects on natural optical activity but rather to focus on a few representative studies in order to illustrate the importance of the solvent effects and the accuracy that can be expected from different theoretical methods. 

The main objection to the use of CMs to describe the solvent effect of an interfacial environment is that such a model neglects the specific effects arising from the interface, thus preventing a faithful description. It is therefore important to test the model and to compare the results obtained with those from other theoretical methods (e.g. simulations) and experiments. 

Here we give an overview of the current status and perspectives of theoretical treatments of solvent effects based on continuum solvation models where the solute is treated quantum mechanically. It is worth noting that our aim is not to give a detailed description of the physical and mathematical formalisms that underlie the different quantum mechanical self-consistent reaction field (QM-SCRF) models, since these issues have been covered in other contributions to the book. Rather, our goal is to illustrate the features that have contributed to make QM-SCRF continuum methods successful and to discuss their reliability for the study of chemical reactivity in solution. 

The MFA [1] introduces the perturbation due to the solvent effect in an averaged way. Specifically, the quantity that is introduced into the solute molecular Hamiltonian is the averaged value of the potential generated by the solvent in the volume occupied by the solute. In the past, this approximation has mainly been used with very simplified descriptions of the solvent, such as those provided by the dielectric continuum [2] or Langevin dipole models [3], A more detailed description of the solvent has been used by Ten-no et al. [4], who describe the solvent through atom-atom radial distribution functions obtained via an extended version of the interaction site method. Less attention has been paid, however, to the use of the MFA in conjunction with simulation calculations of liquids, although its theoretical bases are well known [5]. In this respect, we would refer to the papers of Sese and co-workers [6], where the solvent radial distribution functions obtained from MD [7] calculations and its perturbation are introduced a posteriori into the molecular Hamiltonian. 

In the first contribution, Huxter Scholes present a review of the recent evolution of theory of EET in condensed phase from their earliest and simple formulation, based on the Forster theory to the most recent advances of theoretical and computational methods based on continuum solvation models. In the second contribution, Curutchet reviews the recent developments of PCM towards accurate theoretical investigations of EET in solution. In particular, the modelization of the various contributions of solvent effects in the chromophore-chromophore electronic coupling is presented using quantum-mechanical approaches. 

Solvent effects can significantly influence the function and reactivity of organic molecules.1 Because of the complexity and size of the molecular system, it presents a great challenge in theoretical chemistry to accurately calculate the rates for complex reactions in solution. Although continuum solvation models that treat the solvent as a structureless medium with a characteristic dielectric constant have been successfully used for studying solvent effects,2,3 these methods do not provide detailed information on specific intermolecular interactions. An alternative approach is to use statistical mechanical Monte Carlo and molecular dynamics simulation to model solute-solvent interactions explicitly.4 8 In this article, we review a combined quantum mechanical and molecular mechanical (QM/MM) method that couples molecular orbital and valence bond theories, called the MOVB method, to determine the free energy reaction profiles, or potentials of mean force (PMF), for chemical reactions in solution. We apply the combined QM-MOVB/MM method to... 

Developments in experimental and computational science have shed light on phenomena in bioenvironments and condensed phases that pose significant challenges for theoretical models of solvation [27]. Tapia [22] raises the important distinction between solvation theory and solvent effects theory. Solvation theory is concerned with direct evaluation of solvation free energies this is extensively covered by recent reviews [16,17]. Solvent-effect theory concerns changes induced by the medium onto electronic structure and molecular properties of the solute. Solvent-effect theory is concerned with molecular properties of the solvated molecule relative to the properties in vacuo as such it focuses on chemical features suitable for studying systems at the microscopic level [23]. Extensive reviews of different computational methods are given in a book by Warshel [24]. 

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