Bulk solvation effects

We shall now proceed to the second example of active participation of solvent molecules in reaction mechanisms which has been considered by Rivail et al. (1994), namely the dissociation of HC1. In this study (Rivail et al., 1994 Chipot et al., 1994) the dissociation process has been examined with the aid of two solute models, i.e. HC1H20 and HC1(H20)2 both considered in vacuo as well as in continuum solvents with different dielectric constants. Once again, electronic wavefunctions of good quality have been used (SCF and MP2/6-31+G calculations have been performed). Bulk solvation effects have been described with the SCRF formalism, using an ellipsoid cavity. 

The polarizable continuum model (PCM) by Tomasi and coworkers [77-79] was selected to describe the effects of solvent, because it was used to successfully investigate the effect of solvent upon the energetics and equilibria of other small molecular systems. The PCM method has been described in detail [80]. The solvents and dielectric constants used were benzene (s = 2.25), methylene chloride (g = 8.93), methanol (g = 32.0), and water (g = 78.4). Full geometry optimizations were carried out for the discrete and PCM models. To simultaneously account for localized hydrogen bonding and bulk solvation effects, PCM single-point energy calculations have been conducted on stationary points of the acrolein and butadiene reaction with two waters explicitly... 

Low-cost continuum models are often used to assess bulk solvation effects. The polarizable continuum models (PCM) [20] are continuum solvation models in which the solvent effects are described with induced surface charges. In a PCM calculation, the solutes can be modeled with ab initio methods or force fields, or both. In a combined QM/EFP/PCM calculation [21], the EFP induced dipoles and PCM induced charges are iterated to self-consistency as the QM wavefunction converges. 

Note that reaction 1.1 represents acid dissociation in water, or aqueous solution, yet pK, values can in principal be determined in any solvent. Deprotonation reactions such as reaction 1.1 are most often written with (aq) representing the unknown micro and bulk solvation effects of the solvent, water in this case. The exact nature of the solvated proton in water is unknown at this time and is the subject of much research the standard practice is to use a nominal definition for the solvated proton, and to represent the solvated proton as H+j q, [32,33]. This practice is justified because the solvated proton exists in a cluster of many water molecules, and proton transfer reactions depend on the magnitude of the proton s solvation... 

Kjaer el al. have performed a benchmark study of a combined multipole spin-spin coupling constant polarizability/reaction field (MJP/RF) approach to the calculation of both specific and bulk solvation effects on coupling in solvated molecule. The MJP/RF approach was based on the expansion of couplings of the solvated molecule in terms of coupling dipole and quadrupole polarizabilities and hyperpolarizabilities derived from single ab initio calculations, and on taking into account solvent electric field and electric field gradient calculated by molecular dynamics (MD)... 

Local Versus Bulk Solvation Effects on the Electronic Excitation Energies of Uracil in Aqueous Solution... 

The simulation of molecules in solution can be broken down into two categories. The first is a list of elfects that are not defined for a single molecule, such as diffusion rates. These types of effects require modeling the bulk liquid as discussed in Chapters 7 and 39. The other type of effect is a solvation effect, which is a change in the molecular behavior due to the presence of a solvent. This chapter addresses this second type of effect. 

This chapter focuses on the simulation of bulk liquids. This is a dilferent task from modeling solvation effects, which are discussed in Chapter 24. Solvation effects are changes in the properties of the solute due to the presence of a solvent. They are defined for an individual molecule or pair of molecules. This chapter discusses the modeling of bulk liquids, which implies properties that are not defined for an individual molecule, such as viscosity. 

In this form of catalysis, inclusion of the substrate in the CD cavity provides an environment for the reaction that is different from that of the bulk, normally aqueous, medium. In the traditional view, the catalytic effect arises from the less polar nature of the cavity (a microdielectric effect) and/or from the conformational restraints imposed on the substrate by the geometry of inclusion (Bender and Komiyama, 1978). However, catalysis may also arise as a result of differential solvation effects at the interface of the CD cavity with the exterior aqueous environment (Tee and Bennett, 1988a,b Tee, 1989). 

The fact that these reactions are indeed a plausible approach to interconnect gas-phase and solution reactivity has been contested by Henchman el al. (1983). The basis for their argument is that for n = 1, reaction (30) yields Br -I- H20 + CH3OH as products, and not the solvated anion. Thus, they conclude that bulk solvent effects cannot be properly extrapolated from the data of reaction (30). While the rigorous argument is correct, the rate constant trend is very useful to show that successive solvation of the reagent anion will slow down the reaction even on a thermochemical basis. 

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