Polarizable continuum solvent model

Note that the close association found between gas-phase and aqueous behavior for these carbon acids is in contrast to the behaviors of alcohols, carboxylic acids, and amines, where solvent effects can sometimes reverse the acidity order. See, for example, the discussion of aliphatic amines in Section 6.1. In this study use of a polarizable continuum solvent model (PCM) actually led to slightly worse results for the calculated aqueous pK s. 

N chemical shielding in peptides and proteins is known to be sensitive to secondary structure as well as noncovalent interactions. Cai et al have recently employed DFT calculations with a polarizable continuum solvent model and explicit water molecules in the first solvation shell for A-formyl-alanyl-X amides, where X is one of the 19 naturally occurring amino acids excluding proline. This recent work suggests that the explicit water molecules incorporated in the calculations affect the isotropic amide N chemical shift, but not its anisotropy. A solvation model likewise appears to improve the correlation between calculated and observed C chemical shifts in the complex formed by piperidine-4-carboxylic acid and chlor-oacetic acid. Ksiazek et have evaluated the performance of the... 

For an early benchmark study of OR involving a continuum solvent model, see [174], More recently, Pecul et al. [175] have implemented a polarizable continuum... 

Generally, methods for calculating can be represented by two main categories implicit or explicit solvent models [38, 47-58]. The main difference between these two categories is the representation of the solvent strueture around the solute. Implicit Continuum Solvent Models (ICSMs) treat the solvent around the solvated molecule as a structureless polarizable medium characterized by a dielectric constant, e [49, 59,60]. In turn, in explicit solvent models (ESMs) both solute and solvent molecules in the solute-solvent systems are described at the atomistic level. There are two... 

As can be seen from the histogram in Figure l-l(b), the loose conformation is preferred over the tight one, a result only possible with inclusion of solvent effects. Ab-initio calculations of those conformers show that, without the inclusion of solvent effects, the tight conformer is preferred by 7.4 kcal/mol, while the inclusion of solvent effects (with polarizable continuum model, PCM) shifts the preference towards the loose conformer, which becomes more stable than the tight one by 0.1 kcal/mol. 

Fig. 2.2 Self-Consistent Reaction Field (SCRF) model for the inclusion of solvent effects in semi-empirical calculations. The solvent is represented as an isotropic, polarizable continuum of macroscopic dielectric e. The solute occupies a spherical cavity of radius ru, and has a dipole moment of p,o. The molecular dipole induces an opposing dipole in the solvent medium, the magnitude of which is dependent on e.

C. Amovilli, V. Barone, R. Cammi, E. Cancfes, M. Cossi, B. Menucci, C. S. Pomelli, and J. Tomasi, Recent advances in the description of solvent effects with the polarizable continuum model, Adv. Quantum Chem. 32 227 (1998). 

The most common approach to solvation studies using an implicit solvent is to add a self-consistent reaction field (SCRF) term to an ab initio (or semi-empirical) calculation. One of the problems with SCRF methods is the number of different possible approaches. Orozco and Luque28 and Colominas et al27 found that 6-31G ab initio calculations with the polarizable continuum model (PCM) method of Miertius, Scrocco, and Tomasi (referred to in these papers as the MST method)45 gave results in reasonable agreement with the MD-FEP results, but the AM1-AMSOL method differed by a number of kJ/mol, and sometimes gave qualitatively wrong results. 

In addition to these external electric or magnetic field as a perturbation parameter, solvents can be another option. Solvents having different dielectric constants would mimic different field strengths. In the recent past, several solvent models have been used to understand the reactivity of chemical species [55,56]. The well-acclaimed review article on solvent effects can be exploited in this regard [57]. Different solvent models such as conductor-like screening model (COSMO), polarizable continuum model (PCM), effective fragment potential (EFP) model with mostly water as a solvent have been used in the above studies. 

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

Quantitative models of solute-solvent systems are often divided into two broad classes, depending upon whether the solvent is treated as being composed of discrete molecules or as a continuum. Molecular dynamics and Monte Carlo simulations are examples of the former 8"11 the interaction of a solute molecule with each of hundreds or sometimes even thousands of solvent molecules is explicitly taken into account, over a lengthy series of steps. This clearly puts a considerable demand upon computer resources. The different continuum models,11"16 which have evolved from the work of Bom,17 Bell,18 Kirkwood,19 and Onsager20 in the pre-computer era, view the solvent as a continuous, polarizable isotropic medium in which the solute molecule is contained within a cavity. The division into discrete and continuum models is of course not a rigorous one there are many variants that combine elements of both. For example, the solute molecule might be surrounded by a first solvation shell with the constituents of which it interacts explicitly, while beyond this is the continuum solvent.16... 

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