Reaction field treatments

D. M. Chipmann, Reaction field treatment of charge penetration, J. Chem. Phys., 112 (2000) 5558. 

MD simulations of the aqueous interface and the hydrophobic IL ([C4mim] [NTf2]) were carried out [100], Different liquid models, the Ewald vs the reaction field treatments of the long-range electrostatics, and different starting conditions... 

However, the reaction field treatment cannot account for the third low-energy conformer (fgg). This is within 0.1 kJ/mol of ttt in the gas phase as found by ab initio calculations, the stabilisation being ascribed to an intramolecular weak hydrogen bond between a methyl hydrogen and an ether oxygen at the other end of the molecule. Because of the large dipole moment (1.7 D), reaction-field treatments predict this conformer to be by far the most stable in solution (5), completely at odds with experiment where this conformer is never observed. On the other hand, the presence or absence of this conformer has serious... 

Cances E, Meimucci B (2001) Comment on reaction field treatment of charge penetration . J Chem Phys 114 4744-4745... 

Despite the great scope for rate studies in the fast reaction field, these still constitute a small fraction of published kinetic studies. In part this is because fast reaction kinetics is still in some respects a specialist s field, requiring equipment (whether commercially purchased or locally fabricated) that is not commonly found in the chemical laboratory s stock of instrumentation. This chapter treats the field at a nonspecialist s level, which is adequate to allow the experimentalist to judge if a certain technique is applicable to a particular problem. Reviews and book-length treatments are available these should be consulted for more detailed theoretical and experimental descriptions. 

In spectroscopy we may distinguish two types of process, adiabatic and vertical. Adiabatic excitation energies are by definition thermodynamic ones, and they are usually further defined to refer to at 0° K. In practice, at least for electronic spectroscopy, one is more likely to observe vertical processes, because of the Franck-Condon principle. The simplest principle for understandings solvation effects on vertical electronic transitions is the two-response-time model in which the solvent is assumed to have a fast response time associated with electronic polarization and a slow response time associated with translational, librational, and vibrational motions of the nuclei.92 One assumes that electronic excitation is slow compared with electronic response but fast compared with nuclear response. The latter assumption is quite reasonable, but the former is questionable since the time scale of electronic excitation is quite comparable to solvent electronic polarization (consider, e.g., the excitation of a 4.5 eV n — n carbonyl transition in a solvent whose frequency response is centered at 10 eV the corresponding time scales are 10 15 s and 2 x 10 15 s respectively). A theory that takes account of the similarity of these time scales would be very difficult, involving explicit electron correlation between the solute and the macroscopic solvent. One can, however, treat the limit where the solvent electronic response is fast compared to solute electronic transitions this is called the direct reaction field (DRF). 49,93 The accurate answer must lie somewhere between the SCRF and DRF limits 94 nevertheless one can obtain very useful results with a two-time-scale version of the more manageable SCRF limit, as illustrated by a very successful recent treatment... 

In the case of histamine, inclusion of this reaction field correction was found to change the result by almost 4 kJ/mol. Unfortunately, the highly sensitive nature of the histamine monocation results to protocol and parameters used meant that no definite conclusion could be drawn about the importance of long-range forces in these calculations. It does however appear that they do play a role. The correct treatment of long-range forces is... 

In most work reported so far, the solute is treated by the Hartree-Fock method (i.e., Ho is expressed as a Fock operator), in which each electron moves in the self-consistent field (SCF) of the others. The term SCRF, which should refer to the treatment of the reaction field, is used by some workers to refer to a combination of the SCRF nonlinear Schrodinger equation (34) and SCF method to solve it, but in the future, as correlated treatments of the solute becomes more common, it will be necessary to more clearly distinguish the SCRF and SCF approximations. The SCRF method, with or without the additional SCF approximation, was first proposed by Rinaldi and Rivail [87, 88], Yomosa [89, 90], and Tapia and Goscinski [91], A highly recommended review of the foundations of the field was given by Tapia [71],... 

In chapter 2, Profs. Contreras, Perez and Aizman present the density functional (DF) theory in the framework of the reaction field (RF) approach to solvent effects. In spite of the fact that the electrostatic potentials for cations and anions display quite a different functional dependence with the radial variable, they show that it is possible in both cases to build up an unified procedure consistent with the Bom model of ion solvation. The proposed procedure avoids the introduction of arbitrary ionic radii in the calculation of insertion energy. Especially interesting is the introduction of local indices in the solvation energy expression, the effect of the polarizable medium is directly expressed in terms of the natural reactivity indices of DF theory. The paper provides the theoretical basis for the treatment of chemical reactivity in solution. 

Sections III. 1-III. 3 have described some basic discrete molecular and continuum treatments of solute-solvent interactions. There are many variants and refinements of these that have not been discussed, such as the use of effective dielectric constants66 or the implementation of dielectric screening.155 156 For ionic solutions, it is sometimes preferred to find the reaction field potential via the Poisson-Boltzmann rather than the Poisson equation,132 157 since the effects of the other ions can readily be incorporated into the former.158... 

Recently, the supercritical fluid treatment has been considered to be an attractive alternative in science and technology as a chemical reaction field. The molecules in the supercritical fluid have high kinetic energy like the gas and high density like the Uquid. Therefore, it is expected that the chemical reactivity can be high. In addition, the ionic product and dielectric constant of supercritical water are important parameters for chemical reaction. Therefore, the supercritical water can be realized from the ionic reaction field to the radical reaction field. For example, the ionic product of the supercritical water can be increased by increasing pressure, and the hydrolysis reaction field is realized. Therefore, the supercritical water is expected as a solvent for converting biomass into valuable substances (Hao et al., 2003). 

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