Continuum reaction field

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

The use of isopropylidine acetals (112) as tethers in the intramolecular Diels-Alder reactions of dienes with alkenes facilitates the formation of civ-fused cycloadducts (113) from an endo transition state (Scheme 41).218 The intramolecular Diels-Alder reaction of 4-[tris-(2-mcthylcthyl)silyl]oxy-2//-thiopyran derivatives with potential dienophiles tethered at C(2), C(3), C(5), and C(6) positions yielded cycloadducts when the dienophiles were activated with a carbomethoxy group.219 By the substitution of a phenylsulfonyl group on the dienophile of 2-benzopyran-3-ones, it is possible to enhance exo addition during intramolecular Diels-Alder cyclizations to yield a predominance of trans-fused hexaphenanthrenes related to natural products.220 The intramolecular Diels-Alder reaction of 2-furfuryl fumarates has been investigated by molecular mechanics (SIBFA)/continuum reaction field computations.221 The intramolecular 4 + 2-photo-cycloaddition of A-benzylcinnamamides (114) in the presence of C(,H6 gives 3-azatricyclo[5.2.2.01,5]undeca-8,10-dien-4-ones (115) with high stereoselectivity (Scheme 42).222... 

The preliminary calculations are essentially of the properties of single molecules in the self-consistent continuum reaction field, taking account of the effect... 

The degree of clustering may be determined by a direct comparison of the experimental value of the solvatochromic parameter, for example. Ex, with the value which is calculated for a honx)geneous polarizable dielectric. Diis calculation has been done using the McRae-Bayliss nKxlel(221 which is based on the continuum reaction field theory of Onsager. 

Lange, A. W., and Herbert, J. M. (2010). Polarizable continuum reaction-field solvation models affording smooth potential energy surfaces, J. Phys. Chem. Lett. 1, pp. 556-561. 

One useful method, for taking into account the effect of solvent media upon the conformational properties of glycosidic structures, is the continuum reaction field method. This method is based on the Scaled Particle Theory (SPT) equations and on Onsager s theory of the reaction field, as applied by Abraham by considering the solvent as a dielectric continuum. In this approach the total conformational energy, Gt t, is given by ... 

The electrostatic interaction term 0, between solute and solvent is based on the continuum reaction field which takes into accoimt a reaction potential induced by the solute dipole and quadrupole (third step in Figure 12.2.7) in a continuum medium of dielectric constant e ... 

Most coordination complexes are not well modeled as spheres of uniform charge (e.g., metallo-protein active sites are an extreme example). Therefore, more sophisticated dielectric continuum reaction field models are generally employed to take into account the shape of the complex and even account for the dielectric properties of the ligands themselves. Furthermore, the charge distribution and/or polarity of the complex can be used to model discretely the reaction field energies at various points on the exterior surface of the solute. There are many variations of such solvent models, some of which are available in standard quantum chemistry programs. 

Solvent interactions in the present study have been calculated by following the approach of Sinanoglu [2] who separates mathematically the solvation energies into two parts or steps. In the first step a hole is prepared in the liquid. In the second step a solute molecule is placed in the hole and interacts with its new environment. In calculating the interaction energy of the solute molecule with its environment the concept of continuum reaction field of Onsager [24] has been applied. The solvent is considered as a continuum which possesses the macroscopic dielectric properties of the solution, which coincide with those of a pure liquid in the case of a dilute solution. A fuller description of the theory and computational details will be presented in Section 2.2 and in Section 3. Our procedure differs from that presented by Sinanoglu [2] in the calculation of dispersion interaction, which follows the method developed by Linder [25], which takes into account many body effects and thermodynamic fluctuations. The calculational scheme has been already applied by Nir [26] for the... 

The simplest approach to the calculation of F is that of the continuum reaction field, which implicitly takes into account many-body effects, but considers the surrounding medium to be an uniform dielectric. This approach assumes the vahdity of... 

The results of the application of continuum reaction field method predict a significant effect of the solvent on the conformational stability. A comparison of calculated with observed conformations of N-acetyl L-alanine N-methyl amide in carbon tetra-chloride and in water indicates a qualitative agreement with experiment. 

Any of the methods used in classical Monte Carlo and molecular dynamics simulations may be borrowed in the combined QM/MM approach. However, the use of a finite system in condensed phase simulations is always a severe approximation, even when appropriate periodic or stochastic boundary conditions are employed. A further complication is the use of potential function truncation schemes, particular in ionic aqueous solutions where the long-range Coulombic interactions are significant beyond the cutoff distance.Thus, it is alluring to embed a continuum reaction field model in the quantum mechanical calculations in addition to the explicit solute—solvent interaaions to include the dielectric effect beyond the cutoff distance. - uch an onion shell arrangement has been used in spherical systems, whereas Lee and Warshel introduced an innovative local reaction field method for evaluation of long-... 

N. Gresh, G. Tiraboschi, and D. R. Salahub, Biopolymers, 45(6), 405-425 (1998). Conformational Properties of a Model Alanyl Dipeptide and of Alanine-Derived Ohgopeptides Effects of Solvation in Water and in Organic Solvents - A Combined SIBFA/Continuum Reaction Field, ab Initio Self-Consistent Field, and Density Functional Theory Investigation. 

Gresh N, Roques BP (1997) Thermolysin-inhibitor binding effect of the His -> Ala mutation on the relative affinities of thiolate versus phosphoramidate inhibitors—A model theoretical investigation incorporating a continuum reaction field hydration model. Biopolymers 41 145-164... 

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