Solvent effects reaction-field model

A related methodology that makes use of the calculated surface charges at the cavity surface to estimate the interaction with the solvent has been described in Ref. [54] in addition, the reaction field model can be extended to include the effects of higher order multipoles [55], In the present implementation, only dipole effects are considered. 

As a first approximation, solvent effects can be described by models where the solvent is represented by a dielectric continuum, e.g., the Onsager reaction-field model. 

The experimental data reported in the Table for gas phase have been extracted from measurements in dioxane solution by applying the Onsager reaction field model to eliminate the solvent effect [37], By contrast, the cyclohexane experimental dipole moments have been obtained from those reported in Ref. [37] re-including the proper reaction field factors. Once recalled these facts, we note that the observed solvent-induced changes on both ground and excited state dipole moments are quantitatively reproduced by the calculations. 

We have shown above how the reaction field model can be used to estimate solute-solvent interactions in the absence of external fields. Now we introduce effective polarizabilities that connect the Fourier components of the induced dipole moment (33) with the macroscopic fields in the medium. In the linear case, the Fourier component / induced by an external optical field can be represented by the product of the macroscopic field amplitude " and an effective first-order polarizability a(-tu w) using (93). 

Another model is the reaction field model [83], Here, the solute is placed inside a cavity surrounded by a medium with a dielectric constant that is appropriate for the desired solvent and the effects on the energy of the... 

Solvent effect on tautomers energies was included by means of the simple reaction-field model with MINDO/3 dipole moments. Entropic effects and zero-point vibrational energy differences were neglected. The major results are the following. 

While this result confirmed the feasibility of the general approach, it did not precipitate wider exploration of dielectric medium effects. Recently, however, Wiberg et al. have incorporated the Onsager self-consistent reaction-field model into ab initio MO theory in an implementation which provides analytical gradients and second derivatives. The model considers just the dipole of the solute molecules and a spherical cavity whose radius is chosen for a given solute molecule from the molecular volume estimated at the 0.001 eB electron-density contour (B is the Bohr radius), plus an empirical constant 0.5 A to account for the nearest approach of solvent molecules [164]. Cieplak and Wiberg have used this model to probe solvent effects on the transition states for nucleophilic additions to substituted acetaldehydes [165]. For each... 

A major area of theoretical interest has been on solvent effects, and several techniques have been applied to the calculation of NLO properties. " The most common (and simplest) method is the reaction field model, where the solute molecule is in a cavity of solvent, which is treated as a uniform dielectric medium. Cavity approaches are problematic. How do you pick the cavity size How do you pick the cavity shape How do you model stronger, specific interactions (such as hydrogen bonding) The work of Willetts and Rice " illustrated the inability of reaction field models to adequately treat solvent effects even though they tried both spherical and ellipsoidal cavities. Mikkelsen et al. attempted to provide specific interactions with their solvent model by explicitly including solvent molecules inside the cavity. These and related issues need to be addressed further if computational chemists are to develop truly useful procedures capable of including solvent effects in NLO calculations. Recent work by Cammi, Tomasi, and co-workers " has attempted to address these issues within the polarized continuum model (PCM) and have included studies of frequency-dependent hyperpolarizabilities. 

A. Willetts and J. E. Rice,/. Chem. Phys., 99, 426 (1993). A Study of Solvent Effects on Hyperpolarizabilities The Reaction Field Model Applied to Acetonitrile. 

Standard computations treat isolated molecules (i.e., model the low-pressure gas phase situation). Chemistry in the condensed phase, however, can be significantly different because ionic and polar species are specifically stabilized. Computations can consider solvation explicitly by including the solvent molecules or as a continuous medium effect (reaction field methods). 

Bartkowiak and Misiaszek used the eoupled perturbed Hartree-Fock method and the sum-over-modes formalism to ealeulate the eleetronie and vibrational /i-tensors for 4-nitroaniline, 4-nitro-4 -aminostilbene, 4-amino-4 -nitrobiphenyl and 4-amino-4 -nitrophenylacetylene, all typieal push-pull eonjugated molecules of the kind that have been associated with seeond order optieal non-linearities derived from their large P values. Their ealeulations refer to the gas phase and to chloroform and aqueous solutions, the solvent effects being included through a continuum self-consistent reaction field model. They demonstrate that the solvent effects are much greater for the vibrational hyperpolarizability than for the electronic contribution. 

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-... 

Solvent effects may be modeled via the self-consistent reaction field (SCRF) to arbitrary order in the multipole expansion. 

To model solvent effects, the Onsager self-consistent reaction field model has also been invoked in a study of the hfcc of the H2NO radical, by Barone. Also here, the model seems to be able to describe accurately the shifts in hyperfine couplings of the system. 

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