Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Onsager cavity field

The total local field FA is equal to the vector sum of the Onsager cavity field FA, and the Onsager reaction field.101 The latter is independent of EA, and results from the dipoles own fields.101 In the medium of dielectric constant n2, FA is ... [Pg.205]

Kirkwood generalized the Onsager reaction field method to arbitrary charge distributions and, for a spherical cavity, obtained the Gibbs free energy of solvation in tenns of a miiltipole expansion of the electrostatic field generated by the charge distribution [12, 1 3]... [Pg.837]

The simplest SCRF model is the Onsager reaction field model. In this method, the solute occupies a fixed spherical cavity of radius Oq within the solvent field. A dipole in the molecule will induce a dipole in the medium, and the electric field applied by the solvent dipole will in turn interact with the molecular dipole, leading to net stabilization. [Pg.237]

Several authors have discussed how C(r) can be calculated from the Onsager cavity model. Briefly, we need to consider the time-dependent reaction field, which was related above (Eqs. (12) and (15)) to C(t). For simplicity we consider the case of a probe with ps = 0 and 0. If the probe... [Pg.11]

The key differences between the PCM and the Onsager s model are that the PCM makes use of molecular-shaped cavities (instead of spherical cavities) and that in the PCM the solvent-solute interaction is not simply reduced to the dipole term. In addition, the PCM is a quantum mechanical approach, i.e. the solute is described by means of its electronic wavefunction. Similarly to classical approaches, the basis of the PCM approach to the local field relies on the assumption that the effective field experienced by the molecule in the cavity can be seen as the sum of a reaction field term and a cavity field term. The reaction field is connected to the response (polarization) of the dielectric to the solute charge distribution, whereas the cavity field depends on the polarization of the dielectric induced by the applied field once the cavity has been created. In the PCM, cavity field effects are accounted for by introducing the concept of effective molecular response properties, which directly describe the response of the molecular solutes to the Maxwell field in the liquid, both static E and dynamic E, [8,47,48] (see also the contribution by Cammi and Mennucci). [Pg.172]

This is a generalization of the Onsager reaction field model for a point dipole inside a spherical cavity. For charged solutes, one should also include an ionic Born term, derived by... [Pg.573]

Booth73 used Frolich s108 modification of the Onsager expressions101 for the cavity field in non-associated polar liquids, and corresponding modifications of Kirkwood s equation for associated polar media. Booth s assumptions in deriving Eq. (28) for the Kirkwood case are important to determine the validity of his final expressions. He used the Onsager-Frolich cavity field ratio... [Pg.208]

In the reaction field model (Onsager, 1936), a solute molecule is considered as a polarizable point dipole located in a spherical or ellipsoidal cavity in the solvent. The solvent itself is considered as an isotropic and homogeneous dielectric continuum. The local field E at the location of the solute molecule is represented by (78) as a superposition of a cavity field E and a reaction field (Boettcher, 1973). [Pg.148]

In the case of a static field, the macroscopic relative permittivity e° has to be used in (82) for the cavity field factor, while the optical relative permittivity extrapolated to infinite wavelength e can be applied to estimate the static polarizability a(0 0) in (84). In this way the Onsager-Lorentz factor for a pure dipolar liquid is obtained (87). [Pg.150]

Onsager derived an improved formula by adopting a better model for the calculation of the local field at a molecule. His model consists of a spherical cavity which is excised in the dielectric material and which is just large enough to accommodate one molecule. The molecular dipole is supposed to be a point dipole fj, at the centre of the sphere, radius a. Onsager then said that the local field operating on the dipole at the centre of the cavity could be resolved into two components, a cavity field G and a reaction field R ... [Pg.46]

Among the few determinations of of molecular crystals, the CPHF/ INDO smdy of Yamada et al. [25] is unique because, on the one hand, it concerns an open-shell molecule, the p-nitrophenyl-nitronyl-nitroxide radical (p-NPNN) and, on the other hand, it combines in a hybrid way the oriented gas model and the supermolecule approach. Another smdy is due to Luo et al. [26], who calculated the third-order nonlinear susceptibility of amorphous thinmultilayered films of fullerenes by combining the self-consistent reaction field (SCRF) theory with cavity field factors. The amorphous namre of the system justifies the choice of the SCRF method, the removal of the sums in Eq. (3), and the use of the average second hyperpolarizability. They emphasized the differences between the Lorentz Lorenz local field factors and the more general Onsager Bbttcher ones. For Ceo the results differ by 25% but are in similar... [Pg.49]

In addition, the molecules properties are changed due to the interaction with the surrounding medium. Several computational schemes have been proposed to address these effects. Tliey are essentially based on the extension of the Onsager reaction field cavity model and give effective hyperpolarizabilities, i.e. molecular hyperpolarizabilities induced by the external fields that include the modifications due to the surrounding molecules as well as local (cavity) field effects [40 2]. These condensed-phase effects have, however, not yet been included in the SFG hyperpolarizability calculations, which are therefore strictly gas-phase calculations. [Pg.371]

It is often assumed, although without actual proof, that there exists a definite proportionality between the molecular and the macroscopic relaxation times. A relation between r and r could only be established if the actual field acting on the molecule were known. Equation 27 is obtained if this field is identified either with the external or with the Lorentz internal field. In the first case the macroscopic relaxation time is identical with the molecular one. In the second case r is proportional to t. If Onsager s model is used,88 it may be shown that in first approximation, the cavity field G is itself subject to dispersion ... [Pg.88]

The actual field-parallel contribution of m can be calculated from Onsager s concepts of the cavity field and of the reaction field. At first, Eq. (3.15) is specified as... [Pg.143]

The simplest continuum model is based on die classical Onsager reaction field theory assuming the spherical or ellipsoidal form of cavities for die solute molecules in dielectric... [Pg.655]

The reality of the Onsager cavity is one of the reasons which prompted me to select this paper for this New Century Issue this is one of the further steps in the development of real models using the physical approach. This is not the only reason, however, Onsager introduced new concepts, that of the reaction field and that of the cavity field, with a clear and transparent physical basis, and devised a simple model based on a few parameters with physical meaning, and easily managed computationally. [Pg.39]

So, good models for chemical applications must be robust and flexible, allowing more detailed descriptions of the system and of the property under examination. Onsager s model has these positive features. In the last two decades it has been widely used with important modifications permitting more realistic descriptions of the systems, but keeping the essential points the cavity, the reaction field, and the cavity field. [Pg.40]

The Clausius-Mossotti equation is based on the simplest (Lorentz) form of the local field. In reality, the induced dipole in the selected molecule also creates an additional, reaction field that modifies the cavity field. On account of these factors Onsager has obtained the following equation for dielectric permittivity... [Pg.161]

Here, N stands for the molecular packing density, h = 3el(2e+ 1) is the cavity field factor, e = (ej + 2e )/3 is the averaged dielectric constant, F is the Onsager reaction field, < //> and are the principal elements of the molecular polarizability tensor. [Pg.201]

In this method the field on a dipole in the simulation consists of two parts the first a short-range contribution from molecules situated within a truncation sphere the second from molecules outside the sphere which are considered to form a dielectric continuum (e ) producing an Onsager reaction field at the centre of the cavity. [Pg.532]


See other pages where Onsager cavity field is mentioned: [Pg.39]    [Pg.199]    [Pg.210]    [Pg.39]    [Pg.701]    [Pg.39]    [Pg.199]    [Pg.210]    [Pg.39]    [Pg.701]    [Pg.1468]    [Pg.189]    [Pg.168]    [Pg.261]    [Pg.205]    [Pg.210]    [Pg.218]    [Pg.18]    [Pg.252]    [Pg.155]    [Pg.654]    [Pg.324]    [Pg.39]    [Pg.654]    [Pg.682]    [Pg.5104]    [Pg.121]    [Pg.469]    [Pg.1468]   
See also in sourсe #XX -- [ Pg.701 ]




SEARCH



Cavity fields

Onsager

Onsager cavity

Onsager field

© 2024 chempedia.info