Standard bulk dielectric constants

Table II. Standard Potentials (Molal Scale) of Cell I and the Bulk Dielectric Constants of the Solvents"...

In many centred molecules the interactions between the electro-active centres in a given molecule modify the spacing of the formal Standard Potentials of the successive processes by an amount that depends in the case of coulombic forces in part on the dielectric properties of the local surrounding electrolyte solution. This feature has been observed in Ae case of bimetallic complexes in mixed solvent systems of relatively low bulk dielectric constants, has been used to ascertain Ae impact of electrolyte concentration in particular ion-pairing on electrolyte dielectric behaviour. 

Several conclusions can be drawn from Table 3. First, in accordance with the two-state model, /So and jSj all increase with decreasing HOMO-LUMO gap. Second, the intrinsic second-order polarizability of p-nitroaniline is increased by two-thirds when the solvent is changed from p-dioxane to methanol or A-methylpyrrolidone, even when the values are corrected for the differences in (A ). As we have adopted the value for p-nitroaniline in dioxane as a standard, it should therefore be noted that molecules that truly surpass the best performance of p-nitroaniline should have a second-order polarizability of l. p-nitroaniline (dioxane). As a third conclusion, there is a poor correlation between and the static reaction field as predicted by (91). This is in part due to the fact that the bulk static dielectric constant, E° in (89), differs from the microscopic dielectric constant. For example, p-dioxane has long been known for its anomalous solvent shift properties (Ledger and Suppan, 1967). Empirical microscopic dielectric constants can be derived from solvatochromism experiments, e.g. e = 6.0 for p-dioxane, and have been suggested to improve the estimation of the reaction field (Baumann, 1987). However, continuum models can only provide a crude estimate of the solute-solvent interactions. As an illustration we try to correlate in Fig. 7 the transition energies of p-nitroaniline with those of a popular solvent polarity indicator with negative solvatochromism. 

It should be stressed that the SMC parameterization procedure is very different from standard Born model analysis where an ion of assigned radius is embedded in a uniform continuum dielectric with bulk permittivity. Here, the ion is fully solvated by the model water, and the dielectric constant refiects the solvent s electronic properties. The structural component of dielectric reorganization is treated explicitly. 

Let denote the potential obtained from a standard PB calculation assuming a constant bulk dielectric coefficient and < )var( ) denote that obtained by assuming a variable dielectric coefficient (i.e., Eqs. [385]-[388]). Now let Vb z) be the MC potential (a sum of Lennard-Jones and electrostatic terms) that a cation in the system would experience in a bulk dielectric continuum. The approximate effect of including a variable dielectric coefficient in the Monte Carlo simulation can be found by replacing the bulk MC potential Vb z) with the PB-corrected potential VB(r)- -< )var(r) — < )B(r). Of course the resulting ion distribution will not be self-consistent with the assumed (PB-derived) dielectric coefficient map, but it will be a noticeable improvement on the original MC distribution using a bulk dielectric. 

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