Electrostatic parameter

In some force fields the interaction sites are not all situated on the atomic nuclei. For example, in the MM2, MM3 and MM4 programs, the van der Waals centres of hydrogen atoms bonded to carbon are placed not at the nuclei but are approximately 10% along the bond towards the attached atom. The rationale for this is that the electron distribution about small atoms such as oxygen, fluorine and particularly hydrogen is distinctly non-spherical. The single electron from the hydrogen is involved in the bond to the adjacent atom and there are no other electrons that can contribute to the van der Waals interactions. Some force fields also require lone pairs to be defined on particular atoms these have their own van der Waals and electrostatic parameters. 

Halgren T A 1996b. Merck Molecular Force Field II MMEF94 van der Waals and Electrostatic Parameters for Intermolecular Interactions. Journal of Computational Chemistry 17 520-552. 

Halgren TA. Merck molecular force field. II. MMFF94 van der Waals and electrostatic parameters for intermolecular interactions. J Comput Chem... 

Anisimov VM, Lamoureux G, Vorobyov IV, Huang N, Roux B, MacKerell AD (2005) Determination of electrostatic parameters for a polarizable force field based on the classical Drude oscillator. J Chem Theory Comput 1 (1) 153—168... 

When the two-step process is repeated on the same material, the thermal treatment following the chemical dealumina t ion results in further expulsion of aluminum from the framework into zeolite cages or channels. The solubilization of non-framework aluminum during the first chemical treatment appears to facilitate further framework dealumination during the subsequent thermal treatment due to the altered steric and electrostatic parameters in the zeolite channels. The newly formed non-framework aluminum species can be readily solubilized by acid treatment. This cyclic method has allowed the almost total removal of aluminum from mordenite (5). 

The approach to the mathematical definition of the interface model is very simple. For every layer in the interface, the charge is defined once as a function of chemical parameters and once as a function of electrostatic parameters. The functions for charge are set equal to each other and solved for the unknown electrochemical potentials. Mathematical techniques for solving the equations have been worked out and described in detail (9). 

Fig. 13 shows the B3LYP/6-31- -- -G -calculated optimized structures of neutral diastereomeric [E j-P ] and [E -P ] complexes. Two structures have been predicted for both the homochiral (Ihomo and IIhomo) and heterochiral (Ihetero and IIhetero) complexes. Their electrostatic parameters, binding energies and computed dipole-bound electron affinities EAdb are reported in Table 5. 

Table 5 Calculated energy value and electrostatic parameters for the optimized [E P ] isomers, Ihomo and IIhomo, and [E T ] isomers, Ihetero and IIhetero...

E. Effects of Cosolvent and Temperature on Donnan and Electrostatic Parameters of Protein Crystals... 

Zuidam, N.J., Barenholz, Y. (1997). Electrostatic parameters of cationic liposomes commonly used for gene delivery as determined by 4-heptadecyl-7-hydroxycoumarin. Biochim. Biophys. Acta, 1329, 211-222. 

From these results it would therefore appear that the bulk electrostatic parameters of the solvent, e.g. dielectric constant and dipole moment, do not predict accurately the changes in redox potential that occur in different solvents. 

The importance of solvent parameters such as DN and AN and the advantage of their use over physical-electrostatic parameters was further demonstrated by Mayer et al. [21], who studied correlations between the DN and AN of solvents and redox potentials and their chemical equilibrium and ion pair equilibria. According to the Born theory, redox potentials should depend linearly on the reciprocal of the solvent s dielectric constant. Plotting Em values of a redox such as Cd/Cd2+ versus 1/e of the solvents in which it is measured results in a very scattered picture. In contrast, it has been clearly shown by Mayer et al. [15] that redox potentials of metals (e.g., Zn/Zn2+, Cd/Cd2+, Eu/Eu2+) can be nicely... 

Theoretical studies of such big molecules are at the present time far beyond the computational possibilities, and experimental determination of electron density is the only tool to get accurate electrostatic parameters. However, it may be possible to recover the experimental electron density by performing ab initio fragment calculations [28]. 

The electrostatic parameters for generating partial charges in the calculation of dipole moments were modified to achieve better correlation with experimental... 

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