Polarization, electrostatic

Another method that has been applied by our group to the study of enzymatic reactions is the Effective Fragment Potential (EFP) method [19]. The EFP method (developed at Mark Gordon s group at Iowa State University) allows the explicit inclusion of environment effects in quantum chemical calculations. The solvent, which may consist of discrete solvent molecules, protein fragments or other material, is treated explicitly using a model potential that incorporates electrostatics, polarization, and exchange repulsion effects. The solute, which can include some... 

Kaminski GA, Friesner RA, Zhou RH (2003) A computationally inexpensive modification of the point dipole electrostatic polarization model for molecular simulations. J Comput Chem 24(3) 267-276... 

Bayliss, N.S. 1950. The effect of the electrostatic polarization of the solvent on the electronic absorption spectra in solution. J. Chem. Phys. 18 292-296. 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

The new treatment had its origins partly in ab initio molecular orbital calculations of substituent effects and partly in extensive studies of gas-phase proton transfer reactions from about 1980 (Section V.A). Various aspects of this work essentially drew attention to the importance of substituent polarizability. In 1986 Taft, Topsom and their colleagues252 developed a scale of directional substituent polarizability parameters , oa, by ab initio calculations of directional electrostatic polarization potentials at the 3-21G//3-31G level for a large set of CH3X molecules. The oa values were shown to be useful in the correlation analysis of gas-phase acidities of several series of substrates252, and such work has subsequently been extended by Taft and Topsom151. 

Bayliss, N. S. The Effect of the Electrostatic Polarization of the Solvent on Electronic Absorbtion Spectra. J. Chem. Phys. 18, 292 (1950). 

Another contrast between the zinc proteases and the carbonic an-hydrases concerns the zinc coordination polyhedron. The carbonic an-hydrases ligate zinc via three histidine residues, whereas the zinc proteases ligate the metal ion through two histidine residues and a glutamate (bidentate in carboxypeptidase A, unidentate in thermolysin). Hence, the fourth ligand on each catalytic zinc ion, a solvent molecule, experiences enhanced electrostatic polarization in carbonic anhydrase II relative to carboxypeptidase A. Indeed, the zinc-bound solvent of carbonic anhydrase II is actually the hydroxide anion [via a proton transfer step mediated by His-64 (for a review see Silverman and Lindskog, 1988)]. 

The local version of EHCF method was implemented and used for the analysis of the molecular geometries of complexes of iron (II) in works [29, 148,149]. The satisfactory agreement in the description of complexes geometry with different total spins is achieved when the effect of electrostatic field of the metal ion on the ligands is taken into account through the electrostatic polarization of the ligands. Satisfactory estimates of parameters... 

This observation may result from an usual acceptor effect according to which AE can be identified with the energetic position of acceptors in the forbidden gap. It should be mentioned here that, according to Lyons 75>, AE is connected by Eqs. (52) and (53) with the electrostatic polarization energy P. 

Electret. The electrical equivalent of a permanent magnet. When a block of dielectric material, such as carnauba wax, is allowed to be solidified in a strong electric field it acquires a permanent state of electrostatic polarization (orientation of molecules) in the direction of the field. 

Eq. 9.56 deals with a simplified situation the approach of A and B to the bond distance with a resultant EA U electrostatic energy based on the inherent electrostatic bonding capabilities (dipole-dipole interactions, etc.) and a resultant CACB term based on the inherent covalent bonding capabilities (related to overlap, etc.) This approximation is quite good for neutral species, arid small discrepancies (such as the increase in covalency through electrostatic polarization) could be (and have been) accommodated by incorporating them into the E and C parameters.34... 

The solvation free energy is usually considered to consist of a solvent-solvent cavity term (Gcav), a solute-solvent van der Waals term (Gvsolute-solvent electrostatic polarization term (Ges) (Eq. 2.37). 

In a recent approach that was successfully tested for small hydrocarbons the solvation was treated semianalytically as a statistical continuum1831. The method treats the sum of the solvent-solvent cavity (Gcav) and the solute-solvent van der Waals (Gvdw) terms by determining the solvent-accessible surface1841, and the solute-solvent electrostatic polarization term (Ges) is calculated by a modified version of the generalized Bom equation183,851. 

To summarize, in this chapter, we have seen that the mixed solvents. They express the affinity of a solvent to electrostatic polarities, hb-donors, hb-acceptors, and to nonpolar surfaces, and include the concept of hydrophobicity in a natural way. 

It is true that all molecular and atomic forces ultimately find their root in the mutual behavior of the constituent parts of the atoms, viz., the nuclei and the electrons. They may theoretically all be derived from the fundamental wave equations. It is, however, convenient, as in other branches of physics and chemistry, to treat the various forms of mutual interaction of atoms as different forces, acting independently. We shall therefore follow the usual procedure and treat such forces as the nonpolar van der Waals (dispersion) forces, the forces of the electrostatic polarization of atoms or molecules by ions or by dipoles, the mutual attraction or repulsion Coulomb forces of ions and of dipoles, the exchange forces leading to covalent bonds, the repulsion forces due to interpenetration of electronic clouds, together with the Pauli principle, etc., all as different, independently acting forces. 

All these contributions, however, are certainly low. It is only at active spots, as already mentioned in Sec. V,4, that the electrostatic polar-... 

There have been numerous theoretical and experimental investigations on the adsorption of argon, oxygen, and nitrogen on potassium chloride (126-128) and in this connection we may refer to a survey in Brunauer s book on physical adsorption (129). There seems to be a general agreement that the most favorable positions for the adsorbed atoms or molecules will be found just above the center of a lattice cell. The electrostatic polarization is minimum at such spots, but the nonpolar van der Waals forces are at their maximum and dominate (130). Drain... 

We saw in Sec. V,5 that molecules with peripheral dipoles, such as OH, Nil, and, COOH groups are attracted strongly by the electrostatic field of the surface. These dipole forces form the most important contributions toward the adsorption energy when such molecules are adsorbed on ionic surfaces, nonpolar van der Waals forces and electrostatic polarization giving smaller contributions (133). 

A further analysis of the various contributions toward the adsorption energies (135) has revealed that the adsorption energy of alcohol on rutile consists mainly of the contribution of the attraction of the dipole the nonpolar van der Waals forces contribute less than 40% of this part and electrostatic polarization less than 10%. The adsorption energies of hydrocarbons on rutile are mostly due to the van der Waals forces, and half the amount of the van der Waals contribution (one third of the total) originates from the electrostatic polarization. 

Fio. 25. Isosteric heats of adsorption at 0°K. for nitrogen on rutile (curve A) and for argon on rutile (curve B) contribution due to the quadrupole attraction of nitrogen (curve C) contributions due to the electrostatic polarization of argon (curve D) and to the dispersion forces (curve E) [data from Morrison el al. (280)]. 

We see that at low 0 values the electrostatic polarization is more important than the attraction by van der Waals forces. This would, according to our interpretation, mean that the electrostatic fields force the molecules to be adsorbed on their active spots and not on the active spots of the van der Waals forces. The antagonism of the activities of both kinds of active spots is in this picture reflected in the increase of the van der Waals contributions (curve E) with increasing degree of occupation. 

The undeniable fact that the surface may show a dominating heterogeneity for physical adsorption, hence for van der Waals attraction forces or for electrostatic polarization by local fields of the surface (Secs. VIII,2 and 3), does not mean that they should be heterogeneous for chemisorption as well. As was stated in Sec. V,12 the forces between ions and metal surfaces and the covalent forces between chemisorbed atoms or molecules and metal surfaces are far less influenced by active places of the surface than are some of the forces leading to physical adsorption. It is especially the cracks and fissures of the surface, which may give it a pronounced heterogeneous character for physical adsorption, that do not influence the chemisorption bonds very much (274). 

In this regard, the electrostatic polarization term arises in the case of nonpolar molecules. These nonpolar molecules when within an electric field are polarized, and afterward produce an induced dipole moment. This induced dipole moment, ph interacts with the adsorbent, and the interaction potential is given by... 

It is important to note at this point the quantitative effect on the energy surfaces of solute-solvent interactions. As already anticipated, the dominant effect is due to electrostatic polarization in fact, in the range of distances of Fig. 3, the fluctuations of dispersion, repulsion and cavitation contributions are respectively 0.3 kcal/mol, 1.1 kcal/mol and 0.9 kcal/mol, which must be compared with 100 kcal/mol, the order of magnitude of variation of polarization in this case. 

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