Electrostatic polar effects

These results obtained with modern computational methods demonstrate that the concept of confinement is now widely invoked but not yet clearly quantified, differing with the level of accuracy and approximation. The relative influence of the local structure on the Bronsted acid strengths (large vs. small cavities), of the solid-reactant dispersion interactions, and of electrostatic polarization effects on the reactants has not reaUy been quantified yet. We believe that such a study necessitates first to set up an accurate and also efficient methodology in order to answer the following fundamental questions ... 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

A number of groups have criticized the ideas of Dauben and Noyce, especially the concept of PDC. Kamernitzsky and Akhrem, " in a thorough survey of the stereochemistry of addition reactions to carbonyl groups, accepted the existence of SAC but not of PDC. They point out that the reactions involve low energies of activation (10-13 kcal/mole) and suggest that differences in stereochemistry involve differences in entropies of activation. The effect favoring the equatorial alcohols is attributed to an electrostatic or polar factor (see also ref. 189) which may be determined by a difference in the electrostatic fields on the upper and lower sides of the carbonyl double bond, connected, for example, with the uncompensated dipole moments of the C—H bonds. The way this polar effect is supposed to influence the attack of the hydride is not made clear. 

Although the nature of the general polar effect suggested by Kamernitzsky and Akhrem " to account for axial attack in unhindered ketones is not clear, several groups have reported electrostatic interactions affect the course of borohydride reductions. Thus the keto acid (5a) is not reduced by boro-hydride but its ester (5b) is reduced rapidly further, the reduction of the ester (6b) takes place much more rapidly than that of the acid (6a). Spectroscopic data eliminate the possibility that in (5a) there is an interaction between the acid and ketone groups (e.g. formation of a lactol). The results have been attributed to a direct repulsion by the carboxylate ion as the borohydride ion approaches. " By contrast, House and co-workers observed no electrostatic effect on the stereochemistry of reduction of the keto acid (7). However, in this compound the acid group may occupy conformations in which it does not shield the ketone. Henbest reported that substituting chlorine... 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

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... 

There are two ways of handling the interaction between the QM region and MM region one way is to calculate electrostatic QM-MM interaction with the MM method (sometimes called mechanical embedding, or ME) and the other is to include the QM-MM interaction in the QM Hamiltonian (called electronic embedding or EE). The major difference is that in the ME scheme the QM wave function is the same in the gas phase and the electrostatic interaction is included classically, while in the EE scheme the QM wave function is polarized by the MM charges. The EE scheme is substantially more expensive than ME scheme, as the SCF iteration needs to be performed until self-consistency is achieved for QM electron distribution. Although the polarization effects are called important, as we will show later,... 

That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

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. 

In order to quantify the importance of polarization effects in describing the electrostatic field of the system we compared this... 

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). 

Graf (G8) measured the pressure buildup due to electrostatic charging of stationary drops. He found this pressure to be some 30-60% of the value given by Eq. (4), independent of polarity of charge or dielectric constant of the liquid. This factor, which he terms a charging factor, was a function of both geometry and liquid properties. It may also reflect polarization effects that are neglected in Eq. (4). 

The effect of the relative electronegativity of a substituent (R) and/or the delocalization of electrons on a chemical property of a substance. Thus, for a molecule R—R, the polar effect refers to all nonsteric influences and modifications of electrostatic forces operating at the reaction center (R ), relative to some standard molecule, Rq—R. Hence, the term is synonymous with the electronic effect. 

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... 

The carbonyl group carbon shows the largest upfield shift due to the electrostatic field effect in the dioxolane and dihydroxy derivatives this is countered by the polarization of the free oxygen electron pairs. 

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. 

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