Electrostatic field effects

It has been reported that the selectivity in one case is in accord with a pure electrostatic (field effect) explanation Dneprovskii, A.S. Mil tsov, S.A. Arbuzov, P.V. J. Org. Chem. USSR, 1988,24,1826. See also Tanner, D.D. Arhart, R. Meintzer, C.P. Tetrahedron, 1985, 41,426V, Ref. 113. 

Naray-Szab6, G. 1983. Unusually Large Electrostatic Field Effect of the Buried Aspartate in Serine Proteinases Source of Catalytic Power. Int. J. Quant. Chem. 23, 723. 

By exploiting electrostatic field effects (unfavourable through-space charge-dipole repulsion) to increase the nucleophilic susceptibility of cyclohexanones, more efficient catalysts (16) and (17) for epoxidafion through in situ dioxirane formation have been designed. ... 

The importance of electrostatic effects in protein structure and function has long been recognized (Tanford and Kirkwood, 1957 Peratz, 1978 Warshel and Russell, 1984 Rogers, 1989). It is generally believed that electrostatic field effects are among the first that come into play when two protein molecules approach each other. The reason for this is that electrostatic fields reach much further out than any other force fields that we associate with protein-protein interaction. It is reasonable to expect that motion in the combined electrostatic field of both molecules helps to orient the molecules for subsequent docking. 

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. 

Kim, E. I., Paliwal, S., Wilcox, C. S., Measurements of molecular electrostatic field effects in edge- to-face aromatic interactions and CH-p interactions with implications for protein folding and molecular recognition. J. Am. Chem. Soc. 

Although the interactions of charged, dipolar or polarizable groups have been investigated for various purposes, they have not often been utilized in the context of stereoselectivity. In fact, when coulombic effects were considered in the SN2 or E2 processes, their role was regarded as unimportant (Ingold, 1953 Cristol, et al., 1951). In view of the substantial electrostatic (field) effects estimated for polar substituents on the pA s of carboxylic acids (Tanford, 1958), metal-ion coordination (Basolo and Pearson, 1967), etc., it will be interesting to see what effects there may be on SS. 

The reduction potentials ofthe core iron of A. vinelandii and horse spleen ferritins have been determined electrochemically to be -420 20 mV and -190 mV, respectively, at pH 7 116,117 -pjjg former potential was invariant over the pH range 7.0-9.0 while the latter potential decreased with increasing pH. The Fe +/ + reduction potential at pH 8 of the heme of A. vinelandii ferritin is -475 mV in the presence of the core and -225 mV in its absence. Since magneto-optical spectroscopic properties of the heme are not affected by the core this shift in potential has been ascribed to a core electrostatic field effect. ... 

The double layer theory describes the process of ion-exchange and ion-interaction chromatography from the point view of distributed electrostatic field effect on the charged analyte retention. Other approaches have a more stoichiometric character and describes the analyte retention on the basis of the formation of ionic pairs and their subsequent retention on the adsorbent surface. 

The subscript s denotes isolated ions on the surface. The intrinsic equilibrium constants (K are determined at zero charge and potential conditions, thereby eliminating the electrostatic field effects. 

The rates of saponification of lya-hydroxy-so-oxo-ai-acetoxy steroids and the rates of oxidation of i7a,2i-dihy-droxy-20"Oxo steroids have been examined in relation to the effect of changes in the structures of ring A and C. Again it seems Ukely that electrostatic field effects are responsible for the observed rate differences [ ]. 

A substituent in the steroid molecule can influence reactions at remote centres in three distinct ways, which may be classified as (i) inductive effects (2) electrostatic field effects and (3) conformational transmission. It is difficult to disentangle these effects completely, for all are likely to be operative to some extent, even though for a partioolar reaction one or another may be dominant. The limited number of detailed investigations so far reported make it unwise to generalise at present. 

In an analysis of the influence of charged substituents on the pK values of carboxylic acids, Bjerrum (1923) suggested that, for symmetrical proton transfer reactions such as Equation (4), pK differences can be accounted for in terms of purely electrostatic field effects (and statistical effects, whenever they are warranted). If the proton and charged substituent are treated as point charges separated by a distance r in a solvent continuum whose dielectric constant D is that of the bulk solvent, the work required to transfer the proton from r to infinity in the electrostatic field of the charged substituent can be calculated from classical electrostatic theory. The pK difference between the unsubstituted and substituted acids (ApK) is then given by... 

Schroeder et al (Refs 4—6 8), as a corollary to the study of Tarver et al, have proceeded with a research program in quantum chemistry in an attempt to relate the ballistic and expl properties of compds such as the tetrazoles with their calcd and exptly derived molecular properties and to understand the chemistry of these compds. They reported (ca 1975) progress in their study in that the apparent greater electron withdrawing ability of the 1- rather than the 2- substituted 5-tetrazolyl groups is ... due to both resonance and electrostatic field effects. .. (Ref 8, p 19)... 

The equilibrium between 2)6,3i5-disubstituted 5a- and 5)5-6-ketones (12) shows a surprising and unexplained dependence upon the nature of the C-17 substituent. The presence of an axial 2) -substituent destabilizes the 5a-isomer, shifting the equilibrium towards the 5jS-isomer where the 2)5,10/5 diaxial interaction is relieved. The influence of the C-17 substituent on the equilibrium varied, over a series of eight different compounds, between the extremes represented by 5 5a ratios of 0.13 (17/5-OH) and 7.06 (17/5-COMe). Steric effects, conformational transmission, and inductive and electrostatic field effects are each discussed and dismissed as being incapable of providing an adequate explanation of the data. 

Esters, Ethers, and Related Derivatives of Alcohols.— The mode of transmission of the effect of remote substituents upon reaction rates and equilibria has been discussed previously under three headings, viz. inductive effects, electrostatic field effects, and conformational transmission.A new survey,quoting over 50 references, covers most of the main studies in this field, and suggests direct interactions as a fourth class. Rates of acetylation of 3j3-hydroxy-A"-steroids variously substituted at C-17 show only small variations, which did not permit of any mechanistic interpretation. The formation of 17a,20- and 20,21-cyclic carbonates and their use as protecting groups for diols have been investigated. ... 

High values of the epoxidation rate of 2-furylmethyl 3-cyclohexenecarboxy-late (XII) cannot be explained by the induction effect of the substituent. Apart from this effect, which is transmitted through the 8-hnkages and through the space inside and around the rings (the electrostatic field effect), there are steric effects and the specific solvatation of the XII molecule with acetic acid. 

A most widely used method for the prediction of molecular dipole moments is the empirical vector addition of bond moments (45). It involves the concept of point dipoles situated in the directions of the individual bonds artd assumes that the magnitudes of the dipoles are transferable from one molecule to another. For allenes a somewhat modified approach has been established which is of relevance in connection with the discussion of influences of electrostatic field effects on spectroscopic properties of allenes (24 25) (Section III.D). This model uses fixed origines and Hxed orientations for all the different (point dipole) bond moments in substituted allenes. As the origins of the point dipoles the positions of the hydrogen atoms in allene (11) have been used (24). The directions of the dipole moments are assumed to make an angle of 40 with the C==C=C axis (Fig. 6) which represents an average value of experimental directions of dipole moments in differently substituted allenes (24). 

From Equation 101 (in connection with the values from Fig. 31) one can see that the ionization energies of the W(3) orbitals of allenes must be essentially determined by a long-range electrostatic field effect, as, apart from the small resonance contribution, also the short-range effect must be rather small (c, 0.02). 

Introducing as the angle between the vectors r and i(R) one can write for the electrostatic field effect of the substituent... 

CNDO/S calculations support the assumption that the last mentioned term remains constant. A discussion of the relative importance of polar and resonance substituent effects on the uv bands in allenes therefore would be bound to underestimate the relevance of electrostatic field effects, as such a treatment would reflect the situation expressed by Equation 109. 

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