Electronic effects separation

Why is the exchange phenomenon limited to primary alcohols and is there an electronic effect separate from the steric effect ... 

The question of the extent to which molecules behave ergodically [37, 316, 379, 513, 666] is particularly pertinent in mass spectrometry, where there are no intermolecular collisions to assist, or obscure the observation of, flow of energy among vibrations. The question in mass spectrometry is linked to and must involve the redistribution of electronic energy (Sect. 2.1.1), but there is advantage in discussing vibrational and electronic effects separately. 

On the other hand, the sensitivity of STM to electronic structure can lead to undesired artifacts when the surface is composed of regions of varying conductivity. For example, an area of lower conductivity will be represented as a dip in the image. If the surface is not well known, separating topographic effects from electronic effects can be difficult. 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Research on the nature of substituent constants continues, with results that can bewilder the nonspecialist. The dominant approach is a statistical one, and the main goal is to dissect substituent effects into separate electronic causes. This has led to a proliferation of terms, symbols, and conclusions. A central issue is (here we change terminology somewhat from our earlier usage) to determine the balance of field and inductive effects contributing to the observed polar electronic effect. In... 

The separation of mixtures involving N-methyl-JLtetrahydropyridines into their pure components by means of gas-liquid chromatography was discussed in a report by Holik et al. (87). They found that, using tris(/3-cyanoethoxymethyl)-y-picoline as the stationary phase, the primary factors involved in the specific retention volumes of these enamines is the electronic effect of a methyl substituent and the nitrogen atom on the carbon-carbon double bond. It was observed that 1,3-dimethyl-Zl -tetrahydropyridine (141) has a smaller specific retention volume and, hence, is eluted before... 

One can use an LFER to facilitate the separation and quantitation of the effects of different variables on the reaction. Thus, steric and electronic effects can be explored by selecting a subseries in which only one of them varies. 

It has to date been recognized that the breaking and forming of bonds in solution are in principle influenced by three major factors electronic, steric and solvent effects. Thus, in a quantitative examination to differentiate covalent and ionic bond formation, it is necessary first to investigate the electronic effect alone, separate from steric and solvent effects. 

Although it was elear that separation of an interface into surface and bulk components as in Eq, (19) is artifieial and must disappear in a consistent microscopic analysis, electronic effects were initially diseussed in terms of a compact layer and its capacitance C, It was apparent early on that the eleetrons strongly influence double layer properties [28-33],... 

The requirements for proper disposal of electronic waste appliances are as follows (a) appliances may be broken up (shredded) only if the components containing particularly hazardous substances have previously been removed and (b) since in disposing of electronic appliances the main emphasis is on the recovery of nonferrous metals, nonstripped appliances must not be shredded together with scrap cars. As a rule, electronic appliances are dismantled manually to achieve effective separation of the components containing hazardous substances. 

In many cases it is all but impossible to distinguish, separately, the operation of electronic and steric effects, as they often both operate towards the same end result. Except where crowding becomes extreme, however, it seems likely that the electronic effects are commonly in control. 

Due to the different role of a- and re-effects, we will separate the general description of electronic effects into a- and re-effects in this study. c-Effects include... 

Another approach to evaluating homoaromaticity is to compute various reaction properties such as heats of reaction. A typical example of this approach is a recent paper by Storer and Houk (1992) using molecular mechanics calculations (MM2) of the heats of hydrogenation of triquinacene [118]. In this study they conclude that the anomalous heat of hydrogenation can be explained without invoking homoaromaticity. The use of this type of computational data suffers the same problems as experimentally measured values there is an ambiguity with regard to separation of structural and electronic effects and how to choose appropriate reference systems. 

Substrates 4-9 were chosen by Freeman to study the influence of steric bulk on the free activation energy. As discussed before the substituents also show an electronic effect and it is hard to separate both effects, but at least some comparisons can be made, e.g., for trans-crotonic acid 4 and 4,4-dimethyl-trans-2-pentenoic acid 9. The steric bulk of the t.-butyl group compared to a methyl group should be by far more important than the difference in the electronic effect. 

Two separate computational investigations of the Rh/dppms catalyst and related systems have appeared in the literature. One study [40] concluded that steric effects were important in promoting migratory CO insertion in [Rh(CO)(dppms)l2Me], while the other [41] proposed that an electronic effect, arising from the sulfur donor atom of dppms, was responsible. It is likely that a combination of steric and electronic effects result in the observed reactivity. 

At constant pH, the parameter p is dependent on the electronic effects of the nitrosoarene substituent(s). Using only definite acceptor substituted nitrosoarenes, a Hammett correlation on the a scale was obtained25. Separate investigation of fc2RSH and fc2(rearr) for a wider selection of nitrosoarenes reacting with GSH revealed reasonable correlation with Hammett a constants only for fc2RSH (p = +1.4). However, Utrearr) was found to fit better on the Hammett a+ scale (p+ = —3.5)30 [for further discussion of Utrearr) see Section II.D.l.b]. 

Topsom, 1976) and to treat them separately. In this review we will be concerned solely with polar or electronic substituent effects. Although it is possible to define a number of different electronic effects (field effects, CT-inductive effects, jt-inductive effects, Jt-field effects, resonance effects), it is customary to use a dual substituent parameter scale, in which one parameter describes the polarity of a substituent and the other the charge transfer (resonance) (Topsom, 1976). In terms of molecular orbital theory, particularly in the form of perturbation theory, this corresponds to a separate evaluation of charge (inductive) and overlap (resonance) effects. This is reflected in the Klopman-Salem theory (Devaquet and Salem, 1969 Klop-man, 1968 Salem, 1968) and in our theory (Sustmann and Binsch, 1971, 1972 Sustmann and Vahrenholt, 1973). A related treatment of substituent effects has been proposed by Godfrey (Duerden and Godfrey, 1980). 

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