Polarization effects interaction

The 71-electrons of aromatic rings can interact with charged species, yielding strong cation-71 interactions dominated by electrostatic and polarization effects. Interactions with CH units is also possible. For CH-ti interactions in both alkyl-and aryl-based model systems, dispersion effects dominate the interaction, but the electrostatics term is also relevant for aryl CH-ti interactions. ... 

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

We will address this issue further in Chapter 10, where the polar effects of the substituents on both the c and n electrons will be considered. For the case of electrophilic aromatic substitution, where the energetics of interaction of an approaching electrophile with the 7t system determines both the rate of reaction and position of substitution, simple resonance arguments are extremely useful. 

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... 

The effective dielectric constant can be included to model the effect of surrounding molecules (solvent) and the fact that interactions between distant sites may be through part of the same molecule, i.e. a polarization effect. A value of 1 for e... 

Taft was also able to isolate steric effects. For the acid-catalyzed hydrolysis of esters in aqueous acetone, long (kJh was shown to be insensitive to polar effects. In cases where resonance interaction was absent, this value was proportional only to steric effects (and any others that are not field or resonance). The equation is... 

Synclinal and antiperiplanar conformations of the TS are possible. The two TSs are believed to be close in energy and either may be involved in individual systems. An electronic tt interaction between the stannane HOMO and the carbonyl LUMO, as well as polar effects appear to favor the synclinal TS and can overcome the unfavorable steric effects.161bi 162 Generally the synclinal TS seems to be preferred for intramolecular reactions. The steric effects that favor the antiperiplanar TS are not present in intramolecular reactions, since the aldehyde and the stannane substituents are then part of the intramolecular linkage. 

This stereoelectronic effect also explains the contrasting regioselectivity of cis- and fra x-2-fluoro-4-f-butylcyclohexanone.235 As a result of a balance between its polar effect and hyperconjugation, the net effect of a fluoro substituent in acyclic systems is small. However, in 2-fluorocyclohexanones an unfavorable dipole-dipole interaction comes into play for the cis isomer and preferential migration of the fluoro-substituted carbon is observed. 

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. 

Among the two sorts of interaction mentioned above, one of the reactants happens to be an electron-donor and the other an electron-acceptor, as is the case in most heterolytic reactions. In such cases the charge-transfer effect will perhaps predominate over the polarization effect. Even in homolytic interactions, the importance of the mutual charge transfer is not to be disregarded. 

Finally it has to be remarked briefly that the reactivity and selectivity of free radicals is certainly not only determined by steric and bond energy effects or by the thermodynamic stability of these transients. Polar effects are also important, in particular in those reactions which have early transition states e.g., the steps of free radical chain reactions12. They are either due to dipole interactions in the ground state or to charge polarization at transition states. FMO-theory apparently offers a more modern interpretation of many of these effects13. ... 

With substituents such as OH and OMe that have unshared electron pairs, an electron-donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-position, with the result that the p-substituted aniline is a stronger base than the corresponding w-compound. The m-compound is a weaker base than aniline itself, due to the electron-withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o-substituent remains somewhat anomalous, due to direct interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pAa values as follows ... 

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