Charge controlled interaction

If the MO energy differences - El are large enough and/or the resonance integrals P , (for appropriate nonzero coefficients c and c,J are small, then the second term in Eq. [7] is small, and the interaction is determined mainly by the first (electrostatic) term (charge controlled interaction). In this case, Eq. [7] can be simplified... 

Considering the interaction of a series of related compounds with a given receptor, the electronic parameters q, and c, remain the same, we obtain for charge controlled interactions y -> 0)... 

These concepts play an important role in the Hard and Soft Acid and Base (HSAB) principle, which states that hard acids prefer to react with hard bases, and vice versa. By means of Koopmann s theorem (Section 3.4) the hardness is related to the HOMO-LUMO energy difference, i.e. a small gap indicates a soft molecule. From second-order perturbation theory it also follows that a small gap between occupied and unoccupied orbitals will give a large contribution to the polarizability (Section 10.6), i.e. softness is a measure of how easily the electron density can be distorted by external fields, for example those generated by another molecule. In terms of the perturbation equation (15.1), a hard-hard interaction is primarily charge controlled, while a soft-soft interaction is orbital controlled. Both FMO and HSAB theories may be considered as being limiting cases of chemical reactivity described by the Fukui ftinction. 

Self-consistent field molecular orbital calculations by Fenske and coworkers have confirmed that nucleophilic additions to Fischer and related complexes [e.g., (CO)sCr=CXY, (T)5-C5H5)(CO)2Mn=CXY], are frontier orbital-controlled rather than charge-controlled reactions (7-9). Interaction of the HOMO of the nucleophile with the carbene complex LUMO (localized on Ca) destroys the metal-carbon w-interaction and converts the bond to a single one. 

Another difficulty exists. It has been found that in the gas phase or in dipolar aprotic solutions, small anions (cations) have higher HOMO s (lower LUMO s) than the larger one. It follows that interaction between two hard ions is then favoured by both charge control and frontier control. Therefore, the equivalences suggested by Klopman ... 

Lewis acids are atoms or molecules that accept an electron pair (103). Soft acids have polarizable valence electrons, whereas hard acids do not. Lewis bases are atoms or molecules that are capable of donating an electron pair. Soft bases are polarizable, whereas hard ones are not. Hard acids preferentially interact with hard bases the interaction is mainly ionic and charge controlled (104). Conversely, soft acids tend to interact (or react) with soft bases the interaction has covalent character and may be orbital controlled. 

Interaction between the components in the encounter pair B.X does not necessarily favour either the diffusion-controlled or the pre-association mechanism but, by prolonging the lifetime of the encounter pair, may help to make the formation of this encounter pair the rate-determining stage of the overall reaction.22 Thus, charge-transfer interaction between the components in the encounter pair ArH.NOt may be one factor leading to the easily observed... 

Charge control becomes dominant mostly in compounds containing fluorine or in rigid systems. This can be expected from the foregoing discussion. More interestingly, electrostatic interactions appear to play a prominent role in silicon compounds.99... 

Care must be taken in generalizing gas-phase computational results, which tend to overemphasize charge control while underemphasizing frontier control. Indeed, in the absence of solvent, coulombic forces, which vary as the inverse square of the distance, are stronger at large distances than frontier interactions, which diminish exponentially with the separation. The reaction then becomes charge controlled with an early transition state in which the reagents may still be in their most stable conformations. The role of the conformational effect is then somewhat overstated. This is especially true in older calculations in which O or Cl were replaced by F and Na+ and K+ were modeled by Li+. 

Charge control dominates (a) when the HOMO-LUMO gap is very large (p. 97 see also Exercise 21, p. 230) (b) in molecules whose rigidity prohibits good frontier orbital overlap (p. 156) (c) in very loose transition states (because the Coulomb interaction falls as a function of R 1 whereas the frontier orbital overlap falls exponentially with distance) (d) in the gas phase (p. 99).2 Early transition states (which are not necessarily loose ) can be governed by either charge or frontier orbital factors. 

In the context of computational toxicology, quantum chemical descriptors provide distinct probes to unravel mechanistic causes for the hazardous effects of chemical substances. At the same time, the level of theory employed may be crucial for the molecular property under analysis, which is particularly true for descriptors based on net atomic charges (that, in turn, are not physically observable, despite their intuitive meaning for charge-controlled intermolecular interactions). 

The key point of interest in electrosorption (which the above cited arguments do not take into account) is whether—because such a double layer is formed even when an electric field is not applied—the application of an electric field can enhance or reverse the double-layer-controlled interactions between the charged adsorbent and the adsorbate ions in solution (see Figure 5.6). Therefore, the key issue is whether the following arguments are really applicable when it comes to determining the total capacity of electrosorption [148] ... 

Williams et al. (96JOC1927), by semiempirical AMI calculations, studied the 1,3-di-axial conformation of cA-l,3-diarylcyclohexane in order to investigate in detail arene-arene yr-stacking. Consistent with the experimental results, arene-arene yr-stacking is controlled primarily by electrostatic rather than charge transfer interactions. 

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