Bases Lewis model

Drago and co-workers Introduced an empirical correlation to calculate the enthalpy of adduct formation of Lewis acids and bases ( 5). In 1971, he and his co-workers expanded the concept to a computer-fitted set of parameters that accurately correlated over 200 enthalpies of adduct formation ( ). These parameters were then used to predict over 1200 enthalpies of interaction. The parameters E and C are loosely Interpreted to relate to the degree of electrostatic and covalent nature of the Interaction between the acids and bases. This model was used to generalize the observations involved in the Pearson hard-soft acid-base model and render it more quantitatively accurate. 

For example, students develop an elementary understanding of bonding from the Lewis model. Then they refine it through the valence bond model and finally molecular orbital theory. Some exercises challenge students to refine models further—and to develop new ones. Students will see how current chemical knowledge is based on the authority—and the fallibility—of modern experimental techniques. 

In addition, the two-iron Schiff s base compounds studied by Lewis et al. (160—162) have magnetic properties which indicate a structure which may be similar to that in the active centers of the plant-type ferredoxins. The following arguments set forth criteria on which to base any model for the active site ... 

Enzymatic enantioselectivity in organic solvents can be markedly enhanced by temporarily enlarging the substrate via salt formation (Ke, 1999). In addition to its size, the stereochemistry of the counterion can greatly affect the enantioselectivity enhancement (Shin, 2000). In the Pseudomonas cepacia lipase-catalyzed propanolysis of phenylalanine methyl ester (Phe-OMe) in anhydrous acetonitrile, the E value of 5.8 doubled when the Phe-OMe/(S)-mandelate salt was used as a substrate instead of the free ester, and rose sevenfold with (K)-maridelic acid as a Briansted-Lewis acid. Similar effects were observed with other bulky, but not with petite, counterions. The greatest enhancement was afforded by 10-camphorsulfonic acid the E value increased to 18 2 for a salt with its K-enanliomer and jumped to 53 4 for the S. These effects, also observed in other solvents, were explained by means of structure-based molecular modeling of the lipase-bound transition states of the substrate enantiomers and their diastereomeric salts. 

The Laplacian of the electron density plays a dominant role throughout the theory.191 In addition, Bader has shown that the topology of the Laplacian recovers the Lewis model of the electron pair, a model that is not evident in the topology of the electron density itself. The Laplacian of the density thus provides a physical valence-shell electron pair repulsion (VSEPR) basis for the model of molecular geometry and for the prediction of the reaction sites and their relative alignment in acid-base reactions. This work is closely tied to earlier studies by Bader of the electron pair density, demonstrating that the spatial localization of electrons is a result of a corresponding localization of the Fermi correlation hole. 

The so-called rate-based stage model presents a different way to the modeling of separation processes, by directly considering actual mass and heat transfer rates (Seader, 1989 Taylor and Krishna, 1993). A number of models fall into the general framework of the rate-based stage. In most cases, the film (Lewis and Whitman, 1924) or penetration and surface renewal (Higbie, 1935 Danckwerts, 1951) models find application, whereas the necessary model parameters are estimated by means of correlations. In this respect, the film model appears advantageous due to numerous correlation data available in the literature (see, e.g., Billet and Schultes, 1999). 

Laplacian of the charge density V p(r), along with p(r) and Vp(r), serve to define the conceptual models of chemistry and they provide the necessary basis for the theoretical description of these models. The Laplacian of the charge density V p(r), as demonstrated in the two preceding chapters, plays a dominant role throughout the theory of atoms in molecules. It is shown here that the Laplacian provides a link between theory and the chemical models of geometry and reactivity that are based upon the Lewis model. 

McKim, J. and J. Nichols. Use of physiologically based toxicokinetic models in a mechanistic approach to aquatic toxicology. In Aquatic Toxicology Molecular, Biochemical and Cellular Perspectives, edited by D.C. Malins and G.K. Ostrander, Boca Raton, FL, Lewis Publishers, pp. 469-519, 1994. 

Boron-nitrogen and boron-phosphorous compounds are classical textbook examples of donor-acceptor complexes. The chemical bonds of the Lewis-acid Lewis-base complexes are usually explained in terms of frontier orbital interactions and/or quasiclassical electrostatic attraction in the framework of the Hard and Soft Acid and Base (HSAB) model [73]. We were interested in seeing if the differences between the bond strengths of boron-nitrogen and boron-phosphorous complexes for different boranes, amines and phosphanes can be explained with the EDA method. 

Note that Bronsted-Lowry acid-base reactions (proton donor-proton acceptor reactions) are encompassed by the Lewis model. For example, the reaction between a proton and an ammonia molecule, that is. 

The real value of the Lewis model for acids and bases is that it covers many reactions that do not involve Bronsted-Lowry acids. For example, consider the gas-phase reaction between boron trifluoride and ammonia ... 

I Compare the Arrhenius, Bronsted-Lowry, and Lewis models of acids and bases. 

Perhaps you will not be surprised, then, you to learn that an even more general model of acids and bases was proposed by American chemist G. N. Lewis (1875-1946). Recall that Lewis developed the electron-pair theory of chemical bonding and introduced Lewis structures to keep track of the electrons in atoms and molecules. He applied his electron-pair theory of chemical bonding to acid-base reactions. Lewis proposed that an acid is an ion or molecule with a vacant atomic orbital that can accept (share) an electron pair. A base is an ion or molecule with a lone electron pair that it can donate (share). According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Note that the Lewis model includes all the substances classified as Bronsted-Lowry acids and bases and many more. 

Lewis model (p. 641) An acid is an electron-pair acceptor and a base is an electro-pair donor. 

Lewis model In this model, acids are electron pair acceptors and bases are electron pair donors. 

It s important to understand the distinction between these two models. The Brqnsted-Lowry model was developed when acids and bases were thought to work in aqueous solvents. As a result, it deals only with hydrogen and hydroxyl groups. On the other hand, the Lewis model was developed to show what happens when water isn t the solvent, so it deals with electrons instead. 

In the Lewis model, an acid accepts electron pairs, whereas a base donates electron pairs. Any proton is a Lewis acid, whereas ammonia, for example, is a Lewis base, because the lone pairs of the nitrogen are donated to a proton as shown in the following equation. (In this example the backward pointing arrow is used to signify that the electrons are being donated.)... 

In the Lewis model an Acid is an electron acceptor, and the Base is an electron donor. 

Lewis acid-base theory (Jensen, 1980) is an outgrowth of the Lewis model of chemical bonds. A Lewis acid is a chemical species that can accept an electron pair. Lewis acids can be cations like Fe or Cu or they can be species with empty or partially empty valence orbitals such as CO2 or SO2. Lewis bases can donate an electron pair. Lewis bases are anions like OH" or S or they can be species with lone pairs such as HjO or NHj. The transfer of cations from a solid, such as szomolnokite (FeS04 H20), to form a hydrated ferrous ion in solution is a typical Lewis acid-base reaction. 

Lewis and Kossel s proposals coincided with the shell structure of atoms which resulted firom the hybrid classical/quantum model for the hydrogen atom developed by Bohr [37, 38] and subsequently extended by Sommerfeld [39 1] to other atoms. They did not fully appreciate the physical imphcations of a quantum model. Specifically Lewis based his model on the following postulates ... 

On the other hand, the VB model developed by Pauling was based on a more quantum-mechanical-based model of valence electrons. Instead of simply putting electrons into spatially separated regions (domains), atomic orbitals are involved in the discussion, and all bonding interactions are considered as overlaps between orbitals of different atoms. This approach has been very useful in bridging the Lewis model to the more modem molecular orbital treatment, and similar to VSEPR, the hybridization scheme developed by Pauling has provided an important theoretical underpinning for chemistry over the years. 

The Lewis concept of acids and bases includes proton-transfer reactions all Bronsted-Lowry bases (proton acceptors) are also Lewis bases, and all Bronsted-Lowry acids (proton donors) are also Lewis acids. The Lewis model, however, is more general in that it is not restricted to proton-transfer reactions. 

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