Hard-soft acid-base interaction

The shifts in lEP were observed for all salts, but only some Na and Li salts have an ability to reverse the sign of C to positive over the entire pH range (Figs. 3.102 and 3.103). The critical salt concentration inducing such an effect depends on the nature of the anion, and hard-soft acid-base interactions have been invoked to explain these effects. Small cations (Na, Li ) and large anions (T) show a... 

Solvation of polyaniline is a result of a number of interactions between polymer dopant and solvent (for example. Bom-type solvation, hydrogen bonding, hard-soft acid base interactions, Lewis acid base (donor-acceptor) interactions). 

The hard-soft acid-base principle is not restricted to the usual types of acid-base reactions. It is a guiding principle that for all types of interactions species of similar electronic character interact best. We have already seen some applications (such as the relative strength of HF and HI) of this principle, which we will continue to call HSAB, but we now consider a number of other types of applications. 

The hard-soft acid-base principle just illustrated is one of the most useful principles in all of chemistry for predicting how many types of interactions occur, ft is not restricted to acid-base interactions, so it is better called the hard-soft interaction principle, ft predicts that hard acids (high charge, small size, low... 

Although sometimes referred to as the hard-soft acid-base theory, it is actually a principle that relates to many types of chemical interactions. It provides a good explanation of why HF is a weak acid. If H+ might potentially interact with either H20 or F, the situation with regard to the preferred bonding mode could be shown as follows ... 

In Chapter 9, the hard-soft acid-base principle was discussed, and numerous applications of the principle were presented. This principle is also of enormous importance in coordination chemistry. First-row transition metals in high oxidation states have the characteristics of hard Lewis acids (small size and high charge). Consequently, ions such as Cr3+, Fe3+, and Co3+ are hard Lewis acids that bond best to hard Lewis bases. When presented with the opportunity to bond to NH3 or PR3, these metal ions bond better to NH3, which is the harder base. On the other hand, Cd2+ bonds better to PR3 because of the more favorable soft acid-soft base interaction. 

This chapter is intended to provide basic understanding and application of the effect of electric field on the reactivity descriptors. Section 25.2 will focus on the definitions of reactivity descriptors used to understand the chemical reactivity, along with the local hard-soft acid-base (HSAB) semiquantitative model for calculating interaction energy. In Section 25.3, we will discuss specifically the theory behind the effects of external electric field on reactivity descriptors. Some numerical results will be presented in Section 25.4. Along with that in Section 25.5, we would like to discuss the work describing the effect of other perturbation parameters. In Section 25.6, we would present our conclusions and prospects. 

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. 

Different surfactants are usually characterised by the solubility behaviour of their hydrophilic and hydrophobic molecule fraction in polar solvents, expressed by the HLB-value (hydrophilic-lipophilic-balance) of the surfactant. The HLB-value of a specific surfactant is often listed by the producer or can be easily calculated from listed increments [67]. If the water in a microemulsion contains electrolytes, the solubility of the surfactant in the water changes. It can be increased or decreased, depending on the kind of electrolyte [68,69]. The effect of electrolytes is explained by the HSAB principle (hard-soft-acid-base). For example, salts of hard acids and hard bases reduce the solubility of the surfactant in water. The solubility is increased by salts of soft acids and hard bases or by salts of hard acids and soft bases. Correspondingly, the solubility of the surfactant in water is increased by sodium alkyl sulfonates and decreased by sodium chloride or sodium sulfate. In the meantime, the physical interactions of the surfactant molecules and other components in microemulsions is well understood and the HSAB-principle was verified. The salts in water mainly influence the curvature of the surfactant film in a microemulsion. The curvature of the surfactant film can be expressed, analogous to the HLB-value, by the packing parameter Sp. The packing parameter is the ratio between the hydrophilic and lipophilic surfactant molecule part [70] ... 

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

Hapticity. 629, A77 Hard acids and bases, 344-355 Hard-soft acid-base (HSAB) interaction, 351 Hartree-Fock method, 20 Heavier transition metals, 587-588... 

To predict which of the two alkyne carbons, C1 or C2, HNC will preferentially attack, one now invokes the local hard-soft acid-base (HSAB) principle (cf. [157]), which says that interaction is favored between electrophile/nucleophile (or radical/radical) of most nearly equal softness. The HNC carbon softness of 1.215 is closer to the softness of C1 (1.102) than that of C2 (0.453) of the alkyne, so this method predicts that in the reaction scheme above the HNC attacks C1 in preference to C2, i.e. that reaction should occur mainly by the zwitterion A. This kind of analysis worked for -CH3 and -NH2 substituents on the alkyne, but not for -F. 

More recently, the interaction energy between different molecular systems has been expressed in terms of the chemical potential and molecular hardness (cf. Section IV.B) based on the local hard-soft acid-base principle (Chandrakumar and Pal, 2003), and respective applications include the calculation of acidities and bond dissociation energies of 4-substituted phenols (Romero and Mendez, 2003a, 2003b). 

Chandrakumar, K.R.S. and Pal, S., Study of local hard-soft acid-base principle to multiple-site interactions,. /. Phys. Chem. A, 106 5737-5744, 2002b. 

Regarding the primary coordination sphere of iron, the identity of the axial ligand is critical to determining the chemical reactivity of heme. In general, the interaction between the axial ligand and heme iron follow the hard-soft acid-base principle... 

The products of acid-base interactions such as those shown in Equations (5.36) through (5.39) are not properly considered as salts because they are not ionic compounds. Because in many cases these products are formed from two neutral molecules, they are more properly considered as addition compounds or adducts held together by the formation of coordinate covalent bonds. In that connection, they are similar to coordination compounds except that the latter ordinarily involve the formation of coordinate bonds to metal ions by the electron donors ligands). There are some useful generalizations that correlate to the stability of bonds during this type of acid-base interaction, and these are largely summarized by the hard-soft acid-base principle. 

Pearson s Hard-Soft-Acid-Base (HSAB) priciple is that hard add-base combinations form readily and are generally ionic compounds. The other group of stable compounds and complex ions involves the interaction between soft acid and soft bases. For these, the bonding is primarily covalent with interpenetrating orbitals. The combinations hard acid with soft base, or vice versa, have little stability. 

The preferential interaction between [SiOAl] and Na+ at one hand and between [SiOB]" and TPA+ at the other can be understood on the basis of the hard and soft acid-base interaction. It is well known that hard acids accompany better hard bases and soft acids link preferentially to soft bases. As Na+ is a harder acid than TPA+, [SiOAl] is also a harder base than [SiOB]. The preferential interactions lead then to the TPA+-[SiOB] pairs, as it was demonstrated previously [22]. 

Recently Parr and Pearson have used the b parameter to investigate the hard and soft properties of metal ions and ligands. They have termed this the absolute hardness in comparison to the Mulliken-Jaff6 a parameter which they call absolute electronegativity. They provide strong arguments for the use of the absolute hardness parameter in treating hard-soft acid-base (HSAB) interactions. 

Iwamoto et al. (54) studied the activity of a series of metalion exchanged zeolites for the water-gas shift reaction. The lower water-gas shift activity of the acidic cations was explained in terms of hard-soft acid/base properties. In this model, carbon monoxide, which is a soft base, interacts more strongly with soft acid sites. The adsorption of CO is generally considered to be the rate controlling step in the water-gas shift reaction. Cations of lower acidity are generally softer acids and as such adsorb CO more readily. This would lead to higher surface concentrations of CO, thereby increasing the water-gas shift acitivity of the sample. 

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