Soft acids definition

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. 

The aim of this chapter is to discuss chemical reactivity and its application in the real world. Chemical reactivity is an established methodology within the realm of density functional theory (DFT). It is an activity index to propose intra- and intermolecular reactivities in materials using DFT within the domain of hard soft acid base (HS AB) principle. This chapter will address the key features of reactivity index, the definition, a short background followed by the aspects, which were developed within the reactivity domain. Finally, some examples mainly to design new materials related to key industrial issues using chemical reactivity index will be described. I wish to show that a simple theory can be state of the art to design new futuristic materials of interest to satisfy industrial needs. 

Model Definition. The HSAB model classifies Lewis acids (electrophiles) and bases (nucleophiles) as either "hard" or "soft." Hard acids and bases are relatively small, and exhibit low polarizability and a comparatively low tendency to form covalent bonds. Soft acids and bases have the opposite characteristics (24). Stated simply, the model postulates that hard acids react most readily with hard bases, and soft acids react most readily with soft bases (26). 

Infrared and NMR-spectral analysis, and x-ray diffraction data, testify [42-54] that in case of complexes of the already discussed pseudohalide ions, the competitive coordination can be explained by the HSAB principle hard Pearson acids are bound with hard N-center, and soft acids with soft X- donor (S, Se) centers. This situation allows us to obtain directly the coordination compounds of pseudohalides with a definite localization mode of the coordination bond, i.e., to carry out the regioselective synthesis on the basis of the higher stability of complexes which are obtained as a result of hard-hard or soft-soft interactions [2]. 

Just like Sanderson s electronegativity equalization principle, the Hard and Soft Acids and Bases principle was originally introduced without strong theoretical basis. Nevertheless, it was used widely from its formulation on. The principle states that hard acids prefer to coordinate with hard bases and soft acids with soft bases [82], In 1983, Parr and Pearson provided a definition for the chemical hardness [25]... 

Consistent with the definition of hard and soft metal ions see Hard Soft Acids and Bases) based on the nature of the stable complexes that they form with different ligand donor atoms, these metal ions behave similarly toward donor atoms of nucleic acids and their derivatives (Figure 8). Hard monovalent cations (M+) usually interact with nucleic acid polyanions only in a diffuse ion atmosphere manner, whereas hard and borderline polyvalent (M- +) cations can form both outer- and inner-sphere complexes. Soft metal ions tend to form inner-sphere complexes, however. Hard metal ions (class A) prefer O-donor ligands (usually phosphate oxygens), while soft ones (class B) prefer N-donor atoms of the nucleic acid bases as well as S atoms... 

Stability constant comparisons should be made with some caution, since the accuracies of some values may be no better than 1, and experimental conditions (ionic strengths and the salts used to maintain them) are not identical for all systems. Nonetheless, the Table 3 values give some idea of relative stabilities for a range of ligands. The order of logmA i values for the halide ions should be noted, for comparison of this set of values for Zn + with analogous values for Cd + and Hg + (Table 4) positions these cations according to the Hard Soft Acids and Bases principle, with Zn + just on the hard side, Cd + just on the soft side, of the hard/soft border, and Hg + definitely soft. 

Particularly relevant to the present crmtext is the fact that the olefinic double bond is considered as a soft base in Pearson s theory, while many Lewis acids used in cationic polymerisation (BF3, BCI3, AICI3, etc.) are classed as hard acids. Obviously, n-acceptors like chloranil or tetracyanoethylene are considered as soft acids. Thus, the interactions between Lewis acids and olefins must be considered as very weak in the context of the HSAB theory. This prediction is well substantiated by the tenuous character of the complexes observed in experimental studies (see Chap. IV). On the other hand, carbenium ions are usually placed at the borderline between hard and soft acids and are definitely softer than the Lewis acids mentioned above. Consequently, their interactions with olefins must be rather strong, which suggests that that propagation in cationic polymerisations promoted by Lewis acids should be faster than initiation. 

Our approach was outlined in the framework of the Hard-Soft Acid-Base theory (HSAB, Ref. 90). In a short definition, the HSAB theory states that hard nucleophiles prefer to react with hard electrophiles and soft nucleophiles prefer to react with soft electrophiles. 

Hard and soft acid and base theory gives access to an early part of the slope in a reaction profile like that in Fig. 3.3, just as perturbation molecular orbital theory does. Using the definitions of absolute electronegativity and absolute hardness derived in Equations 3.5 and 3.6, the (fractional) number of electrons AN transferred is given by Equation 3.14. 

Definition of hard/soft character is the result of empirical observations and trends in measured stability of complexes. For example, hard acids (such as Fe3+) tend to bind the halides in the order of complex strength of F > Cr > Br > I, and soft acids (such as Hg2+) in the reverse order of stability. However, as with any model with just two categories, there will be a grey area in the middle where borderline character is exhibited. This is the case for both Lewis acids and Lewis bases. Selected examples are collected in Table 3.2 below a more complete table appears later in Chapter 5. 

However, in many cases, electronegativity difference alone cannot account for the stability of the molecule. For example, according to the electronegativity criterion, the CsF molecule should be very stable as the electronegativity difference between Cs and F is very large. But the reaction enthalpy data indicate that Fil and CsF will react to form Csl and FiF. In order to predict the direction of acid-base reactions and to account for the stability of the products, Pearson introduced two parameters hardness and softness in the vocabulary of chemistry. The qualitative definitions of hard and soft acids and bases are as follows [28-32] ... 

Consistent with the definition of hard and soft metal ions see Hard Soft Acids and Bases) based on the nature of the stable complexes that they form with different ligand donor atoms,these metal ions behave similarly toward donor atoms of nucleic acids and their derivatives (Figure 8). ... 

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