Global molecular electronegativity

Figure 3. Comparison between gas and solution phase molecular electronegativities (a), and global hardness (b). Experimental values (+) rom reference [35], Gas phase values (X) from CNDO/2 calculations and solution values ( ), from SCRF calculations.

By contrast, quantum chemical methods allow, in principle, the analysis of the electronic structures of molecules with regard to chemical attacks at every site. Moreover, global reactivity parameters such as molecular electronegativity (see below) can be quantified. In addition, besides... 

Local or global molecular descriptors related to the electronic distribution in the molecule they are fundamental to many chemical reactions, physico-chemical properties, and ligand-macromolecule interactions. The theory of electronic density is based on a quantum-mechanical approach however, - electronegativity and charges, which are not physical observables, are also important quantities for the definition of several electronic descriptors. 

The present review paper picks up the line of thought where a previous review on electronegativity equalization and its applications left it [3]. Since then, many applications of the EEM-method have been realized (e.g. see Ref. [4]). Also, within the same framework, new discoveries have been made, allowing the direct computation of several local as well as global molecular responses [5,6]. The local properties referred to are all atom-in-molecule quantities, obtained by integration of the equivalent r-dependent functions. 

The proposal of a hardness equalization principle [33] possibly arises from a dual consequence of the electronegativity equalization principle and the fact that global molecular softness (S) can be expressed as a mean of the corresponding local softness (Sj) of the constituent atoms [34], It is already known that the hardness (t]) varies directly with the electronegativity (%). i.e., T a X, for both atoms [30, 34] and moleeules [34], Therefore, Eq. (1) ean be similarly expressed in terms of hardness as [33,35, 36] ... 

In this chapter, we present different global and local reactivity descriptors vis-a-vis the associated electronic structure principles for analyzing structures, properties, reactivity, bonding, interactions, and dynamics in the contexts of various physicochemical processes such as molecular vibrations, internal rotations, chemical reactions, aromaticity, stability of isomers, ion-atom collisions, atom-filled interactions, solvent effect, etc. Global reactivity descriptors are presented in Section 2, whereas Section 3 provides their theoretical basis. Section 4 delineates the local reactivity descriptors. Electronic structure principles associated with electronegativity and hardness are given in Sections 5 and 6, respectively. Section 7 presents the reactivity... 

Although information about the overall reaction can be obtained from knowledge of global parameters such as electronegativity and hardness, the reactivity of a particular site of a molecular species can be explained by local quantities such as electron density (p(rj), Fukui function (/(F)) [75], local softness [64], or local hardness [76,77], The dependence of these local quantities on reaction coordinate reflects the usefulness of these quantities in predicting the site selectivity of a chemical reaction. The most important local descriptor is the density p(F) itself, the basic variable of DFT [78], given as ... 

The third volume highlights chemical reactivity through molecular structure, chemical bonding (introducing bondons as the quantum bosonic particles of the chemical field), localization from Hiickel to Density Functional expositions, especially how chemical principles of electronegativity and chemical hardness decide the global ehemieal reactivity and interaction ... 

Electron transfer is governed, in global terms, by two important parameters of atomic or molecular systems, the concepts of which have been introduced, modified, redefined, and extended from time to time. These concepts of absolute electronegativity " and chemical hardness form only a subset of the larger set of concepts that chemists have introduced over the years to understand, interpret, systematize, and rationalize chemical knowledge. 

Once defined the global measure characterizing molecular systems, i.e., the absolute electronegativity, there is realized that the chemical hardness is not constant over the entire molecule. Therefore, also the local sizes can be defined, which have been called as Fukui functions by Parr and Yang (1984) ... 

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