Softness, protonation indices

Evaluation of the only appropriate Fukui function is required for investigating an intramolecular reaction, as local softness is merely scaling of Fukui function (as shown in Equation 12.7), and does not alter the intramolecular reactivity trend. For this type, one needs to evaluate the proper Fukui functions (/+ or / ) for the different potential sites of the substrate. For example, the Fukui function values for the C and O atoms of H2CO, shown above, predicts that O atom should be the preferred site for an electrophilic attack, whereas C atom will be open to a nucleophilic attack. Atomic Fukui function for electrophilic attack (fc ) for the ring carbon atoms has been used to study the directing ability of substituents in electrophilic substitution reaction of monosubstituted benzene [23]. In some cases, it was shown that relative electrophilicity (f+/f ) or nucleophilicity (/ /f+) indices provide better intramolecular reactivity trend [23]. For example, basicity of substituted anilines could be explained successfully using relative nucleophilicity index ( / /f 1) [23]. Note however that these parameters are not able to differentiate the preferred site of protonation in benzene derivatives, determined from the absolute proton affinities [24],... 

In this work, we have developed a model for the evaluation of proton affinity in terms of some akin conceptual reactivity descriptors which can be concepmally linked and associated with the physico-chemical process of protonation. The akin descriptors are the ionization energy (I), the global softness(S), the electronegativity (x), and the global electrophilicity index (co). 

Hence, it is clear that fundamentally and operationally the physico-chemical process of protonatirMi can be linked to the above akin descriptors — the ionization process, the hardness, softness, electronegativity and electrophilicity. Recently, we (Islam et al. 2010, 2011a, b Ghosh et al. 2011a) have published good number of papers where we have discussed that the three descriptors, the electronegativity, the hardness and the electrophilicity index of atoms and molecules are fundamentally qualitative per se and operationally the same. All three represent the attraction of screened nuclei towards the electron pair/bond. Thus, we can safely and reasonably conclude that the proton affinity and the three descriptors have inverse relationship. 

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