DEVELOPMENT OF LOCAL HARDNESS RELATED REACTIVITY INDEXES - THEIR APPLICATION IN A STUDY OF THE SE AT MONOSUBSTITUTED BENZENES WITHIN THE HSAB CONTEXT

The local hardness, ($) over tilde eta(lambda)(r), a local quantity de scribing reactivity following Pearson's hard and soft acids and bases (HSAB) principle, is analyzed in detail. These considerations lead to the introduction of two possible working equations for the local hardn ess and a new quantity, the hardness density, h lambda(r) better fitti ng the picture of a counterpart of the local softness. On the basis of the study of intra- and intermolecular reactivity sequences for an el ectrophilic attack on monosubstituted benzenes PhX (X = O-, NH2, OH, F , CN, NO2, CHO, NH3+, and CHCH2) a better insight into the physical me aning of these density functional theory related quantities, and the m olecular electrostatic potential, is acquired. These new insights in t urn lead to the introduction of a general form of a new global reactiv ity index combining the local softness and the newly defined hardness density.