QUANTUM COMPUTATIONAL STUDIES ON PORPHYCENE - Theoretical and Computational Research in the 21st Century - page 163

9.1.1.4 ELECTROPHILICITY INDEX

The electrophilicity, ω is a descriptor of reactivity that allows a quantita-
tive classification of the global electrophilic nature of a molecule within
a relative scale. Parr et al. [24] suggested that electronegativity squared
divided by hardness measures the electrophilic power of a ligand its pros-
perity to “soak up” electrons.
Thus,

ω = μ 2 /2η=χ 2 /2η (11)

It is further anticipated that electrophilicity, ω should be related to elec-
tron affinity, because both ω and electron affinity measure the capacity
of an agent to accept electrons. Electron affinity reflects the capability
of an agent to accept only one electron from the environment, whereas
electrophilicity index measures the energy lowering of a ligand because
of maximal electron flow between the donor and acceptor. The electron
flows may be either less or more than 1. Thus, the electrophilicity index
provides direct relationship between the rates of reaction and the electro-
philic power of the inhibitors [25] .

9.2 THE LOCAL REACTIVITY PARAMETERS

9.2.1 FUKUI FUNCTION

Fukui functions play a prominent role in the field known as conceptual
density functional theory (DFT). Parr and Yang [26] based on the original
ideas of Fukui [9], introduced Fukui function that reflects the response of
a molecular system toward a change in the number of electrons ( N e ) of the
molecular system under consideration .The Fukui functions is a measure
of local reactivity and defined as

f ( r ) = (∂ρ( r )/∂ N ) v (12)

where ρ( r ) is the electron density.
As is clear from previous equation, Fukui functions measured the re-
sponse of the electron density at every point r , in front of a change in the
number of electrons, N under the constant external potential, v . The sites
with the largest value for the Fukui functions are those with the largest