QUANTUM COMPUTATIONAL STUDIES ON PORPHYCENE - Theoretical and Computational Research in the 21st Century - page 159

to predict the properties of molecules. Medicinal chemistry is a particu-
larly glaring example of our use of structure-function relationships. There
is a tremendous need to be able to quickly design new drugs for curing
human disease. The rapid prediction of the activities of compounds for use
as drugs and the discovery of new compounds is an important goal.
Since porphyrins and porphyrin metal complexes play a fundamen-
tal role in many biochemical processes, we believe that it is important to
make use of quantum chemical methods to calculate the global and local
reactivity parameters for porphyrin and substituted porphyrin system in
order to carry out a more detailed analysis of the different effects that
have an infl uence on the chemical reactivity at the carbon atom and at the
N atom in terms of the local version of the Hard Soft Acid Base (HSAB)
principle [7]. The failure of HSAB principle [8] in case of biological envi-
ronments of metal ions is reported [9]. Although there are a good number
of literatures [10, 11] discussing the study of the global quantum chemi-
cal reactivity parameter of porphyrins, but the study of the local quan-
tum chemical reactivity parameters of porphyrins are limited [12]. Thus
we feel it necessary to study of the local reactivity parameters for better
understanding of the preferred sites for coordination with the metal ion
(electron acceptor) of the porphyrins [13, 14]. We also venture to study the
global reactivity parameters for them for the better understanding of the
stability of these systems. We have invoked some semiempirical methods
to realize the charge distribution on the different atomic sites of porphyrins
to analyze the use of different reactivity descriptors for the prediction of
the coordination sites for them.
In a recent work, we have established the fact that the largest and
smallest value of the Fukui function and local softness do not necessarily
correspond to the softness and hardness regions of the molecule porphyrin
[13, 14].
In a previous work [14], we have adopted semiempirical AM-1 calcu-
lation [15, 16] for this venture. Our fi nding was “neither the Fukui func-
tions nor the local softnesses can predict the preferred donor sites of por-
phyrins toward metal ions”. The Local version of the HSAB is found to be
very diffi cult to apply to study the preferred site for coordination at least
through AM-1 method. However, when we look at the atomic charges,
computed using both methods, we see that the results show a good corre-
lation between the chemical observations and the mathematical formulae.