Semi-empirical AM-1 Studies on Porphyrin - Modern Trends in Chemistry and Chemical Engineering - page 135

where the metal Fe(II) is a borderline acid having some intermediate hardness value
between hard acids and soft acids, we can pointed out that our computed results (Table
9.3) show that the N-atoms have some intermediate f + value. This local version of
HSAB principal was applied to rationalize the regioselectivity in several Diels-Alder
reaction [44]. Mendez and Gazquez [14] pointed out that this interaction can be proved
by the minimization of the grand canonical potential without assuming the local soft-
ness equalization that is, S A = S B . This is a general statement of local HSAB principal
to determine the atom through which the interaction between A and B takes place. The
regions of a molecule where the Fukui function is large are chemically softer than the
region where the Fukui function is small, and by invoking the HSAB principal in a
local sense, one may establish the behavior of the different sites with respect to hard
or soft reagents. But in this type of work, where one concentrate only to the ligand and
try to examine the electronic distribution on the atoms in the molecule, local HSAB
principle [14] cannot be applied because the Fukui functions for the acceptor cannot
be computed in the same computational environment.
Although there are views [43] that the Local HSAB principle [14] cannot be ap-
plied to study the biological systems and their functions, we can point out that the
interaction of the border line acids Fe 2+ will takes place through the borderline local
centers (the N-atoms) of the porphyrin ring, but this is not enough explanation because
this conclusion is too qualitative.
Thus we can say that both the Fukui functions and local softnesses fail to explain
the maximum reactive or more precisely the donor site of the molecule/ligand por-
phyrin. But when we look on the pi charge density, ZDO and Mulliken charges on the
atomic sites we surprisingly noted that two sets of the trans N atoms of the molecule
has the same value and higher in magnitude than all C atoms. Again one set of trans
N atoms has the highest value. We know that in porphyrin, two trans sets are initially
different, one set contains two non-protonated N atom and other contains two proton-
ated N atoms (N-H). This is due to the fact that in case of the non-protonated free lone
pair of electron is present while in the case of other set, the lone pair is utilized for H + .
CONCLUSION
In this work, on the basis of semi-empirical AM1 model, we have calculated the eigen
values and eigen functions of molecules. We have also calculated the Global DFT
descriptors for porphyrins. The Fukui functions, local softness, and atomic charges
for each center of the porphyrin are also calculated. We have found that all the centers
are not equal in chemical reactivity. The differences in chemical reactivity of different
sites for are nicely represented in this work. It is well known fact that the host por-
phyrins bind the guest into their cavity. But this fact cannot be predicted by the Fukui
function and the local softness values. From the above discussions it is distinct that the
largest values of the Fukui function and local softness do not necessarily correspond
to the softest regions of the molecule. Based on our results, we can conclude that it
is more useful to interpret the ZDO atomic charge and Mulliken charge as function
that measure the local abundance or concentration of the charges on the atom in the
molecule. The purpose of mathematical definitions of the two very important concep-