QUANTUM COMPUTATIONAL STUDIES ON PORPHYCENE - Theoretical and Computational Research in the 21st Century - page 188

dices follow the trend porphycene > pyrrol. Thus, porphycene tend to un-
dergo electrophilic reaction more than pyrrol.
It is worth nothing to mention that the frontier orbital energy is not the
only criterion that governs the chemical characteristics of a compound.
Nevertheless, the energy criterion may be applied to deduce gross fea-
tures. Compounds that have a large HOMO-LUMO gap will be stable
against self-reaction, for example, dimerization, polymerization, and in-
tramolecular rearrangements.
If the HOMO is low in an absolute sense, the compound will be chemi-
cally resistant to reaction with Lewis acids. If the LUMO is high in an
absolute sense, the compound will be chemically resistant to reaction with
Lewis bases:
Compounds with a high LUMO and a low HOMO will be chemi-
cally inert. Compounds with a low HOMO and LUMO tend to be stable
to self-reaction but are chemically reactive as Lewis acids and electro-
philes. Compounds with a high HOMO and LUMO tend to be stable to
self-reaction but are chemically reactive as Lewis bases and nucleophiles.
Compounds with a narrow HOMO-LUMO gap are kinetically reactive
and subject to dimerization or reaction with Lewisacids or bases.
Larger the HOMO-LUMO energy gap more hard is the species and
thus is less reactive. A big advantage of using hardness as the function
is that the quantities concerned are physical. Larger η means larger I and
smaller A , which implies that the system is more stable than its competitor.
From Table 9.5, we have surprisingly noted that the calculated results
of the AM1study are numerically same with those values calculated on the
basis of HMO theoretical procedure. Thus, the AM1studies also reveal the
same result.
Now let us analyze the various atomic sites on the molecule on the
basis of some very important local reactivity descriptors.
9.4.1 PYRROL
We invoked simple HMO theory for the computation of π charge density
on each atomic site of pyrrol. The computed results are presented in Table
9.2(A). The analysis of the densities of HOMO and LUMO shows that
both the orbital are centered on the four C atoms of the ring.
It is distinct from Table 9.2(A) that the π charge density of the posi-
tions P(1,1) and P(1,4); and P(2,2) and P(3,3) is the same for pyrrol. The