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1.2.8 THE FRONTIER ORBITAL INTERPRETATION OF HSAB
PRINCIPLE: KLOPMAN
Despite its empirical nature, it is still possible to summarize a great deal
of diverse chemical information summarized by the HSAB principle. This
usefulness of the HSAB principle mainly attracts chemists to study further
on it. Klopman and Hudson [31] in an attempt to look more deeply into
the physical implications of the HSAB principle showed that the polyelec-
tronic perturbation treatment of chemical reactivity leads to a reasonable
definition of hardness and softness including a general interpretation of
ambident reactivity.
In his landmark paper, Klopman [24(b)] opined that when two reac-
tants approach each other, a mutual perturbation of the molecular orbitals
of both reactants occurs. The resulting change in energy can be estimated
from SCF-MO calculations. When the bonds are completely formed and
when the systems are simple enough, then good accuracy can be obtained
for the calculations of the heats of formation.
Klopman [24(b)] considered two systems; R and S interact through
their atomic site labeled r and s, respectively. The total perturbation en-
ergy (ΔE
total
) is produced by two distinct effects: (i) the neighboring ef-
fect, which accounts for the interaction due to the formation of an ion pair
without any charge or electron transfer and (ii) the partial charge transfer,
which is usually accompanied by covalent bonding.
()
i
( )
ii
−
−
−
+
δ
−
δ
+
(7)
RS
+
→+
RS
→ −
)(
RS
)
(
)
(
)
(
)
(
)
(solv)
solv
solv
solv
solv
This treatment of chemical reactivity was based on the limits of total en-
ergy equation for small perturbations (β) as follows:
mn
∑
E
mn
ΔE
total
= ΔE(1) + ½
(8)
(
) (
)
⎡
2
2
⎤
2
CC
m
EE
β
n
2
∑∑
⎢
r
s
⎥
or, ΔE
total
= q
r
q
s
(Γ/ε)Δ
solv
(1)+
(9)
m
n
⎢
*
−
*
⎥
occupied
occupied
m
n
⎣
⎦
where
E
m
*
and
E
n
*
are the energy of the highest-occupied orbital of the
electrophile or acceptor, and the nucleophile or donor, respectively, Ψ
m
and Ψ
n
, are the lowest empty orbital of the electrophile or acceptor, and the
nucleophile or donor, respectively,
q
is the total charge, Γ is the coulomb
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