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2 Actual Potentials of Theoretical Chemistry: What Can Be Obtained
Molecule A
Molecule B
Electron transfer
Molecule A
Molecule C
Molecule B
HOMO
HOMO
LUMO
LUMO
(a)
(b)
Fig. 2.63 a HOMO-LUMO interaction between molecules A and B resulting in electron transfer
from the HOMO of A to the LUMO of B, and b schematic representation of formation of a new
molecule C
Fig. 2.64 Schematic
representation of the
HOMO-LUMO interaction
between the sites r of A and
s of B
r
s
B
A
where S rs stands for the overlap integral of the AO’s at the sites r and s, c r the coefficient of the AO at r in the HOMO of A, c s the coefficient of the AO at s in the LUMO
of B, and ε HO-LU defined by ε LU(B) – ε HO(A) (normally > 0) the energy difference
of the HOMO of A and the LUMO of B. This energetical stabilization promotes
favorableness of the initial stage of the reaction and possibly lowers the transition
state of the reaction as illustrated in Fig. 2.65, when the following conditions are
satisfied:
I. S rs is positive and large,
II. ε HO-LU is small,
and
III. c r,HO(A) and c s,LU(B) are large.
Orbital-interaction approach basically utilizes the above principle based on perturbation theory and often affords prediction as well as understanding of the chemical
reaction mode such as stereospecificity or stereoselection influencing the molecular structures of the reaction products. A couple of examples of orbital-interaction