K. Fukui
19
random system can localize. In a molecule, there are potential barriers between
atoms which should be got over by the aid of a certain condition to be satisfied,
in order for an electron to move around it freely. Although the question how
valence electrons can delocalize in a molecule have not yet been solved
satisfactorily under the condition of unfixed nuclear configuration, the in-phase
relation of HOMO and LUMO at the junctions of the two parts of the
molecule seems to be at least one of the conditions of intramolecular delocaliza-
tion of electrons.
Generally speaking, the electron delocalization gives rise to a stabilization
due to “conjugation” which is one of the old chemical concepts. If so, similar
stabilization mechanisms must be chemically detected in other systems than
aromatic compounds. The discussion of this delocalization stabilization at the
transition state or on the reaction path was nothing but the reactivity theory
hitherto mentioned. The term “delocalizability” was attached to the reactivity
indices we derived,” and our reactivity theory itself was sometimes called
“delocalization approach.“
14
The “hyperconjugation” of various sorts is ex-
plained in the same manner. The stabilization due to homoaromaticity or
bicycloaromaticity of Goldstein,
57
the stability in spirocycles, pericycles,
58
“laticycles” and “longicycles” of Hoffmann and Goldstein,
59
that of spirarenes
of Hoffmann and Imamura,
60
and so on, are all comprehended as examples of
the stabilization due to the delocalization between HOMO and LUMO,
although other explanations may also be possible.
You may be doubtful to what extent such a qualitative consideration is
reliable. In many cases, however, a considerably accurate nonempirical deter-
mination of the stable conformation of hydrocarbon molecules
61, 62
results in a
conclusion qualitatively not much different from the expectation based on the
simple orbital interaction argument mentioned above.
CHEMICAL REACTION PATHWAYS
It has already been pointed out that the detailed mechanism of a chemical
reaction was discussed along the reaction path on the basis of the orbital
interaction argument. For that purpose, however, it is required that the prob-
lem as to how the chemical reaction path is determined should have been
solved. The method in which the route of a chemical reaction was supposed on
the potential energy surface and the rate of the reaction was evaluated by the
aid of a statistical-mechanical formulation was established by Eyring.
63
Many
people wrote papers where the rate expression was derived wave-mechanically
with the use of the potential energy function. Besides, the problem of obtaining
the trajectory of a given chemical reaction with a given initial condition was
treated by Karplus.
64
The centre line of the reaction path, so to speak, the idealized reaction
coordinate - which I called “intrinsic reaction coordinate” (IRC)
65
- seemed
to have been, rather strangely, not distinctly defined till then. For that reason, I
began with the general equation which determines the line of force mathemat-
20
Chemistry 1981
i c a l l y .
3 4 , 6 6 , 6 7
Al h
t ough my papers themselves were possibly not very
original, they turned out later to develop in a very interesting direction.
68-74
These papers opened the route to calculate the quasistatic change of nuclear
configuration of the reacting system which starts from the transition state
proceeding to a stable equilibrium point.
66
I termed the method of automatic
determination of the molecular deformation accompanying a chemical reaction
as “reaction ergodography.”
34 67
This method was applied to a few definite
examples by Kato and myself
67
and by Morokuma.
72, 73
Those examples
were: abstraction and substitution of methane hydrogen by hydrogen atom,
67
nucleophilic replacement in methane by hydride anion,
72
and isomerization of
methylcarbylamine to acetonitrile.
73
All of these reactions thus far treated are
limited to the simplest cases, but there seems to be no principal difficulty in
extending the applicability to larger systems. Once IRC was determined in this
way, the driving force of a chemical reaction was analyzed on the basis of the
orbital interaction argument.
66
In the reacting system with no angular momentum it is possible to obtain the
IRC by the use of the space-fixed Cartesian coordinate system. All of the
calculated examples mentioned above belong to this case. However, in the
reaction in which rotational motion exists, it is required to discuss the IRC
after separating the nuclear configuration space from the rotational motion.
74-
77
For that purpose, it is essential to derive the general classical Hamiltonian of
the reacting systems and then to separate the internal motion which is deter-
mined only by the internal coordinates. The nuclear configuration space thus
separated out is in general a Riemannian space. The classical Lagrangian form
to be obtained in that process of constructing the Hamiltonian is used to deriye
the IRC equation in the presence of rotational motion. It is thus understood
that the rotational motion of the reacting system generally causes a deviation of
I R C .
7 4
Once the method of determining unique reaction pathways is obtained, the
next problem we are concerned with is to see if the calculated pathways are
interpreted in terms of the frontier orbital interactions. A method referred to as
the “interaction frontier orbitals” or “hybrid molecular orbitals” has been
developed very recently by Fujimoto and myself in order to furnish a lucid
scheme of frontier orbital interactions with the accuracy of nonempirical calcu-
lations now and in the future.
78-80
By including properly contributions of other
MOs than the HOMO and the LUMO, we realized in terms of orbital
diagrams how ingenious the empirically established chemical concepts - “reac-
tion sites” and “functional groups”- and the empirical concept of reaction
pathways could be. Fig. 5 compares the HOMO of styrene and its interaction
frontier orbital for protonation to the olelinic double bond. The latter is seen to
be localized very efficiently in the frontier of chemical interaction. The double
bond is evidently the functional unit in this case. Innovation of the frontier
orbital concept will hopefully be continued by young people to make it useful
for one of our ultimate targets: theroretical design of molecules and chemical
reactions.