F.R.S. 1989
9 January 1998: Elected
−−
Kenichi Fukui. 4 October 1918
A.D. Buckingham and H. Nakatsuji
, 223-237, published 1 November 2001
47
2001
Biogr. Mems Fell. R. Soc.
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KENICHI FUKUI
4 October 1918 — 9 January 1998
Biog. Mems Fell. R. Soc. Lond. 47, 223–237 (2001)
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KENICHI FUKUI
4 October 1918 — 9 January 1998
Elected For.Mem.R.S. 1989
B
A.D. B
1
, C.B.E., F.R.S.,
H. N
2
1
Department of Chemistry, Cambridge University, Cambridge CB2 1EW, UK
2
Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan
L
Kenichi Fukui was born in Oshikuma, Nara, Japan, on 4 October 1918. He was the eldest of
three boys of Ryoukichi, his father, and Chie, whose family name before marriage was
Sugisawa. Ryoukichi Fukui, who graduated from the Tokyo Commercial Institute (later
Hitotsubashi University), was a merchant who traded with foreign countries and also
managed a factory making precision instruments. He liked fishing and often took Kenichi
with him, and was a member of the National Geographic Society—the
National Geographic
Magazine
was one of the most important magazines of Kenichi’s childhood. Chie graduated
from Nara Women’s College and was an affectionate mother of her boys. She never forced
them to study but provided a studious environment. For example, she bought for her children
the complete works of Souseki Natsume, a famous Japanese novelist, whose books Kenichi
was very fond of reading.
Shortly after Kenichi’s birth, the family moved to their new house at Kishinosato, Osaka,
and lived there until he was 18. In his childhood, he loved playing in the natural environment
and spent almost every vacation at his mother’s native house in Oshikuma. He liked walking
near or sometimes far from the house. There were many ponds in the vicinity and he enjoyed
fishing with his brothers. Even in Osaka, there was much in nature that entranced him.
Kenichi liked collecting many different kinds of things, like postage stamps, match labels,
leaves and buds of plants, and mineral stones. There were many beautiful butterflies and
mysterious insects; the imaginative and sensitive heart of Kenichi was enchanted by their
225
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Biographical Memoirs
beauty. This interest in nature remained with him even when he went abroad to give lectures at
international symposia many years later; he was excited to catch a splendid cicada at
Ryukabitos near Athens, and butterflies in Florida and Istanbul.
Kenichi entered Tamade Daini Primary School in 1925. He was not strong physically. He
enjoyed field work in the summer school at the seashore to the south of Osaka. A beautiful
Halinga ornata
(
hana-densha
in Japanese) in seawater in bright summer sunshine enchanted
Kenichi.
Kenichi moved to Imamiya Middle School in 1931. He joined the Biological Circle, whose
senior members were experts and were good leaders. They went hiking in nearby mountains in
the suburbs of Osaka and gathered many different kinds of insect. This introduced Kenichi to
the works of Jean Henri Fabre, who continued to influence him through his book series
Entomological souvenirs
. Kenichi read it in the Japanese translation by Y. Yoshida and
T. Hayashi and eagerly awaited the publication of each volume. The statements and
observations written in this book were in harmony with his own experiences. This was in some
sense a surprise for him because Osaka and Provence are so distant. Much later, Kenichi was
elected a member of the International Academy of Quantum Molecular Science, whose
headquarters are located in Menton, France. While attending an academy meeting, he enjoyed
travelling in Provence with Tomoe, his wife, because this was the place where Fabre had spent
his life with his insects. Kenichi and his wife dreamt of living in Provence after retirement, but
this was not realized because the Nobel Prize caused him to be so busy.
Henri Fabre was a gifted chemist as well as an eminent entomologist. He succeeded in
preparing alizarin dye from plant madder on an industrial scale, but this was not used because
of the success of the synthetic method introduced by German chemists. Fabre devoted the last
chapter of his
Entomological souvenirs
to this story and the last sentence was a declaration of
starting again from the beginning: ‘Laboremus!’, and this impressed Kenichi. Chemistry did
not seem to give happiness to Fabre. This cast a shadow on Kenichi’s impression of chemistry.
The chemistry course started in the third year of the middle school, but he did not like it
mainly because of its dependence on memory work, but also partly because it did not give
happiness to Henri Fabre.
Kenichi wrote at the age of 65 how important these boyhood experiences were in his
becoming a natural scientist. However, in those early days he never thought of becoming a
scientist, but rather a doctor of literature. His favourite subjects were history and literature. A
reason for this is that his birthplace, Oshikuma, was located between Nara and Kyoto, where
there are many historical monuments.
In 1935 Kenichi entered Osaka High School. He joined the Science Department and took
German as a second language. In those days, students had to take one sport as a specialty and
he took Japanese fencing (Kendo). Kenichi enjoyed doing exercises for this sport almost every
day, but it made him tired so he did not do much study. When he started fencing he was
unable to win, but one day his master said to him ‘never expect to win, rather only do your
best’. After taking this suggestion on board, he found he could win a match and he gradually
got into this sport. It made him strong in body, but rather apart from his studies.
In the spring of 1938, his last high school year, his father visited Genitsu Kita, a relative
and native of the same district of Nara and Professor of Chemistry at Kyoto University, at his
home in Kitashirakawa and consulted him about the course for Kenichi after graduation from
high school. He explained that his son had taken German and liked mathematics. Kita
advised that both mathematics and German are important for chemistry, so ‘please send your
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Kenichi Fukui
227
son to my laboratory in Kyoto’. This advice was rather unexpected because it was thought in
those days that mathematics was not necessary for chemistry. When Kenichi heard this advice
from his father, namely ‘if he likes mathematics, he should do chemistry’, he immediately
decided to do chemistry. Kenichi was very happy to have received such good advice and to
have met such an excellent teacher.
Professor Kita was born in Nara in 1883, graduated from the Department of Applied
Chemistry of the Imperial University of Tokyo in 1906 and became an associate professor in
that department in 1908. After two years in Europe and America, he became a professor in
the Department of Industrial Chemistry of Kyoto Imperial University in 1921. Kita was not
only an eminent chemist who published more than 1000 articles, but also an excellent teacher
who taught a number of leaders of chemistry in Japan, including J. Sakurada, S. Kodama,
M. Horio and J. Furukawa. After his retirement from Kyoto University in 1944, he became
President of Naniwa University (later Osaka Prefectural University) and a member of the
Japan Academy.
Kenichi entered the Department of Industrial Chemistry, Faculty of Engineering, Kyoto
Imperial University, in 1937. He would often visit Professor Kita at home where he and his
wife, an excellent violinist who had played in a US orchestra and once performed for the Meiji
Emperor, warmly welcomed him. Kita was a rather silent person and even looked rustic. His
advice was, ‘You must study basic science if you want to do excellent applied chemistry’.
Although he never suggested an actual field, there was a clear recommendation to study
fundamental chemistry. The Department of Industrial Chemistry emphasized applied fields
of chemistry such as ceramic chemistry, electrochemistry, fermentation chemistry and the
chemistries for synthetic dyes, fibres, rubbers and plastics. The lectures in the department were
strongly application-oriented. Kenichi, who was oriented towards basic science, took lectures
in the Faculty of Science, which was located nearby. In due time, Kenichi decided to study
quantum mechanics, which was only about 12 years from its birth. There were almost no
lectures on quantum mechanics, so he went to the library of the Physics Department and
borrowed books. His policy was to try to understand all the equations, so he went back to the
original literature. The world of atoms and molecules enchanted Kenichi. A problem was that
he could not borrow books such as
Handbuch der Physik
, so he wrote the essence of the
articles in his notebook. For recreation he enjoyed, once in a while, imported movies like
Under the roofs of Paris
. His undergraduate days in Kyoto gave him much pleasure.
Kenichi’s method of study was to read deeply a small number of selected papers, rather
than a lot of literature in a wide range of fields. The field of mathematical physics was already
established. Courant & Hilbert’s
The methods of mathematical physics
was one of his favourite
books. He wondered then why ‘mathematical chemistry’ did not exist and thought that the
empirical nature of chemistry should decrease through cultivating mathematical chemistry.
‘Decreasing the empirical nature of chemistry’ was a phrase that Professor Fukui was often to
use in his lectures.
In the third undergraduate year he started graduate study under the guidance of Associate
Professor Haruo Shingu, because Professor Kita was approaching retirement. This
experimental study was important for his later theoretical study: different hydrocarbons
showed different reactivities to hexachloroantimony, which was rather mysterious and
interesting. Kenichi was also interested in the different reactivities in aromatic hydrocarbons,
such as naphthalene and anthracene. This was the subject through which the frontier electron
theory was first cultivated and to which the first applications of the theory were so
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Biographical Memoirs
successfully made. It was perhaps fortunate that the results of his experimental studies could
not be explained by existing theories.
Kenichi graduated from Kyoto University’s Faculty of Engineering in March 1941 and
entered the graduate school in the Department of Fuel Chemistry of the Faculty of
Engineering. The supervisor of his graduate study was Professor Shinjiro Kodama, who was
also a student of Professor Kita. Professor Kodama considered fundamental science to be
important, probably even more so than Professor Kita. Kodama had studied in Germany
from the age of 24 and owned many books on subjects such as quantum mechanics and
electromagnetism. Kenichi was able to study basic physics in the free atmosphere of Professor
Kodama’s laboratory.
In August 1941, Kenichi went to the Fuel Institute of the Japanese Army in Tokyo. In
1943, he became a lecturer in the Department of Fuel Chemistry, Kyoto University, and in
1944 Associate Professor in the same department. Kenichi spent much time studying quantum
mechanics. Particularly impressive to him were R.H. Fowler’s
Statistical mechanics
(1936) and
Hideki Yukawa’s
Introduction to quantum mechanics
(1947) and
Introduction to particle physics
(1948).
The Fuel Institute was concerned with the synthesis of hydrocarbons that improve the
performance of gasoline. In the USA they used 2,2,4-trimethylpentane, and Kenichi had to
synthesize similar compounds from butanol that had been made by the fermentation of sugar.
In September 1944 his team succeeded in this synthesis and was awarded a prize by the Army.
After World War II, Kenichi returned to Kyoto University and engaged in chemical reaction
design under the guidance of Professor Kodama. He worked on basic aspects of engineering
in the high-pressure synthesis of polyethylene. From this emerged the subject of his doctoral
thesis, namely the theoretical study of temperature distributions in the reactors of chemical
industry. It was a volume of about 200 pages; when he showed it to Professor Kita, who had
already retired, he only responded how thick it was! It was a hot summer in 1948 when he
submitted his doctoral thesis. Three copies were required, all handwritten in those days. As his
thesis was long, his wife, Tomoe, helped him to make a copy; her handwriting was mixed with
his.
Tomoe (whose maiden name was Tomoe Horie) and Kenichi married in the summer of
1946. She had dreamt of becoming a scientist after reading the biography of Marie Curie, and
graduated from the Physical Chemistry Department of the Imperial Women’s University of
Science in Tokyo. Before their marriage, Kenichi took her to a concert with all the scores of
what was probably the Ninth Symphony of Beethoven. After the concert, he proudly pointed
out that they did not play some parts of the symphony as written on the score. Her concern
was with how he could spoil the pleasure of the concert. Those were difficult days in Japan,
but Tomoe did her best to let him concentrate on science. Their son, Tetsuya, was born on
8 January 1948 and their daughter, Miyako, on 19 May 1954.
After completing his doctoral thesis, Kenichi turned to theoretical studies on chemical
reactions. In those days, chemical reactions formed the major subject of the Department of
Chemistry in the Faculty of Science of Kyoto University. In particular, S. Horiba, T. Lee and
S. Sasaki were active in this field. This was in sharp contrast to the Faculty of Science of the
University of Tokyo, where studies on molecular structure were favoured. In this atmosphere
it was natural for him to take chemical reactions as his main subject. The experimental studies
on the reactions of hydrocarbons that he did in his undergraduate study and in the Fuel
Institute in Tokyo formed the backbone of his theoretical studies.
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229
In 1951 Fukui was promoted to Professor in the Department of Fuel Chemistry. In
February of that year a fire occurred at the department and as a result he had to share a
laboratory with Professor Shingu and others. It was in this room that the frontier electron
theory was born. He imagined that, in the course of a chemical reaction, the electron in an
outermost molecular orbital should have an important role, as it is the outermost region of
the molecule that meets the other molecule. This orbital was called a ‘frontier orbital’.
Fukui first calculated the frontier electron density of naphthalene and found that the den-
sity was largest at the position where chemical reaction took place. He proceeded, with the
help of Teijiro Yonezawa, who was at that time his graduate student, to study more complex
hydrocarbons such as anthracene, pyrene and perylene. The frontier orbital theory correctly
showed the positions of chemical attack by an electrophile such as NO , thus giving confi-
dence in the validity of the theory. Details of the theory are explained later in this memoir.
+
2
The collection of many experimental results and their interpretation were due to Professor
Shingu, an organic chemist with a deep knowledge of the electron theory of organic reactions.
They discussed the naming of the new theory and it was Shingu who suggested ‘frontier’
electron theory. In October 1951 the first paper of the frontier electron theory was submitted
to the
Journal of Chemical Physics
; it appeared in April 1952 (1)*, with T. Yonezawa and H.
Shingu as co-authors. Two years later, a second paper appeared and then the frontier electron
theory was changed to the frontier orbital theory to include the lowest unoccupied orbitals in
the frontier orbitals (2). Kenichi and his students T. Yonezawa, C. Nagata, H. Kato,
K. Morokuma, A. Imamura and H. Fujimoto in particular developed frontier orbital theory
and identified the special role of the frontier orbitals of molecules in the course of chemical
reactions. Later, R.B. Woodward (For.Mem.R.S. 1956) and Roald Hoffmann (For.Mem.R.S.
1984) also considered the role of orbitals in chemical reactions (Woodward & Hoffmann
1965, 1969
a
,
b
). The orbital was the key common concept of their theories.
The frontier orbital theory explained not only the reactivities of hydrocarbons but also
those of many types of molecule. This showed that the frontier orbital theory had a wider
validity than the electron theory of organic reactions that had previously existed. Hammett’s
rule (Hammett 1940), one of the important consequences of the electron theory, was
explained by the frontier orbital theory.
In the same year as the frontier orbital theory was published, the charge-transfer theory of
Robert Mulliken appeared (Mulliken 1952). His theoretical formulation on the interaction of
two molecules leading to a charge-transfer complex had some similarity to the frontier orbital
theory. This similarity gave Kenichi much insight in the later development of frontier orbital
theory. Mulliken visited Japan in 1953 and gave a series of lectures referring to the frontier
orbital theory, thus enhancing the importance of this theory to Japanese chemists. It took
about 10 years for the frontier orbital theory to be recognized worldwide.
In 1962 he received the Japan Academy Prize for the study of the electronic structure and
chemical reactivity of conjugated compounds; this was 10 years after the first appearance of
the frontier orbital theory. Yoshio Tanaka, Professor Emeritus of the University of Tokyo,
who was one of his supporters, told him, ‘This theory may even deserve the Nobel Prize’.
Tanaka continued to ask him whether the frontier orbital theory was applicable to
stereospecific reactions. That was three years before the Woodward–Hoffmann rule was
published.
* Numbers in this form refer to the bibliography at the end of the text.
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In 1964 he attended the Sanibel Symposium and met Roald Hoffmann for the first time.
Hoffmann was 19 years younger than Fukui and was already well known for his extended
Hückel method, a subject of his PhD dissertation. From that meeting a warm friendship
continued until the end of Kenichi’s life. After Sanibel, he travelled for almost two months in
the USA and Europe with Tomoe. This was his first travel abroad and they celebrated their
19th wedding anniversary at a restaurant in Paris.
In 1964 P.-O. Löwdin and B. Pullman invited him to contribute a chapter to a book that
was to be a tribute to Robert Mulliken at the age of 60. He accepted, and wrote a paper
entitled ‘A simple quantum theoretical interpretation of the chemical reactivity of organic
compounds’ (9). In this article he studied the Diels–Alder reaction and related for the first
time the symmetries of the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) to the selectivity of the reaction. This concept was
shown to be important by Woodward and Hoffmann through their presentation of the theory
of the conservation of orbital symmetry, the so-called Woodward–Hoffmann rule. This
theory, presented in 1965, relating the chemical reactivities of molecules directly with the
natures of their HOMOs and LUMOs, was quickly understood by chemists and opened a
new field in organic chemistry. As a result, the frontier orbital theory, especially in its
application to the Woodward–Hoffmann rule, became famous and was recognized by the
joint award of the Nobel Prize for Chemistry in 1981.
Chemical and Engineering News
,
published weekly by the American Chemical Society, commented as follows:
So if Fukui had not developed frontier orbital theory, Hoffmann and Woodward might not have hit
upon their own theory when they did. And if Hoffmann and Woodward had not presented their ideas
in simple, instantly usable form, Fukui might have been much longer in gaining recognition.
In 1970 Fukui was in Chicago for six months with Tomoe. He took the opportunity to visit
R.B. Woodward at Harvard University. The purpose of his visit to Chicago was mainly to
lecture for a semester in the graduate school at the Illinois Institute of Technology. It was his
first graduate lecture course outside Japan and therefore he prepared hard. However, the
reaction of the students was that his lectures were difficult to understand. This was also the
general impression even for Japanese students in Kyoto. After a discussion with a group of
the students, he agreed to make the lectures easier. It was disappointing for him. Nevertheless,
he enjoyed life in Chicago, where it was possible to sample restaurants from all over the world.
It was in Chicago that they experienced a tornado that hit their apartment—they saw a big
tree pulled from the earth and several cars turned over like toys.
Fukui completed a small but important study in Chicago (12). It was about the definition
of the chemical reaction coordinate, which he called the ‘intrinsic reaction coordinate (IRC)’.
He thought it necessary to define the route of a chemical reaction, and the formulation itself
was quickly completed. However, anxiety came over him; his idea was so simple and obvious,
and it was already about 30 years from the famous book of Henry Eyring, so surely a similar
idea must have been published somewhere. He did his best to look for such a publication but
failed. At the apartment, which faced west, he struggled with the anxiety about the priority of
this study. Later, Tomoe told him that he was like a Deva King facing sunset and trying with
all his power to produce his cerebrospinal fluid. He finally decided to submit this small article
to the
Journal of Physical Chemistry
. The referee’s report was fun: this article had no
originality but was worthy of publication. He had never before had the experience of a paper
having no originality being acceptable for publication. However, it was indeed pleasing for
him that this short study turned out to initiate a series of studies by other scientists.
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Kenichi Fukui
231
Fukui was a scientist with plenty of ideas, most of which came early in the morning. He
suddenly woke up with an inspiration that would quickly disappear if it were not put down on
paper. Such papers were always beside his bed, just like a samurai’s sword, and when some
inspiration came he could record it without light.
Kenichi Fukui was Chairman of the third International Congress of Quantum Chemistry
held in Kyoto, and Teijiro Yonezawa was its general secretary. It was in the autumn of 1979
and Kyoto was brilliantly coloured by maple trees. Many scientists gathered from both
outside and inside Japan to the Congress Hall located north of the city.
It was at about 10 p.m. on 19 October 1981, soon after his 63rd birthday, that Fukui had a
telephone call from a newspaper in Tokyo asking for an interview as a Nobel laureate. He was
astonished but became confident when he saw his name on television with Roald Hoffmann’s.
That night he had many visitors at home: cameramen, newspapermen, friends and students.
Kenichi and Tomoe were surrounded by newspapermen until midnight.
On 10 December 1981 Kenichi Fukui received the Diploma and Medal of the Nobel Prize
for Chemistry from King Gustav of Sweden. The prize was shared with Roald Hoffmann.
The pictures on the Diploma were purple crocus flowers. At that moment he recalled the
favours and the benefits that he had received from Professor Genitsu Kita and Professor
Yoshio Tanaka. It was impressive for him that many important events after the ceremony
were held under the auspices of the Student Union of Sweden.
After the Nobel Prize, Fukui became very busy and was constantly in the public eye in
Japan. This made his life less flexible, but he still liked walking among nature and he did so
early in the morning. He became President of the Kyoto Institute of Technology. As this
position was not a scientific one but an administrative one, he could not have a laboratory in
the university. Three years later he became President of the Institute for Fundamental
Chemistry that was built for him in Kyoto with funds donated by the Japanese chemical
industry. He also became chairman of many important organizations and committees, leaving
little time for active science.
Fukui was frequently invited to deliver lectures, not very specific but rather general ones.
In such lectures, he discussed several topics. He suggested that chemistry would become one
of the most popular fields of science. Although the problem of environmental pollution
damaged the image of chemistry, this had also forced chemistry and chemical industry to
change. Now it was clear that without chemistry the problems of resources, energy and food
that the human race must face could not be resolved. Young scientists and students should
study this important subject by less empiricism and through more fundamental concepts.
Advances in computer science should be of great benefit in such non-empiricism in chemistry;
he proposed the name ‘molecular engineering’ for the field that exploits the intrinsic
properties of molecules. He encouraged young scientists to be creative and a source of new
science and engineering.
In the winter of 1997, cancer was diagnosed in his stomach. He immediately underwent
surgery but had to return to hospital in the summer. On 9 January 1998 he died at the age of
79. His tomb is on the hillside of Higashiyama, where his teacher Genitsu Kita also lies.
R
On graduation from Kyoto Imperial University in 1941, Kenichi Fukui was engaged in
experimental research on synthetic fuel chemistry. He therefore started his research career as
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Biographical Memoirs
an experimentalist. This might be thought to have been a diversion from the path to becoming
a theoretician. However, not much time was required to make up for this because he had been
immersing himself since his student days in quantum mechanics, theoretical physics and
mathematics. By 1956 Fukui had built up a subgroup of theoreticians in his research group.
His experience in experimental chemistry and his associations with experimentalists were in
fact central to his theoretical work.
The thrust of his main contribution to chemistry can be recognized in his more than 300
publications in English, of which about 230 concern the theory of chemical reactions and
related subjects. Other papers relate to the statistical theory of gelation, organic synthesis by
inorganic salts, and polymerization kinetics and catalytic reactions.
One of the most important papers was his first on the theory of chemical reactions (1). He
found a correlation between the reactivity of aromatic hydrocarbons to electrophilic reagents
and the square of the atomic orbital coefficients of the linear combination in the HOMO. The
spatial distribution of electron density in the HOMO was parallel to the order of reactivity in
the molecule. A little later, a similar correlation was found with respect to the reactions with
nucleophilic reagents between the reactivity and the distribution of the LUMO. The reactivity
with free radicals was determined by the summed density of both the HOMO and LUMO (2).
Fukui considered this result as coming from a general feature of chemical reactions, that
is, a general orientation behaviour. He attempted to extend the range of compounds to which
this rule was applicable to different types, namely organic and inorganic, aromatic and
aliphatic, saturated and unsaturated. He found that the range of chemical reactions thus
treated covered substitutions, additions, abstractions, bond fissions, eliminations and
molecular complex formation.
Fukui’s 1952 paper (1) was published in the same year as Mulliken’s important paper on
the charge-transfer interaction in donor–acceptor complexes (Mulliken 1952). Under the
influence of Mulliken’s paper, Fukui gave a theoretical foundation for his findings. The basic
idea was essentially the consideration of the importance of the electron delocalization
between the HOMO and LUMO of reacting species. These particular orbitals are known as
‘frontier orbitals’.
The frontier orbital approach was developed in various directions by Fukui’s own group
and by other scientists, both theoretical and experimental. Useful reactivity indices,
particularly ‘super-delocalizabilities’, (3) were derived from the theory and applied to various
special topics, for example comparison of the chemical reactivity of different molecules,
polymerization kinetics and copolymer structures (4, 6), antioxidants (8) and various
biochemical substances (5).
However, it was after the discovery of a relation connecting the HOMO and LUMO with
stereoselective phenomena that Fukui’s theory attracted more attention. In 1961 the
importance of the nodal property of the frontier orbital was indicated in the study of silver
complexes of aromatic compounds (7). In 1964 Fukui correlated the symmetry of the HOMO
and LUMO of the reactant molecules with the occurrence of a cycloaddition reaction (9).
This was a result of the simple application of the frontier orbital approach to so-called
‘concerted’ two-centre reactions.
A more decisive illumination of Fukui’s theory was brought about by Woodward &
Hoffmann (1965), who pointed out a specific control by the HOMO and LUMO for the
formation of stereospecific products in thermal cyclizations and photocyclizations of
conjugated polyenes. This discovery was the start of their establishing rules for stereoselection
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Kenichi Fukui
233
in various concerted reactions. They interpreted all of these rules in a unified manner as the
consequence of ‘conservation of orbital symmetry’ (Woodward & Hoffmann 1969
a
,
b
).
All the results explained by the Woodward–Hoffmann rules were interpreted later by
Fukui through the frontier orbital approach (14, 27–29). However, there is no doubt that
Fukui’s work became more widely understood by the chemical community through the
incisive work of Woodward and Hoffmann.
The HOMO–LUMO interaction rationale of the type made by Fukui in 1964 in respect of
cyclic additions was applied later by his own group and by other chemists (Houk 1973) to a
variety of chemical reactions, cyclic and acyclic additions, eliminations, rehybridizations,
multicyclizations, various intramolecular rearrangements, benzyne reactions, ring openings
and closures, and so on, including thermally induced and photoinduced reactions
(13, 14, 20, 27–29). Particular use was made of frontier orbitals with respect to complicated
regioselectivities and various sorts of secondary stereochemical effects in concerted
cycloadditions. These were explained in the same unified manner by the particular interaction
of the frontier orbitals. The charge and spin transfers in chemical reaction paths were
discussed from the same viewpoint (17).
Fukui and his co-workers extended their orbital interaction rationale from two-orbital
problems to three-orbital interactions (16). The important role of orbital mixing and polari-
zation and that of a favourable phase relation in three-orbital interactions were derived to ex-
plain various experimental results. This theory of three-species interaction was introduced to
discuss the role of catalysts in terms of HOMO–LUMO analysis (19), and the concept of
‘pseudoexcitation’ was applied to the interpretation of several chemical phenomena (18).
In addition to these fundamental but rather qualitative successes, Fukui and his group
tried to give the frontier orbital rationale a more quantitative character (10). In 1968 a general
theory of intermolecular reactions was proposed to disclose the general principles governing
the reaction path, pointing out the increasing importance of the HOMO–LUMO interaction
with the progress of the reaction (11). The mechanism of bond interchange essential to the
occurrence of the reaction and the origin of stabilization of the reacting system along the
reaction path were made clear (15).
In this connection the formulation of the reaction coordinate made by Fukui in 1970 (12)
was important. The concept of the IRC was defined as the steepest-descent path from the
transition state. In this way, a method was proposed for discussing the origin of the favourable
or unfavourable character of a reaction path by an analysis of the potential gradient along the
reaction coordinate. A method of investigating qualitatively the contribution of frontier
orbitals along the reaction coordinate was provided by this formulation. By the use of the
IRC concept the geometry change of reacting systems could be calculated. The process was
named ‘reaction ergodography’ (21).
The IRC approach was extended to a wave-mechanical estimation of the absolute rate of
chemical reactions (24). The case of proton migration in the enol form of malonaldehyde was
used as an example (23). In relation to the IRC approach a differential geometrical study of
the configuration space of chemically reacting systems was developed. ‘Normal coordinates’
of reacting systems were defined in a coordinate-transformation-invariant form. The ‘cell’
structure of that space was indicated in relation to the absolute rate calculation (24).
Variational principles, including the geodesic principle implicitly involved in chemical
reactions, were elucidated (25). Thus, a particular Riemannian space could be defined for any
reacting system (23).
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234
Biographical Memoirs
Fukui tried to combine the frontier orbital concept with the IRC approach. This attempt
was successful in obtaining the frontier orbitals of interacting molecules—the interaction
frontier orbitals (IFOs) (26).
The range of Fukui’s research extended to many fields of molecular science that are
related to chemical reactivity. These included the theories of biradicals, the behaviour of
crown ether complexes, chemisorption on a solid metal surface, the formation of charge-
transfer complexes, the role of solvent molecules, interatomic long-range forces, solvated
electrons and the theoretical interpretation of nuclear magnetic resonance and electron spin
resonance spectra. They also included the nature of some intramolecular and intermolecular
bonds, electronic structures and spectra of compounds including high polymers, a theory of
resonant states, the problem of complex eigenvalues and quasistationary molecular systems,
some vibronic problems, and some polymers of practical importance such as linear
superconductors and semiconductors and chalcogenides.
Fukui’s work, particularly the frontier orbital approach, exerted a strong influence on
chemists. The term ‘frontier orbital’ is now used in many papers without a citation of Fukui’s
paper. The book by Ian Fleming (F.R.S. 1993) has the title
Frontier orbitals and organic
chemical reactions
(Fleming 1976). Fukui and Fujimoto summarized selected papers of Fukui
in a book (33) that is useful for understanding Fukui’s contributions to chemistry. A
memorial volume for Fukui was published in
Theoretical Chemistry Accounts
(1999) and
included a complete list of Fukui’s publications in English.
Fukui did not stop studying after he retired from Kyoto University. His recent work on the
IRC and IFO approaches should be mentioned (24, 26). With his co-workers he applied the
IRC theory to an analysis of chemical laser systems as well as to mode-selective chemical
reactions by using vibrational correlation diagrams (30). A succession of IFO plots—‘IFO
correlation diagrams’—is useful for visualizing the mode of formation and the breaking of
chemical bonds in a reaction through a completely non-empirical calculation.
Fukui also tried to extend his theories of chemical reactions to more general rate
processes. He named the resultant equation of the rate process the ‘equation of wandering
mutation’, which was presented at the Fifth Institute for Fundamental Chemistry (IFC)
Symposium in 1989 and was summarized in Japanese and English (32). This was probably the
last systematic scientific study he made.
Fukui’s name is appropriately associated with a useful function in density functional
theory (Parr & Yang 1984). If the nuclei in a molecule give rise to a potential
V
(
r
) acting on
N
electrons and the total ground-state electronic energy is
E
[
N
,
V
(
r
)], then
d
E
=
m
d
N
+
〈
r
(r)
d
V
(r)
〉
,
where
is the chemical potential and
the electron density; the angle
brackets
〈 〉
denote an integral over all
r
;
m
= (
û
E
/
û
N
)
V
r
(r)
d
m
=
h
d
N
+
〈
f
( r)
d
V
(r)
〉
,
where
is the hardness (equal to the inverse of the softness) and
h
= (
û 2
E
/
û
N
2
)
V
f
(r) =
(
û r
( r)
/
û
N
)
V
=
û 2
E
/
û
N
û
V
(r) =
(
û m
/
û
V
(r)
)
N
is the Fukui function (Parr & Yang 1984; Parr & Parr 1999), which can be used as a reactivity
index.
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Kenichi Fukui
235
We end this memoir by quoting from Kenichi Fukui’s Nobel Lecture (28):
In my opinion, quantum mechanics has two different ways of making contributions in chemistry. One
is the contribution to the nonempirical comprehension of empirical chemical results just mentioned.
However, we should not overlook another important aspect of quantum mechanics in chemistry. That
is the promotion of empirical chemistry from the theoretical side. But, also for this second purpose, as
a matter of course, reliable theoretical foundations and computational methods are required. The
conclusions of theories should be little affected by the degree of sophistication in approximations
adopted.
On the other hand, for theoreticians to make the second contribution, the cases where predictions
surpassing the experimental accuracy are possible by very accurate calculations are for the present
limited to those of a very few, extremely simple molecules. In order to accomplish this object in regard
to ordinary chemical problems, it becomes sometimes necessary to provide qualitative theories that
can be used even by experimental chemists. If one can contribute nothing to chemistry without
carrying out accurate calculations with respect to each problem, one cannot be said to be making the
most of quantum mechanics for the development of chemistry. It is certainly best that the underlying
concepts are as close to experience as possible, but the sphere of chemical experience is steadily
expanding. Quantum chemistry has then to perform its duty by furnishing those concepts with the
theoretical basis in order to make them chemically available and serviceable for the aim of promoting
empirical chemistry.
A
The personal part of this memoir is based on Kenichi Fukui’s book
Creation of an academic discipline
(31),
which is very much his autobiography. We thank Mrs Tomoe Fukui for permitting us to use this book as a
source and also for providing other information, including the frontispiece photograph. The section on
Professor Fukui’s research owes much to a manuscript found among his papers. We are grateful to Professor
Hiroshi Fujimoto, Professor Roald Hoffmann, Professor Keiji Morokuma and Professor Robert G. Parr for
their helpful comments.
The frontispiece photograph was taken in 1981 by Sato Photo Co., Tokyo, and is reproduced with
permission.
R
Fleming, I. 1976
Frontier orbitals and organic chemical reactions
. Chichester: Wiley.
Fowler, R.H. 1936
Statistical mechanics
. Cambridge University Press.
Hammett, L.P. 1940
Physical organic chemistry
. New York: McGraw-Hill.
Houk, K.N. 1973
J. Am. Chem. Soc.
95
, 4092–4094.
Mulliken, R.S. 1952
J. Am. Chem. Soc.
74
, 811–824.
Parr, R.G. & Yang, W. 1984
J. Am. Chem. Soc.
106
, 4049–4050.
Parr, R.G. & Parr, J.B. 1999
Theor. Chem. Accts
102
, 4–6.
Theor. Chem. Accts
1999
102
, 1–400. [Pages 13–22 list Kenichi Fukui’s publications in English.]
Woodward, R.B. & Hoffmann, R. 1965
J. Am. Chem. Soc.
87
, 395.
Woodward, R.B. & Hoffmann, R. 1969
a Angew. Chem. Int. Edn Engl.
8
, 781–853.
Woodward, R.B. & Hoffmann, R. 1969
b The conservation of orbital symmetry
. New York: Academic Press.
Yukawa, H. 1947
Introduction to quantum mechanics
. Tokyo: Khobundo. [In Japanese.]
Yukawa, H. 1948
Introduction to particle physics
. Tokyo: Iwanami. [In Japanese.]
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236
Biographical Memoirs
B
The following publications are those referred to directly in the text. A full bibliography
appears in
Theor. Chem. Accts
102
, 13–22 (1999).
(1)
1952 (With T. Yonezawa & H. Shingu) A molecular orbital theory of reactivity in aromatic
hydrocarbons.
J. Chem. Phys.
20
, 722–725.
(2)
1954 (With T. Yonezawa, C. Nagata & H. Shingu) Molecular orbital theory of orientation in
aromatic, heteroaromatic and other conjugated molecules.
J. Chem. Phys
.
22
, 1433–1442.
(3)
(With T. Yonezawa & C. Nagata) Theory of substitution in conjugated molecules.
Bull. Chem.
Soc. Japan
27
, 423–427.
(4)
(With T. Yonezawa, K. Hayashi, C. Nagata & S. Okamura) Molecular orbital theory of
reactivity in radical polymerization.
J. Polym. Sci.
14
, 312–314.
(5)
1955 (With C. Nagata & T. Yonezawa) Electronic structure and carcinogenic activity of aromatic
compounds. I. Condensed aromatic hydrocarbons.
Cancer Res.
15
, 233–239.
(6)
1961 (With T. Yonezawa & K. Morokuma) On cross termination in radical polymerization.
J. Polym.
Sci.
49
, S11–S14.
(7)
(With A. Imamura, T. Yonezawa & C. Nagata) A theoretical treatment of molecular complexes.
I. Silver-aromatic hydrocarbon complexes.
Bull. Chem. Soc. Japan
34
, 1076–1080.
(8)
1963 (With K. Morokuma & H. Kato) The electronic structures and antioxidizing activities of
substituted phenols.
Bull. Chem. Soc. Japan
36
, 541–546.
(9)
1964 A simple quantum-theoretical interpretation of the chemical reactivity of organic compounds.
In
Molecular orbitals in chemistry, physics and biology
(ed. P.-O. Löwdin & B. Pullman), pp. 513–
537. New York: Academic Press.
(10) 1968 (With H. Fujimoto) An MO-theoretical interpretation of the nature of chemical reactions. I.
Partitioning analysis of the interaction energy.
Bull. Chem. Soc. Japan
41
, 1989–1997.
(11) 1969 (With H. Fujimoto) An MO-theoretical interpretation of the nature of chemical reactions. II.
The governing principles.
Bull. Chem. Soc. Japan
42
, 3399–3409.
(12) 1970 A formulation of the reaction coordinate.
J. Phys. Chem
.
74
, 4161–4163.
(13)
Theory of orientation and stereoselection
. Berlin: Springer-Verlag. [Revised edn 1975.]
(14) 1971 Recognition of stereochemical paths by orbital interaction.
Accts Chem. Res.
4
, 57–64.
(15) 1972 (With H. Fujimoto, S. Yamabe & T. Minato) Molecular orbital calculation of chemically
interacting systems. Interaction between radical and closed-shell molecules.
J. Am. Chem. Soc.
94
, 9205–9210.
(16) 1973 (With H. Fujimoto, S. Kato & S. Yamabe) Orbital symmetry control in the interaction of three
systems.
Bull. Chem. Soc. Japan
46
, 1071–1076.
(17) 1974 The charge and spin transfers in chemical reaction paths. In
Proc. First Int. Congr. Quant.
Chem., Menton, France, 1973
(ed. R. Daudel & B. Pullman), pp. 113–141. Dordrecht: D. Reidel.
(18) 1975 (With S. Inagaki & H. Fujimoto) Chemical pseudoexcitation and paradoxical orbital interaction
effect.
J. Am. Chem. Soc.
97
, 6108–6116.
(19)
(With S. Inagaki) Mechanism of
cycloaddition and related reactions between electron-
donors and electron-acceptors. Perepoxide quasi-intermediate and its roles in the reactions of
1
∆
g
molecular oxygen with olefins.
J. Am. Chem. Soc.
97
, 7480–7484.
2
+
2
(20) 1976
Chemical reaction and orbitals of electrons
. Tokyo: Maruzen. [In Japanese.]
(21)
(With S. Kato) Reaction ergodography—methane–tritium reaction.
J. Am. Chem. Soc.
98
, 6395–
6397.
(22) 1977 (With S. Kato & H. Kato) A theoretical treatment on the behavior of the hydrogen-bonded
proton in malonaldehyde.
J. Am. Chem. Soc.
99
, 684–691.
(23) 1979 (With A. Tachibana) Intrinsic field theory of chemical reactions.
Theor. Chim. Acta
(
Berl.
)
51
,
275–296.
(24) 1981 The path of chemical reactions. The IRC approach.
Accts Chem. Res.
14
, 363–368.
(25)
Variational principles in a chemical reaction.
Int. J. Quantum Chem. Symp.
15
, 633–642.
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Kenichi Fukui
237
(26)
(With N. Koga & H. Fujimoto) Interaction frontier orbitals.
J. Am. Chem. Soc.
103
, 196–197.
(27) 1982 The role of frontier orbitals in chemical reactions. In
Nobel Foundation: Les Prix Nobel 1981
,
pp. 146–166. Stockholm: Almqvist & Wiksell International.
(28)
Role of frontier orbitals in chemical reactions.
Science
218
, 747–754.
(29)
The role of frontier orbitals in chemical reactions (Nobel Lecture).
Angew. Chem. Int. Ed.
21
,
801–809.
(30)
(With K. Yamashita & T. Yamabe) Dynamic behavior of the IRC in chemical laser systems.
Theor. Chim. Acta
(
Berl.
)
60
, 523–533.
(31) 1984
Creation of an academic discipline
(
Gakumon no souzou
). Tokyo: Kousei. [In Japanese.]
(32) 1989 Possibility of chemical creation of definite-sequence polymers—a contribution to the discussion
about the origin of life. In
Proceedings of the 5th IFC Symposium, 26 May 1989, Institute for
Fundamental Chemistry, Kyoto
, pp. 29–42. Kyoto.
(33) 1997 (With H. Fujimoto)
Frontier orbitals and reaction paths
. Singapore: World Scientific.
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