Kenichi Fukui (1983) - Chemical Reaction and My Life

Mr. Chairman, ladies and gentlemen. First of all, I should like to express my heart felt gratitude to Count Bernadotte and Das Kuratorium, and also the Oberbürgermeister Steurer and the city of Lindau for the kind invitation and generous hospitality shown to me here. This morning I have the pleasure to talk about chemical reaction and my life. I was born in 1918, the same year as Professor Barton, in Nara,Japan. Nara was an old capital of Japan and the house of my birth was an old family, lasting for many generations at the same place. I spent a large fraction of my childhood in that house, intimately conversing with the nature. My father was not a scientist, he finished a first rank college of commerce and was engaged in foreign trade. I selected chemistry as my speciality by a casual occasion. When I was a high school boy,my father consulted one of his acquaintances to shape his son’s course. My father’s acquaintance was a chemistry professor at Kyoto Imperial University. That professor became later my life teacher. That professor suggested my father to send me to his department when my father told him that I was fond of mathematics and physics. At that time, in 1937, mathematics was believed not so important to chemistry in Japan. Professor’s suggestion was very unusual but really far seeing. About twenty years after that, the character of chemistry changed rapidly. At present, both mathematics and physics are effectively used for the purpose to give chemistry a side of logical science. Such situation of chemistry was brought about through the following path, in my opinion. The progress of computers was unexpectedly rapid. In addition to this, the outstanding development of physical chemical instruments facilitated chemical analysis and structural determination a great deal. These two striking trends accelerated the change in the fundamental character of chemistry in cooperation with the progress in quantum mechanical theories. It was fortunate for a mathematically oriented high school boy to have selected chemistry as his special field as early as in 1937, owing to the professors’ suggestion. Such a story indicates a non-autonomous nature of my decision. But I myself believe that I decided my course autonomously, because it was myself that learnt something keenly from the professors foresight. The industrial chemistry department of Kyoto Imperial University I entered in 1938 was of strongly empirical character at that time. But professor told me frequently to study basic sciences for the future. I again followed his suggestion. The self study of quantum mechanics in my student age was extremely useful for me for the subsequent forty years research life. In this way my research life was sustained by the two suggestions made by that professor, before entering university and in my student age, which I followed faithfully. The first experimental study was carried out in my third year student age. That is in 1940. It was the reaction of paraffinic hydrocarbons with antimony pentachloride, the remarkable difference in the activity between tertiary carbon and a primary or secondary carbon drew my interest. Incidentally this problem was later explained by my reactivity theory in 1961. After my graduation from Kyoto Imperial University in 1941 I was engaged in the study of a new method of hydrocarbon synthesis involving routine ionisation process. By the successful result of this study I was awarded a prize in 1944, when I was 25 years old. While studying the butane isomerisation I learned that a carbon group easily shifted to the neighbouring carbon in an electron-deficient carbon-carbon bond. Corresponding to a kind of Wagner-Meerwein or Whitmore rearrangement. This problem was again incidentally treated in the theory of 1,2 sigmatropy in the Woodward Hoffman rule in 1969. And such a type of this reaction was included in the work of professor Roald Hoffmann, who shared the 1981 Nobel prize with me. The experience of experimental studies of hydrocarbons favoured my start of study on the theory of chemical reactions. In 1951 I became full professor of the fuel chemistry department of Kyoto University. That department was specified in hydrocarbon chemistry. The existing electronic theory at that time was very powerful but not so convenient for the purpose to explain the reactivity of hydrocarbons. Because, as you know, usual hydrocarbons, the charge density is almost uniform in the molecule. More decisively some hydrocarbons react with both electron attracting and electron repelling agents, at the same position. Evidently such a result is difficult to explain by the electronic theory in which the electro-static force between electric charges is a principle factor. This question attracted my attention, I did not hesitate to attack this problem. Because hydrocarbons had long been an intimate subject with me since 1940, as I told you before. Assisted by an analogy with the role of variant selections in the bond formation between two atoms simply, I tried to calculate the electron distribution in the highest energy occupied orbital of aromatic hydrocarbons. I found that the position of attack in the substitution reaction by an electron attracting reagent known from experimental data agreed completely with the position of the largest density of this particular orbital. I was delighted with this result, of course, and I named this particular orbital as ‘frontier orbital’, by the analogy with border guards of a country. This was in 1951, and the paper appeared in Journal of Chemical Physics in 1952. As you will see later, my research career, during which I wrote more than 470 scientific papers, was made up of four strides, big steps. The work of Frontier Orbital was the first stride in my research life. I adopted two ideas of somewhat physical nature. One was that I discarded all electrons other than of the highest energy, the other was that I regarded a chemical reaction as an exchange of electrons between molecules. That is a mutual delocalisation of electrons. The concept of mutual electron delocalisation easily led me to an idea that in an electron accepting reaction of an acceptor molecule the lowest energy vacant orbital will become the frontier orbital. That is to say the reaction will occur at the position of largest amplitude of the lowest unoccupied orbital. The frontier orbitals are frequently abbreviated as ‘HOMO’ and ‘LUMO’. That is the abbreviation of ‘the highest occupied molecular orbital’ and ‘the lowest unoccupied molecular orbital’ respectively. For research work in science, to reach to a new result, generally speaking, it happens frequently that several selections are essential. That is selection of the subject models, premise, method of logical thinking and so on. Coming back to my case, I selected as a subject the reaction of hydrocarbons from the beginning, which was hardly tractable with existing electronic theory. I adopted as a model for a chemical reaction a mutual delocalisation of electrons between reactant molecules. The detailed mechanism of the bond formation and breaking was easily understood by the aid of this model. In order to interpret the position of attack in a chemical reaction, you can put the following premise which was derived straight forwardly. That is, the chemical reaction likes to take place at the position of large amplitude of the frontier orbitals. I selected the method of calculation in which the degree of approximation might exert not so serious as an influence upon the theoretical result. In this manner I was able to make a successful selection in each item. The object of the theoretical treatment was extended from plainer compounds to non-plainer compounds and from substitution reactions to each other, to other reactions. The theory seemed to have a general character. May I have the first slide, please. This is the highest occupied molecular orbital, that is HOMO of naphthalene, an electron-deficient reagent like NO2+ attacks position 1, where the amplitude of HOMO is larger than position 2. Next slide, please. This is the lowest unoccupied molecular orbital, that is LUMO of naphthalene, an electron-rich reagent like NH2- also attacks position 1, where the amplitude of LUMO is larger than position 2. Next slide, please. This is a frontier electron density and LUMO density of aromatic hydrocarbons. In this case, the frontier electron density in HOMO and in LUMO are the same. The dot indicates the position of largest frontier electron density which agrees with the position of attack. Next slide, please. This is the LUMO density of halogenated non-planar hydrocarbons. The position of attack each hydrogen is interpreted by the LUMO density value. If you want to compare the reactivity of different molecules,you need some theoretical index. I derived an index called superdelocalisability from my theory in 1954. This idea was used in many papers for the purpose to compare the chemical reactivity of different molecules. I was glad that a book was published in 1981, this book collects various numerical data of reactivity indices for biological molecules. And two thirds of the whole content comes for my theory. Next slide, please. The second stride in my research life was made in 1964. I calculated the phase relationship of frontier orbitals in one typical organic reactions and found an interesting relation. That reaction I discussed was an additional reaction usually called diene synthesis. By the discovery of diene synthesis, professors Otto Diels and Kurt Alder were awarded the 1950 Nobel prize in chemistry. A few years in advance to the start of my theoretical work. The most remarkable difference of the orbital itself from the orbital density is that the orbital function has a sign + or –. The orbital density is a square of orbital function. The border of two regions with different signs is called node. On the nodal plain the orbital density vanishes. The regions with the same sign in one orbital are bonding in that orbital. The regions with different signs are anti-bonding to each other in that orbital. Therefore the electron delocalisation from HOMO or one molecule to LUMO of the other molecule in a bimolecular reaction will cause a change in bonding and anti-bonding situations. This slide shows the frontier orbitals of butadiene and ethylene. The reaction of butadiene and ethylene is hypothetical but stands for the prototype of diene synthesis. Butadiene is the standard of diene and ethylene is that of dienophile. The phase relationship indicates a favourable situation for simultaneous bond formation and for the bond exchange. This is HOMO of diene and this is LUMO of dienophile, in which you can find +, this full line is part of + sign. And the dotted line is part of – sign. You can see the orbital phase situation + / -, + / - in favour of diene synthesis between butadiene and dienophile. In this case also LUMO of diene and HOMO of dienophile, which are favoured in orbital phase situation. In this way you can understand that the mutual electron delocalisation will bring about an exchange of chemical bonds, which is nothing but a chemical reaction. I investigated the electronic mechanism of the Diels-Alder reaction in detail. I noticed that the symmetry relationship of HOMO and LUMO of diene and dienophile were favourable for the simultaneous bond formation between these reactants. This was a first notice that the orbital symmetry was connected with easiness of occurrence of chemical reaction. You can understand that frontier orbitals have more nodes than lower orbitals, and hence most effectively contribute to the bond exchange. This paper appeared in some monograph published in 1964 as a tribute to Professor Robert Mulliken, who is the 1966 Nobel Prize winner in chemistry. But it was after the advent of a well-known 1965 paper of Professor Woodward and Professor Hoffman that this paper of mine attracted attention of a majority of chemists. Robert Woodward was awarded the Nobel Prize in chemistry in 1965 by his alcaloide study, but very regrettably passed away in 1979. Roald Hoffman, who is 19 years younger than I, has been a friend of mine since 19 years ago. I met him for the first time in Florida in 1964. His research had a close connection to mine and served to make my work better known to public. Such a situation was very fortunate to me. My 1964 paper was developed by many researchers in two directions. One was the relation of the frontier orbital phase and the selection rule of chemical reactions. In various cyclic additions, the favourable situation is that the frontier orbital phases of reactants are in conformity with each other at both sites of reaction. The other was the regioselection, that is regional selection in cycle additions. The effect of a substituent group at the reactant molecule upon the regioselection was discussed theoretically and compared with experimental data. It was my pleasure that two books, which collected mainly the results of application of frontier orbital interactions, were published in the United Kingdom in 1976 and in 1979. One of them had the title of Frontier Orbitals and Organic Reactions. You may be interested to learn what is the origin of electron delocalisation in the interaction of two reactants. It is the molecular vibration that helps to pump electrons from HOMO and pour them into LUMO, at the beginning of interaction. The direction of favourable interaction is determined as that of maximum overlapping of HOMO and LUMO. Once the reaction starts, the electron delocalisation reduces the reparation energy to advance the reaction. And at the same time it forms the reaction site in reacting molecules, if the frontier orbital phase relationship is favourable. The idea of HOMO / LUMO interaction was applied to various chemical facts including molecular stability. In this connection I have to say I am much obliged to Roald Hoffman because he showed the usefulness of isolobal concept in the theoretical interpretation of properties of complicated method complexes and he used the frontier orbital in his definition of isolobal concept. He presented the result beautifully in the Nobel lecture in 1981 in Stockholm. Next slide, please. The third stride in my research career was taken in 1970. I proposed a concept of reaction path that is called intrinsic reaction coordinate. Which is usually abbreviated as IRC. Intrinsic reaction coordinate. This is to define the centre line of the reaction path on the potential surface. Which is given as a solution of a differential equation called IRC equation. The solution curve is chosen as one which starts from a stable point and reach another stable point passing the transition state. This slide shows a schematic diagram of potential energy surface and the reaction path. The zig-zag line represents an actual classical path and the dotted line, dotted smooth line is IRC, that is intrinsic reaction coordinate path. You may feel interesting if I tell you the following story. Well, when I was in Chicago in 1970 I submitted this paper to the Journal of Physical Chemistry of the American Chemical Society. The letter from the referee read, this paper, is not much original, nevertheless it is publishable. The referee’s judgement was quite correct, because my paper was too simple to be said original. But it was later developed in many directions, and therefore it was truly publishable. Shortly speaking, IRC is the steepest decent path from the transition state defined mathematically. That is IRC can be compared with path on which one can go down most steeply from the col to the base. In the case of actual mountain, it is easy to find the valley path, but in the path of reaction, the problem is that the measure of distance varies with the mass of atoms involved in the reaction. The space which determines the internal configuration of the reacting molecular system closely assembles the space in which the general theory of relativity was discussed. That is space conveniently treated by differential geometry. Next slide, please. The theory of IRC has many applications. One of the practical merits of the theory is to obtain the height and the shape of the potential barrier of a reaction, theoretically. By the reason already mentioned, the breadth of potential barrier may be affected by the mass of atoms that is a kind of isotope effect. In addition to this, you can determine the change of shape of reactant molecules in the course of reaction. You can also calculate the magnitude and direction of force acting on each atom. I named such a non-empirical theoretical analysis as “reaction ergodography”, of course “ergodo”means path of energy. This slide indicates the reaction of a hydride anion with a methane molecule. That is a model reaction of SN2 type substitution. And you can see how the molecular shape is changed from here to here, going to the final state. Next slide, please. This is a potential barrier of methane tritium reaction, that is radical reaction. The full line is non-deuterated methane, that is CH4 reactions and the dotted line is deuterated methane, that is CD4 reaction. Next slide, please. This shows the IRC diagram for the HNC-HCN reaction, that is isomerisation of hydrogen cyanide. This is nitrogen atom fixed at the origin and this is carbon atom and this is a path of a hydrogen atom along the reaction path. This is the transitional state point. You can see the carbon atom moves, changes very little in almost, just in this range. The second practical application is to plot against IRC any quantity which can be theoretically calculated with respect to every point on IRC. The quantity can be energy of each electron electronic state, energy of each molecular orbital, vibration frequency and mode of each normal vibration, curvature of IRC and so on. The diagram of these plots may be called correlation diagram in general and the molecular orbital correlation diagram of the reacting molecular system could serve to make the Woodward Hoffman correlation diagram quantitative. The vibrational correlation diagram and the curvature correlation diagram may be important for the study of laser selective reactions. Because these correlation diagrams are helpful to discuss which mode should be activated in order to cause a desired chemical reaction selectivity. The third application is to calculate or qualitatively discuss the absolute rate of a chemical reaction. For this purpose the internal configuration space for the molecular reacting system is divided into cells by the assembly of solutions of IRC equation. That is mathematical division of the space. In each cell you have a stable equilibrium point which corresponds to a stable molecular system. You can treat the chemical reaction theoretically as a wave mechanical transfer, as a process of wave mechanical transfer wave packet, initially confined to the initial cell from that cell to the final cell. This model is helpful to get a theoretical picture of our chemical reaction including that tunnelling effect. The fourth application of the IRC theory is to catch the general feature of chemical reactions. You can show that the chemical reaction follows the valuation principle as any other physical laws do, like Fermat’s Principle, Maupertie’s Principle, Principle of Wrist Action, Hamilton’s Principle and so on. In this model, the chemical reaction is regarded as a sort of large amplitudinal motion, which is essentially indistinguishable from an ordinary molecular vibrational motion. The situation remains unchangeable also in a very large system, like DNA or protein molecule. The self motion of DNA could be connected with a large amplitude motion in the configuration space. In the large amplitude motion of such a large number of freedom, a large amount of dissipated energy should be supplied by a separate route. Next slide, please. My research life experienced a fourth stride in the last two years of my professorship at the Kyoto University. That is in 1981 and early 1982. This was a study of the frontier orbitals of two reacting molecules on the way of IRC. That is to say interaction frontier orbital. Usually abbreviated as IFO. I succeeded in that study by anthologising delocalisation energy between the two molecules to obtain the molecular orbitals in the form of pairs. Each orbital of a reactant molecule obtained in this way can interact with only the corresponding orbital of the partner molecule. The most contributive pair is called IFO, the most contributing to IFO is a frontier orbital of each isolated reactant. This slide shows the IFO, that is interaction frontier orbital, of protonation of hexatriene. The upper right orbital is the frontier orbital of hexatriene indicated together with other hexatriene orbitals. While the left is an IFO, where the position of the proton approaching other point, which is 1.5 Å above the left end carbon-carbon bond. Next slide, please. This slides shows a interaction frontier orbital of dimerisation reaction of acroleins. You can see the orbitals, essential to the addition reaction, are made visualised. Next slide, please. This slide indicates the model proceeding of the reaction of methane and the hydride anion already mentioned, shown in nine diagrams. The reaction starts this top left and to right bottom, like this. The mode of bond exchange is most clearly understood by an orbital picture. But this dotted area is an area of anti-bonding IFO. So at the beginning of the reaction anti-bonding region is produced here, but after that the bonding region is produced here to make a new bond here and one of the hydrogen of methane is expelled as hydride anion. Obviously this work was to connect the work of the third stride to that of the first stride. You can draw a succession of IFOs along IRC by a graphic display automatically obtained from computer without the use of empirical numerical data. If you had enough money, you can easily make an animation film of a chemical reaction showing how a bond is formed and is broken. Such a film will be no doubt instructive for chemical education. Serving as a visualisation of the chemical reaction. In this way, the theory known as frontier orbital theory has been made grown by the aide of a certain idea of physical nature, I had conceived in atmosphere which favoured me strongly. My selection of what to do and how to do was also luckily appropriate for the chemical world in which the selection was essential. I dreamed in my student age a very self considered wish in which I could contribute to reduce rather empirical character of chemistry which I felt at that time in Kyoto University. I believe that my desire has been - I hope that my desire has been attained at least to some extent. The circumstances surrounding the art and mankind particularly the situation of resources and energy seem to impose a heavy burden upon chemistry. This implies that chemistry has to be responsible for a contribution of science of saving, collecting, recycling, newly developing and creating resources and energy. For chemistry, to carry out this duty, the wisdom of nature hidden in it should hereafter more precisely and in more details be elucidated to copy the nature’s wisdom. Such is needless to mention, but it is not enough for chemistry to fulfil its responsibility, it happens to become also necessary to devise and utilise the wisdom and mechanism which do not exist in nature as it is. In order to devise our wisdom out of our mind without direct recourse to the wisdom of nature, you have to invite a new principle and assess the effect of its realisation. Desirably without relying upon the assistance of experience. In this sense it is desired that at least some areas, some parts of chemistry, particularly of physical chemistry, will be required by this reason to increase its less empirical character. Thank you very much.

Kenichi Fukui (1983)

Chemical Reaction and My Life

Kenichi Fukui (1983)

Chemical Reaction and My Life

Comment

Many lectures of Nobel Laureates at Lindau are highly interesting but somewhat formal presentations of scientific ideas and results. As I remember it from my years in the audience, each time a Laureate included some facts or stories about his or her life, the 700 young students and scientists sat as lighted candles and followed each word with maximum interest. Those present in 1983, when Kenichi Fukui gave his one and only presentation in Lindau, should have experienced this effect. The lecture starts with a charming account of Fukui’s personal life story and how a young boy interested in mathematics and physics became a chemist at the Kyoto Imperial University during the 1930’s and early 40’s. Even though he started in the Department for Industrial Chemistry, his professor encouraged him to continue and develop his interest in basic science. After some experimental work on hydrocarbons, Fukui concentrated on trying to understand chemical reactions from a theoretical point of view. With the help of quantum mechanics and the basic concept of molecular orbitals, he developed a set of rules defining which of the many orbitals are really important in a chemical reaction. These orbitals he named “frontier orbitals” and most of the scientific work he describes in his lecture is based on this concept. Through the whole lecture, Fukui now and then tells a little personal story, how he met Roald Hoffmann, what the referee wrote in his report on a certain manuscript, etc. As in his Banquet Speech on the Nobel Day in Stockholm 1981, he ends his lecture in Lindau 1983 with a general appeal for the development of an even deeper understanding of basic chemistry, in order to help mankind survive on Earth. Applause!

Anders Bárány

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