Dr. Anders: Professor Salem, besides being a quantum chemist by training you have written and co-authored a great number of successful books in all kinds of fields (1-8). In quantum chemistry you wrote at least two well-known books: In 1982 you issued Electrons in Chemical Reactions: First Principles (9). Earlier on, in 1966, you had published The Molecular Orbital Theory of Conjugated Systems (10) about which one still hears how well written it was. It was and still is certainly well received by the students, even nowadays it is, for instance at the University of Constance, much in use - it's always out on loan. This probably being due to the fact that quantum chemistry is quite often presented in a fairly abstract, dry manner, and which is, to the ordinary chemist at least, not always appreciable in its practical orientation. So students still pick up your book for that very reason: in order to find out what they are really doing and why.

Furthermore, generally speaking, I am searching in my interviews time witnesses of the early days of quantum chemistry. I will try to tell a story of the beginning of quantum chemistry starting at and about Hückel.

Prof. Salem: Just in parenthesis: Berson at Yale has written a book about Hückel (12) and he wrote a big Angewandte Chemie article on Hückel very recently (13). He is really one person who has gone into it.

A: It can be seen from the literature that you have worked with Longuet-Higgins. What kind of a person was Longuet-Higgins? How do you remember him?

S: First, well, he was a rather complex person. And as to your second question: Being born in 1937, I went over to England in 1957 when I had passed my BA degree at the Sorbonne. I had a mathematician father who had a colleague at Chicago University, Antoni Zygmund (14). So my father wrote to him for advice. Zygmund asked Mulliken with whom I should go to work with. I really wanted to work with the best people. My father wanted me to do biology and biochemistry. So Mulliken gave two names. He gave the name of Francis Crick - that was in 1957 (15) - and if I really wanted to do Theoretical Chemistry, that of Longuet-Higgins . So I went to see Longuet-Higgins to ask whether he would take me.

And I realized that I had come from an entirely different world because I discovered the world of true science where knowledge was not only memorizing - not like in school and also unlike in graduate classes: where you learned about science just by learning definitions. You didn't really know what a molecule would look like, the molecules were abstract. It's like knowing a country in terms of physical ways and not of its people. And I remember what Christopher Longuet-Higgins said to me in '57, when he said: "Think of molecules as they are and not as you like them to be." It always gets to my mind. Although he was very unimpressed, he gave me articles to read. I was staying somewhere in southern France and I went to the library of Marseille and tried to read some of the articles of semiempirical methods, by Pople (1998 Nobelprize, added in proof by L.S.) and others. I didn't understand a single word. I realized how far behind I was. So as for himself he was clearly - - - (thoughtful, I's. n.).

Now, I don't know whether he had learned a lot, but he had really a very creative mind. In other words: he would think up of a problem and he would have an idea of how to solve it. I mean, people think of problems less than the people who already see the solution while thinking of the problem. In other words: it comes at the same time. He was really most capable doing that. Creatively think of a problem and he would already know how to do it. And he always had a good feeling how the problem would end up, he wouldn't be surprised.

And in terms of personality he was sort of a shy man. I think he had a big complex with women, possibly because back in those days in Cambridge, between 1957 and when I majored in 1960, it was a men's college where all men spent all the evenings together. I would say he was sort of a lone wolf, in a sense.

And why he left the field is also difficult to know, but I think he felt that he had done major discoveries, that he was not recognized at the international level. There were well known feuds between him and Löwdin, of their schools, I mean. Longuet-Higgins was very critical of Löwdin's work on the hydrogen bond. As I remember it, Löwdin had brought forward the idea that proton tunneling between the H-bonds could be responsible for mutations and for the splitting of the DNA (16), I don't remember. There were lots of those. Everybody had taken up DNA because it was very fashionable. Some of the ideas were really very, very extravagant. And Löwdin himself had been realizing that .... There were quite a bit of hard feelings between Löwdin and Longuet-Higgins.

When Woodward and Hoffmann made their discovery of the conservation of orbital symmetry (17a,b), Longuet-Higgins was the first to really say that you had to deal with states and not with orbitals. This was in a paper with Abrahamson (18). And when he ... ahead of that time was that - - - (thoughtful, I's. n.). But Longuet-Higgins was telling me that Woodward and Hoffmann sent him their paper, their first paper (17a), it had just come up, and he, in two days, he was reflecting that probably it will be the states. And apparently they held back his paper, they were referees - or at least they didn't react to it immediately. Mentioned him in their second paper (17b), but he was very upset about this. And you know, he felt that he contributed as much to this discovery as they had, sort of. Maybe he felt that he should have shared their Nobelprize, I don't know. He certainly is a highly original thinker - - - I would say (thoughtful, I's. n.) we are going a bit off the field - the two best theoretical chemists in the world still alive are Longuet-Higgins and Martin Karplus (20).

Now, with his students, he was extremely - - - (thoughtful, I's. n.). I would see him once a week. I could go and see him sometimes in between. And I would see him at the morning tea and at the afternoon tea where people talked nothing else but science. He might ask you or Leslie Orgel (21) or John Pople (22) might ask something - that was an incredible group: There was Longuet-Higgins, there was Pople and Orgel. Longuet-Higgins in his office, Pople and Orgel had a little, tiny, tiny office, a 3 m2 room in a room. The other people included Clemens Roothaan (23) who was there for a year, Robert Sack (24) - he was a polymer chemist, he's dead now, and Andrew McLachlan (25), John Murrell (26) and John Griffith (27).

A: Transition metals.

S: Yes. You had a fantastic group of people in this: Longuet-Higgins, Pople, Orgel, Griffith, McLachlan, John Murrell was there, and so you had six or seven of the really leading quantum chemists in the world. So it was really very exciting. I was really very proud to be in this group. And so you receive a lot. And you would really learn, day in and day out. (thoughtful, I's. n.).

He wanted me to do a thesis on intermolecular forces. And again, you know, it was clear the difference between what we had learned and what we wanted to accomplish when dealing with things was 100 %. And one day he would like me to tell him what I had done in this and this research. And one other day he had said: "Lionel, how do you measure this intermolecular force?" Of course I didn't know. So I had to go home for two weeks and write a dissertation of 25 pages on how to measure intermolecular forces and so I plunged into the thing. And after this I knew all about how to write and how to measure intermolecular forces. He wanted me to get into contact with experiment; he wanted to know that people have at least some contact with experiment!

And the work we did, which was most relevant, was the work on bond alternation, where we published this article (28), saying that the long polyene would, after a certain length, would have alternate double and single bonds.

Now it turned out that he discovered just as the article was in press, that Peierls had done the same thing in solid state physics when working on one dimensional chains (29), so we mentioned this work though we had been unaware of it when we did our own. There had also been similar efforts in Japan by Ooshika (30a,b)- but we had made the fundamental work.

This was his idea and how could a student come out with something like that! And he found the solution to it, his name being associated with this. Here it begins to be very important. Because the instabilities in physics are very important - - - .

So that's about it. He (Longuet-Higgins, I's n) liked music a lot, went to concerts. I was a bit scared of it. I enjoyed it quite well for a few years - - -

A: Do you consider it true that he suppressed Boys?

S: Ah, there was also Boys, of course - I didn't mention Boys, I made an omission there! It was incredible, he essentially considered what Boys was doing (31) as non-important. There were two types of theoretical chemistry: one who had qualitative ideas, that was with a little backing of calculations; and then the other - things like Boys was doing. And he didn't - - -. You know - it's like buying on the stock market, it's good to know when things go up. He didn't judge Boys properly, he didn't realize that what Boys was doing on Gaussians was going to be extremely important. So he did not - - - . The true fact was that Boys was - no, not suppressed: in the same way that Longuet-Higgins has not been given international recognition, he certainly did not give Boys his due recognition. That must be true. But you have to say also about recognition that Boys was not Longuet-Higgins, that generally speaking, most theoretical chemists are only recognized at a later date. Except people who were already there involved in computing in an early stage like J. D. Roberts (32). I mean, even John Pople who finally went into Gaussians and all that - only not in 1948 as Boys did, but after some years.

But I mean, that when John Pople was in the lab I think he was more like Longuet-Higgins, he would not see that what Boys was doing anything really important. He caught up later when he introduced his GAUSSIAN programs.

A: One of the stories about Longuet-Higgins is that he considered only ideas worth while which could be written on the back of an envelope - was that really true?

S: No - but he often did do things on the back of an envelope.

A: So one can maintain that sort of thing?

S: Yes, absolutely. And if you are going to write you should go and see him - - - .

A: Yes. But at first I didn't have his address. But I was also warned by some people that he might not answer my letter.

S: Well it's likely that he doesn't like to talk about his old days. I don't know if he has been successful in artificial intelligence or not, I haven't seen his name very often. It's been years that I wrote to him. You know - - - (thoughtful, I's. n.). Many theoretical chemists feel that we are a bit like mathematicians, if we are 30 or 35 years old then we are at our best. And I myself, when I was 40, I tried to move into banking, I tried to do politics, I came back into theoretical chemistry and never did as well as before. And finally now I am the head of the Center for the Popularization of Knowledge (xx). I did leave. There are few people who stay in like John Pople or Roald Hoffmann. Even then I think probably the best work was done earlier on. I think you got very few cases where a guy is say 60 years old, and comes out with a fantastic paper. And because it's very close to mathematics. And then some people have to change because they don't like to do the same thing all the time.

What else is there to add to the story? Not very much. If you want to have another picture of Longuet-Higgins then talk to people who were closer to him like Pople or Orgel. These people were much closer, they were colleagues not students like myself.

A: Apparently Longuet-Higgins doesn't want to talk about quantum chemistry - McWeeny said something like that.

S: It's possible, he doesn't go to any of those meetings in theoretical chemistry, I don't know it either. I was talking to him on the phone 10 years ago when I had a book out and sent it to him. He was very nice about it.

A: Murrell?

S: Murrell is having his 60th anniversary in December (1997, I.'s n.). I had promised to go - I can't go finally, I don't find the time. He's sort of retired. Murrell was a very nice fellow liking to play tennis very much.

A: Many early semiempiricists went to him like Heilbronner, Trinastic among many others.

S: Let me just add one thing, more of a general comment (thoughtful, I's. n.). As I told you in the beginning: I had an enormous handicap in terms of physical chemical knowledge. The young students, PhD's, coming out of the British universities were extraordinary good physical chemists. The reasons of my being so poor in the field was one that I'd never really be excited about science, in a sense. I wasn't fanatical about science when I was 9 years old, I didn't do experiments in the kitchen. I was pushed into science by my parents, and second my formation on the whole was very bad. Now, however, with a little hindsight I discovered in life that some people were extremely logical, knowledgeable people, they would tell me where you put this electrode there, they reasons why a car goes and all that.

So I don't even know why the sky is blue. But they would tell you why the sky is blue - or they would tell you anything. These people are not necessarily the people who have original ideas. And though I have seen people who were really outstanding physical chemists, they would not necessarily become outstanding researchers. And I'm not saying I became an outstanding researcher. But I was lucky enough to be a man who liked to discover, to begin to think. So in spite of my not having the know-how I was able to discover the things to say like that didn't fit with that, there must be a problem here and all that. So this detective part in research, in discovering nature which had allowed me despite of this enormous handicap in knowing things. And I think of one person who was an outstanding physical chemist, it was Andrew McLachlan. And he did for a few years very, very good work on the theory of EPR. But after that when he went into biochemistry, and I think for the next years, it didn't work so well. So he was - - - he could speak to you about any physical chemical subject. He was perfect, he had some fantastic background. But the background was not used to ask himself questions on this or that. Put him on something - he would do it. But he was not himself sort of engaged in the constant questioning which you have to have in order to discover things.

A: My training in Europe was also missing some of the hard facts of science you describe. Maybe the European way of thinking is much less concerned with the production of numbers but quite often is trying to think what these numbers mean.

S: Yes, but that's another pair of things. That's the idea that some people are just cranking out numbers in the computers without trying to see what is in them and then again we get back to the Longuet-Higgins version of Boys' type of approach. That is one thing. But I think that, again, is more like saying people will be trying to look qualitatively what these numbers mean, rather than just being happy to get a lower number for the free energy.

This is not quite the same thing as saying that people who have extremely - - - . I don't know what example to give. A man who would be a fantastic tennis man as long as he plays against his mirror. He has the best score and all that, against the machine. But the day he plays on a championship meadow of court but he didn't get through. It's not quite saying, well - - - . And that is something different to know whether you are going to do qualitative or quantitative work, people who are just giving out numbers or not. That's the idea of whether people are innovative or not innovative. And there is also part of luck in research and discovery. The people who have innovated are the better champions.

A: You are specialist in the popularization of knowledge.

S: Yes.

A: Do you have a book out on which you base your lectures?

S: All right - there is no book. Because popularization of knowledge is not - - -. It's a field of more like making violins, then learning how to play the violin. In other words making of the violin goes further. It's a field where you learn by doing it. And some day you write a book on how to popularize. First of all there is no unique way for popularizing.

What we've done is the following: The most interesting thing we made were very big posters precisely saying a number of very simple things. Why are your socks red, why do boats float, why does the water in the freezer explode glass bottles, why is the sea salty? And we even did that in the Paris underground in March until June. And they have been quite popular. Three million people are taking the underground every day, so a lot of people phoned up, and we have had a big success. We had a prize from the ministry and now people are thinking putting them in the airplane terminals in the USA. So it's getting quite some success and we don't do it just on the natural sciences. We popularize political science, we did a series of books, small guides, on the citizen, on law, the town, the region - part of it, the state, contemporary art. We try to do all fields because we feel that everything in this world is getting more and more specialized and it doesn't pay. We want to get culture to the general public.

So I started this, because when I wrote my book The Marvelous Molecule, Molécule la merveilleuse, in 1978, it was really a success and it was translated into many languages (xx). And then I wrote another popularization book on math with one of my daughters who is a graphic artist, and that's still selling in the states very, very well (xx). And so these two books put me in the mood of trying to do the Center of Popularization. It's not easy, a lot of people think it's a waste of time. But I think the trends in that direction at the moment - - - it was that people doing pure research realize that there aren't people who are translating their gains to the general public.

A: I think you got a point there. Every field branches into more and more subbranches, and so on. I think in this context that quantum chemistry has to explain itself in a similar manner, since it is paid for by the general public.

S: Of course, the general public will be more and more demanding about this and - - - (thoughtful, I's. n.) - - - I think also it's a way of giving the general public some form of culture, the taste of culture.

A: You wrote me that you wanted to prefer to talk about the energy surfaces of photochemistry in quantum chemistry rather than about your very first book. How did you proceed then from the empirical Hückel molecular orbital theory to the theory of reactions?

S: I went to a Gordon Conference in the United States, an organic chemistry symposium. They have one a year, this one was at Brandeis and it was all about reaction mechanisms. So I got this idea about reaction mechanisms. And so the first thing we did was an enormous calculation on the isomerization of cyclopropane and we published the first transition structure with all the 21 coordinates (11a), nobody having ever done this type of calculation before. Then after that, what I think, Paul de Mayo, a grand photochemist, who had used to call me "Baby Machiavelli", came to a meeting at Orsay. My boss in Physical Chemistry, Magat, also wanted me to get interested in the field.

So I wanted to go, in '74, to that photochemistry meeting - and I was very perplexed. I remember there was this fellow, he was a very nice chemist called Hans Schmidt. He gave a talk and I was only half listening because I just couldn't understand him. He would start, he put hn - and he would have this excited molecule. And then before you knew it, the final thing would be the ground state molecule. I didn't know where it was coming down. He decided that this is the ground state. For these people, you know, there were no such things as allowed states for the chemists. They were sort of empirical people. So I kept worrying about this a lot. I talked about it with Dauben and Turro. And I was using very simple valence bond structures. This was the hydrogen distraction photochemical reaction. And finally when I went back to our house and my first wife was already in bed. And so one evening in October while working upstairs, the house was kind of dead, she was already in bed, and I thought for the tenth time over the valence bond structure of the ground state of a ketone and then in the end to find the excited states, I put a CH bond beside it. And then I tried to write the diradical of the other side, and there must be something excited on the other side.

And it turns out I'd been working also on the electronic theory of diradicals. I published an article in 1972. That is a main classic paper. I mean it is the second most quoted theoretical paper in that journal after the Woodward Hoffmann paper. It says essentially that diradicals take up four states: a singlet diradical, a triplet diradical and two ionic states, higher. But anyway people took a diradical and gave it an ionic excited state; then something would show that the ground state of the reactant was correlated to this side of the product and the excited state of the reactant to the ground state of the products. In other words: it went up here and then it would just ski down, like a skier, and would end up here. So there you had the surface crossing! So this is what I considered - - - . Now Turro and I published the surface crossing and there may be even indications for chemical reactions and so that sometimes they are called Salem's crossings and sometimes they are called Salem's rule, but anyway. And so again - - - (thoughtful, I's. n.) - - - if I hadn't been bored at Hans Schmidt's lecture and if I wouldn't have been listening to him with a half ear I would have never found it. I then realized there was something there which just wasn't right. I talked about it to Turro and Turro wrote out for me a typical reaction, not the one of Hans Schmidt, and I worked on Turro's reaction.

And as it shows, congresses are important, talking to people is important as well, then you have to have a nice evening, relax, with a wife in bed, then you discover something like that.

A: For how many years did you pursue this?

S: Quite a bit, and it's recognized that it all was done by Turro, Dauben and myself. It was important because the Woodward Hoffmann rules had said what happens if you have thermal reactions which are allowed, others are forbidden. And then you have certain photochemical reactions which were allowed and forbidden, but they were not reactions the photochemists use. The photochemist uses carbonyl in chemistry with a -cleavage or abstraction and these are not at all involved in the Woodward Hoffmann rules. This really was setting up the photochemistry framework which was different from the rules and it was one which would essentially involve the states. And essentially the idea is that things cross and once you are there, your skier, your molecules, just ski down.

A: And then you had at least three famous students who became well known.

S: I had Devaquet. He was a very good theoretical chemist. He spent two years at De Mayo's, he has worked a lot in photochemistry. And then, I think, he went once to a meeting of the Gaullist movement and he met Jacques Chirac, in the toilettes, and Chirac said: "Would you like to work with me?" And that's how it started. But he had a very unfortunate political career because he was extremely honest, he was one of the few honest politicians. And he was involved in a great upheaval as soon as he became minister of Universities in 1986, this was when Mitterand lost the legislative elections. Mitterand was still President but Chirac became Prime Minister. He took Devaquet as Minister of the Universities. And then the people who were on the right pushed right away very hard a law which said that you had to have selection at the entrance of the universities. Students walked in the streets. And finally one student got killed by the police. There were lots of things and in the end Devaquet had to resign and he never was able to come back, even when Chirac was President of the Republic. Even then, after, it must have been 9 years later, he was not taken as minister. His political career was really killed by this. Maybe he will come back in 10 or 15 years, now he's 55. So he had a very promising career, he was a bit of an intellectual, and so politicians don't like this. He was really a great fellow, very intègre as we say, ethical and all. So really fate was unfair to him and maybe I did bring bad luck to my students.

A: Well, I wouldn't think so. Then there were other second generations students of yours.

S: Well, there is X. Chapuisat, who is a socialist, and he has not gone on as much as Devaquet. He is number two of the university. He is President of the Executive Board of the University and he has a brother who has been on the cabinet of Jospin. Chapuisat tried to be elected several times as member of parliament; he was from one of the big départements in the Southwest but where he has never been able to win for some reason. He is not involved as much as Devaquet was. (Added as postscript in Jan. 1999: Chapuisat has just been elected President of the University of Paris-Sud.)

And then there is a third one called Bernard Bigot. And Bigot was a sort of grand-student of mine, because he worked with Devaquet. Chapuisat is on the left, Devaquet on the right, Bigot was, as student I guess on the right also, also a bit of a student of mine - - - . Bigot was a student of Devaquet and he - - - (thoughtful, I's. n.), he essentially was first - he was Director of Studies at ENS de Lyon and after that, when the right wing came back to power four years ago, with still Mitterand being President, Bigot was appointed Director of Research in the ministry that engaged him. And now he is Director General of Research and Technology. An extremely important post. Because this means that all the research and technology of France is somehow dependent on him. Now, with the Socialist Government coming in and with a socialist minister, it's not clear whether he will stay or not. But as a young lad, he is maybe 38 or 40, very dedicated, a real top notch civil servant and - - - . So I think he has a lot of future. (Added as postscript in Jan. 1999: Bigot left and is now head of the Institute of Catalysis at Villeurbanne).

Those are the three I think of. I may have forgotten someone. They all go into politics. I would have loved to go into politics, but - - -.

A: Berson at Yale, does he have a chair of History of Chemistry?

S: No, he is an outstanding organic chemist.

Well - I think that's pretty much - - -

A: Yes. Thank you, Professor Salem, that you have given me the possibility to talk to you. It was very interesting. Thank you very much.

(1) L. Salem, Molécule, la merveilleuse. Inter editions, Paris, 1979.

(2) L. Salem, Le Krach de 1987. Edition no. 1, Paris, 1987.

(3) L. Salem (ed.), Le Dictionnaire des sciences. Hachette, Paris, 1990.

(4) L. Salem, F. Testard, C. Salem, Les Plus belles formules mathématiques.

InterEditions, Paris, 1990

(5) C. Cabrol, La Bataille pour la Vie: La chirugie au quotidien.

( sur une idée du Prof. L. Salem). Hachette-Carrère, Paris, 1993

(6) A. Blocker, L. Salem, L'homme génétique. Dunod, Paris, 1994.

(7) L. Salem, La parenthèse. Université Paris-Sud, Paris, 1997.

(8) I. de Maison-Rouge, J.-M. Prevost, L. Salem, L'art contemporain. Milan, Toulouse, 1997.

(9) L. Salem, Electrons in Chemical Reactions: First Principles.

John Wiley & Sons, New York, 1982.

(10) L. Salem, The Molecular Orbital Theory of Conjugated Systems.

Benjamin, New York, 1966.

(11a) L. Salem, C. Rowland, Die elektronischen Eigenschaften von Diradikalen.

Angew. Chemie 84, 86-106 (1972).

The Electronic Properties of Diradicals.

Angew. Chem. Int. Ed. 11, 92 (1972).

(11b) L. Salem, Intermolecular Orbital Theory of the Interaction between Conjugated Systems.

I. General Theory. J. Amer. Chem. Soc 543-552 (1968).

(11c) L. Salem, Intermolecular Orbital Theory of the Interaction between Conjugated Systems.

II. Thermal and Photochemical Calculations. J. Amer. Chem. Soc 553-566 (1968).

(12) J.A. Berson. in press, VCH-Wiley, New York, 199x.

(According to the records of The Library of Congress, Washington, Internet-Catalogue,

this book could not yet {as of Dec. 1998} be found in its records.)

(13) J.A. Berson, Erich Hückel - Pionier der organischen Quantenchemie: Leben, Wirken und

späte Anerkennung. Angew. Chemie 108, 2922-2937 (1996).

(14) Antoni Zygmund was a well-known mathematician and author of many books, eg:

A. Zygmund et al., Contributions to Fourier Analysis. Princeton Univ. Press, Priceton, 1950..

A. Zygmund, Trigonometric Series. 1968.

A. Zygmund, Intégrales Singulières. Springer, Berlin, 1971.

Antoni Zygmund. Sein Leben und sein Beitrag zu der Entwicklung der Mathematik

im 20. Jahrhundert. German Thesis D83/3039, Univ. Regensburg, 1983.

(15) Crick, Watson and Wilkins were honored in 1962 with the Nobelprize for their discovery of

the configuration of the DNA.

For an early popular account see:

J.D. Watson, The Double Helix. Weidenfeld and Nicolson, London, 1968.

F.H.C. Crick. Of Molecules and Men. Univ. of Washington Press, Seattle, 1966.

F.H.C. Crick What Mad Pursuit. Basic Books, New York, 1988.

(16a) P.-O. Löwdin, Proton Tunneling in DNA and its Biological Implications.

Rev. Mod. Phys. 35, 724-732 (1963).

(16b) P.-O. Löwdin, Effect of Proton Tunneling in DNA on Genetic Information and Problems

of Mutations, Aging, and Tumors. Biopolymers Symposia 1, 161-181 (1964)..

(16c) P.-O. Löwdin, Some Aspects on DNA Replication; Incorporation Errors and Proton

Transfer. Electronic Aspects of Biochemistry, Academic Press, New York, 1964.

(16d) P.-O. Löwdin, Quantum Genetics and the Aperiodic Solid. Some Aspects of the

Adv. Quant. Chem. 2, 213-360 (1966).

(16e) P.-O. Löwdin W.M. MacIntyre, Electronic Energy of the DNA Replication Plane.

Int. J. Quant. Chem. 2S, 207-217 (1968).

(17a) R.B. Woodward, R. Hoffmann, Stereochemistry of Electrocyclic Reactions.

J. Amer. Chem Soc. 87, 395-397 (1965).

(17b) R. Hoffmann, R.B. Woodward, Selection Rules for Sigmatropic Reactions.

Reactions. J. Amer. Chem Soc. 87, 2511-2513 (1965).

(17a) R.B. Woodward, R. Hoffmann, Stereochemistry of Electrocyclic Reactions.

J. Amer. Chem Soc. 87, 395-397 (1965).

(18) H.C. Longuet-Higgins and E.W. Abrahamson, The Electronic Mechanism of Electrocyclic

J. Amer. Chem. Soc. 87, 2045-2046 (1965).

(19a) R.B. Woodward, R. Hoffmann, Die Erhaltung der Orbitalsymmetrie.

Angew. Chemie 81, 797-870 (1969).

Angew. Chem. Intern. Ed. 8, 781 (1969).

(19b) R.B. Woodward, R. Hoffmann, The Conservation of Orbital Symmetry.

Verlag Chemie, Weinheim, 1970.

(20) Some references on M. Karplus during this period:

M. Karplus, G.K. Fraenkel, Theoretical Interpretation of Carbon-13 Hyperfine

J. Chem. Phys. 35, 1321-1323 (1961).

M. Karplus, R.G. Lawler, G.K. Fraenkel, Electron Spin Resonance Studies on

J. Amer. Chem. Soc. 87, 5260-5261 (1965).

(21a) H.C. Longuet-Higgins, L.E. Orgel, The Possible Existence of Transition-metal Complexes

of cycloButadiene. J. Chem. Soc. 1969-1972 (1956).

(21b) L.E. Orgel, An Introduction to Transition-Metal Chemistry Ligand-Field Theory.

Methuen, London, 1960.

(22) Some references on J.A. Pople during this period:

J.A. Pople, D.P. Santry, G.A. Segal, Approximate Self-Consistent Molecular Orbital

Theory. I. Invariant Procedures.

J. Chem. Phys. 43, S129-S135 (1965).

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