Professor Daudel: So you are doing a book about quantum chemistry?

Anders: About semiempirical methods, in a non-technical, historical context.

D: Semiempirical methods - -

A: Starting with Hückel and going on from there including your early work of the loges which you published between 1950-1970.

D: Well, these reprints here (1,2) could be useful to you. They are something I have written for the American Encyclopedia - kind of a review of quantum chemistry.

A: Yes, thank you, I can certainly use them. My idea is to write about the semiempirical methods, but not in a technical way. I want also to include the manner in which persons remember it, how persons got to this kind of chemistry. So I would like you to think back in what you remember of the old events.

D: Yes, I remember this period because I started my work with Alberte Bucher who became Alberte Pullman after her marriage. She was my first student in the field of quantum chemistry, together we introduced a new index in the field of quantum chemistry which has been called the free valence number.

A: Ah, yes - you introduced - -

D: Yes - I believe it was in this paper (10), Daudel and Pullman. Pullman, because she had in the meantime been rapidly married to B. Pullman.

A: That was in 1945.

D: Yes. And after that I have shown that there was in some cases a relationship between the free valence number and the potential barrier which occurs in some chemical reactions, mainly in chemical reactions on alternant hydrocarbons. I believe that was the first precise relationship between electronic structure and chemical reactions. If you are interested I can find the reprint for you (1). It is also in the book I have written for the American universities a long time ago (3).

A: Was that the book of 1965?

D: Of 1959, I think. This problem is completely discussed here in this book (3). We have a lot of things about the non-empirical as well of the empirical methods.

A: Why did you write the book?

D: Because the only important book in America was Pauling and Wilson (17). But Pauling and Wilson do not go far into the field of chemical reactivity.

Furthermore, I believe the second idea we introduced here were density difference functions (5;109), for use with any method you like, including the empirical one. That is also in this paper (1).

A: From when is this paper?

D: That's a review from 1992, but I had introduced this notion a very long time ago (14-16).

A: Did you do these calculations by hand?

D: The first calculations were done by a small hand calculator. I remember that once with A. Pullman we drew some small diagram - one had to turn very hard by hand for doing it all. That was before the computer time. I think we used a computer for the first time during the 1950's. That was our first touch with the world of electronic computers.

Then there is the second paper which shows the change of the electronic density which appears when two atoms are bounded (2;630). The difference between the molecule and the sum of the electronic densities of the atom - so it is very well used, this function.

But in this paper (1), as you can see a more general review, I do not give the detailed references. And after that comes the loge theory which is completely devoted to lobes because it was an homage to Charles Coulson. And I started discussing that with Coulson.

A: When did you start to work on the loge theory?

D: Near 1950? In 1953! The exact references are in this paper (7).

A: The loge theory - how did it develop?

D: The idea was - well, it is explained in reference (1). The starting point was that in the field of wave mechanics it is not possible to distinguish between K-electrons and L-electrons because all electrons play the same role in a given molecule. This being the undistinguishability principle of wave mechanics. Therefore it was kind of a compromise between the way chemists are thinking, because they were thinking about K-electrons and valence electrons, and physicists. In quantum chemistry it is not possible to speak of valence electrons. The question simply was: how had the old chemists made their model?

Therefore I tried to find a concept which was rigorous from the wave mechanical view point and which had something in relation with the old idea of valence electrons, bonds and so on. All these chemical notions were not correct in the field of wave mechanics. And for that we started in a very simple way. We decomposed the space into many regions and we tried to find the regions which minimized the missing information function. That is to say: what is the decomposition of the space which gives the maximum amount of information. From this starting point the ideas developed. So you see the definition is there (2;233 and 5;18); you have to minimize the missing information function (12):

You have to minimize I! If you do that, for example in the case of the molecule (1;114 and 15), you obtain 2 spheres about each nucleus, and the missing information function has its minimum value when R = 1.53. And the probability of finding two electrons with opposite spin in each loge is 0.95, so it's very high!

A: Are you a chemist by training or a physicist?

D: I had both trainings. Because I started to be a physicist in what is called in France "high school", grand école. After that I started medical training because I was the assistant of professor Irène Joliot-Curie in the Radium Institute. She asked me to work on cancer by using radioelements in order to precise the mechanism of the formation of cancer cells. Therefore I worked in the field of medicine. And as a consequence I moved from mathematics to medicine. But it was experimental work. My training is mainly experimental. But I can do calculations with the computer as well.

A: Back to the loge. Is it a chemical concept?

D: It is to save the chemical intuition in the field of the rigorous point of view of wave mechanics. And that was not easy to do because a priori the two ways of thinking were in contradiction.

A: What is the present state of the loge theory in France.

D: I don't know.

A: It was never so well accepted - - there was no computer program ever made?

D: There was a Canadian chemist who did that - Bader. He has done some calculations. There are many references about loge theory in my second textbook on quantum chemistry(5). You see, all the facts, drawings, etc. in this book have been done in cooperation with Bader (5;22 and 5;107).

During the year 1947 I met a strange work which established a bridge between chemistry and radioactivity (4;69 and 5;111). While I was working at the Radium Institute I made a very simple quantum mechanical calculation showing that the period, that is to say the half life of the radioactive nucleus, may depend on the molecule in which they are. That was rather strange because usually it is said that the rate of transformation of nuclei does not depend on the environment. And I showed that it was not the case in some kinds of radioactivity. After that I discovered the phenomenon experimentally with some colleagues.

A: Do you still hold this idea that a chemical environment can influence - -

D: Oh yes! It's published. It's a new phenomenon which I discovered in 1947. But probably it's not in this kind of book because it is - - I don't know why, it's more considered physics than chemistry. But it really is a bridge between the two. Here is the paper: Altération des periodes radioactives sous l'influence des méthodes chimiques (8). But that's not the experimental work, it's the calculation. As for the experiment - - it was a rather complicated experiment which we did immediately after the calculation. That was in 1948, I think. And this paper here of 1952 (9) may well contain the exact reference to the paper of 1948.

A: What else can you think of?

D: Then I am also working in a completely different field, namely in Aids, in connection with the European Academy. We are trying to improve the treatment of patients who are infected by the virus and we are also preparing a vaccine. So you see: a completely different field.

A: Do you work theoretically on this or also in practice?

D: I believe mainly practically but using theoretical ideas. We are presently doing a theoretical essaying. We have a hospital especially organized to do that.

A: Why did you change into this field? Did you think it's more worthwhile?

D: One of the reasons is the following: I was mainly working in cancerology, as I said, by using the idea of quantum chemistry and the method of radioactive tracers. From the experimental chemistry view point as well as from the quantum theoretical view point. For example: we discovered that the carcinogenic molecules like alternant hydrocarbons are not really carcinogenic molecules. They are modified by enzymes, they are oxidized by enzymes. And the molecules which are the really carcinogenic causes are the result of this oxidation. Epoxides. This may be somewhat surprising. You see the organism tries to eliminate the compound by oxidation. We have shown that experimentally: if you put an alternating hydrocarbon into nitric acid in vitro, nothing appears. But when you do this on the skin of mice, then you have your reaction. We found this out with tracers, radioactive carbon. So you see, this was the end of my work in the field of cancer. Furthermore I did also some treatment by using radiation.

After that when I had been named President of the European Academy it happened that one day we were asked by the director of UNESCO to do something in the field of Aids. And with Montagnier - you know Montagnier?

A: All I know is that he is the discoverer of the Aids virus.

D: Yes, and he is a member of the European Academy and he was also my colleague during the days in the Radium Institute. Therefore we decided together to organize a group of laboratories in Europe and in North America which were interested in the field of Aids. Right now we are coordinating the research of that rather big group. This is why I am now working in this field of Aids. Together with Montagnier I've written 4 or 5 reports in this field.

A: Could you provide me with some literature on the epoxides?

D: The last work was done by my wife and some English workers. The reprint is in FEBS Letters (6).

A: Would quantum chemistry really have influence on the thinking of physical chemists in the sense that they use it in their daily work or do they only use it afterwards to give some explanations? How do you see this in retrospect?

D: If I consider my particular case, I must state that initially I was in a group which did both, theoretical quantum chemistry and experimental work. I believe in fact that somehow experiments have been inspired by quantum chemistry. So in some cases I know that very certainly there is some relation.

Let me give you an example: It concerns one of my former students, Prof. Dannenberg, who is now professor of quantum chemistry at New York University and who did also some syntheses of chemical compounds. New kinds of syntheses which have been inspired by quantum chemistry since he is both, an experimentalist and a quantum chemist. Therefore he believes that, provided a student has had both trainings, then and only then will there be certain fruitful interaction between the two fields.

And you see, in the field of Aids we are now looking for inhibitors of enzymes because what we try to do is to really beat the enzymes used by the virus to proliferate. This case is another example - there are indeed uses of quantum chemistry. Because with quantum chemistry we can roughly calculate the bond energy between a molecule and an enzyme like the protease which act in the virus to produce what is called the maturation of the virus. And if this protease does not act the virus is not active.

So you see that in this field quantum chemistry has been useful to inspire experimentalists. But usually those people are doing both: They are doing the calculations and they are doing the experiments. That is not only the case with Dannenberg, it is generally the case of American workers who use quantum chemistry to produce synergators of protease.

A: That is a great success of quantum chemistry if chemists really use it on these very modern needs.

D: Perhaps it is less real for experimental chemists who are not really doing quantum chemistry. I am clear or not?

A: They are - -

D: Those chemists who only have a rough idea about quantum chemistry - I am not sure that they are able to use it to do experiments. There are some quantum chemists who are doing only calculations. Then there are chemists who are doing experiments but who are not able to do real quantum chemistry. In all these cases I believe the interaction between theory and experiment is not so high.

A: What about the didactics of quantum chemistry? Too much general theory? What is the situation in France?

D: I don't know. It was not the case in the school in which I did my studies in chemistry and physics. Because we were working 5 hours a day in the laboratory and I believe that it is extremely important. For myself I believe that it is important to try to have the students to work in the laboratory a very long time each year.

We had 3 hours of lectures and 5 hours of experiment each day - I believe it was a good ratio. I am under the impression that the time of work in the laboratory is very important, Otherwise we can do only those types of calculations which are not really useful.

Do you agree?

A: Yes, maybe because my upbringing as a chemist went in the way you just described.

D: Because I have been in relation with various workers in the field of quantum chemistry who were doing very long calculations. But in my opinion they did not obtain very useful information. And if both, experimentalists and theoretical chemists, are doing calculations, it should serve the experiment. Therefore there is a creed between theoretical and experimental activities that we are not doing calculations which are not useful.

We want to do calculations in order to better organize some experiments. And this is rather different.

A: Do you consider commercial programs to be of some help? Would you use those programs in your research?

D: Not now. Formerly we had the following situation at my institute: I was the director of laboratories in which about 50 people were working - all of them coming from different countries of the world. Therefore they were bringing along and using a lot of programs of all kinds from empirical to theoretical. Others were using no programs at all, producing new theories - - .

In my impression they were interesting people because they came from important research laboratories. Our setup at the institute was a mixture between various approaches and of various forms of thinking. Produced perhaps by the diversities of cultures - because we were completely international. There were always two or three workers from the States, due to my books. A lot of them, after having worked through my book, were coming to my laboratory to apply what they had learned. Then they also came from England, Germany, Sweden, Latin America, Tunisia. And at least it was interesting for that main reason: You see because of all these diverse backgrounds all kinds of thinking were admitted; we were not polarized towards one single method. Each week we had a general discussion about one special topic in which every kind of thinking was proposed and discussed.

A: You were at the Sorbonne?

D: No. I was a university professor but I was a Director of a laboratory of the CNRS (Centre National de la Recherche Scientifique). And that was much better because the CNRS was very rich.

A: ENS, CNRS, Sorbonne - I do have a problem relating them.

D: The CNRS is a government organization of research which has a special budget, completely independent from the universities. The CNRS has laboratories which only depend on the CNRS. And by chance some laboratory can be directed by a university professor but it's not the rule, it's a possibility, it was my case.

A: The CNRS has laboratories everywhere in France?

D: The CNRS has laboratories in all parts of France. There are two kinds of laboratories: laboratories which only depend on the CNRS - as in my case - and associated laboratories. That is kind of an association between a university and the CNRS. In this case the CNRS helps some universities budgetwise.

A: Does the CNRS belong directly to some ministry?

D: Yes - to the Ministry of Research.

A: It's like the Max-Planck Gesellschaft in Germany?

D: Yes, something like that.

When I was director of a laboratory it was very easy to do because we had plenty of money. I had 5 secretaries, we had a very big computer and a lot of technicians to manage the calculations.

A: Prof. Daudel, should I have any other questions I can perhaps contact you in writing?

D: All right.

A: It was a very interesting interview which gave me some idea about French quantum chemistry.

You are indeed one of the active parts of that French quantum chemistry. And you were and, as one can see, you still are very active in publications.

D: I believe I have written some 30 books, but not only on quantum chemistry. One of them being an especially interesting experience because we did this book in organizing an international symposium during 3 years and the book arose from the various contributions to the seminar. It was completely programmed in such a way that in the end a full book resulted, completely organized in chapters and so on.

And another book here: A lot of people have contributed to this: A book for the secondary school in Frances. Molecular Vision of the World (13), with particular intensity on the relation between science and the arts.

A: Prof. Daudel, once again, thank you very much for this interview.

(1) R. Daudel, About the nature of the chemical bond.

J. Mol. Struct. (Theochem) 261, 113-114 (1992).

(2) R. Daudel, Quantum Chemistry.

Encyclopedia of Physical Science and Technology, Vol. 13, p. 629-640 (1992).

(3) R. Daudel, R. Lefebvre and C. Moser, Quantum Chemistry.

Interscience, New York, 1959. 2nd. printing 1965.

(4) R. Daudel, L'Empire des Molécules. Hachette, Paris, 1991.

(5) R. Daudel, G. Leroy, D. Peeters and M. Sana, Quantum Chemistry.

John Wiley and Sons, Chichester, 1983.

(6) R. Daudel, Recent Work of Pascaline Daudel.

Polycyclic Hydrocarbons and Cancer, Vol. 2, p. 37-41.

Acad. Press, New York, 1978.

(7) S. Odiot and R. Daudel, Relation entre le volume occupé en moyenne par un électron

Compt. Rend. Acad. Sci. 238, 1384-1386 (1954).

(8) R. Daudel, Altérations des périodes radioactives sous l'influence des méthodes chimiques.

Rev. Sci. 85, 162 (1947).

(9) R. Daudel, Relation entre les propriétés radioactives des noyaux et la structure

chimique des molécules qui les renferment. J. Physique 13, 557-562 (1952).

(10) R. Daudel and A. Pullman, Sur le calcul de la répartition du nuage électronique

Compt. Rend. Acad. Sci. 220, 888-889 (1945).

(11) B. Werth, Das Milliarden-Dollar Molekül. Verlag Chemie, Weinheim, 1996.

Orig. Title: The Billion-Dollar Molecule. - , - (1994).

(12) Pi : Probability P of i events. For more details, see Ref. (5), p. 18f.

(13) R. Daudel, Vision moléculaire du monde. Hachette, Paris, 1981.

(14) R. Daudel, Remarque sur la rôle de l'indiscernabilité des électrons en

chimie théorique. Compt. Rend. Acad. Sci. 235, 886-888 (1952).

(15) M. Roux and R. Daudel, Effet de la liaison chimique sur la densité electronique.

Cas de la molécule Li2. Compt. Rend. Acad. Sci. 240, 90-92 (1955).

(16) M. Roux, S. Besnainou and R. Daudel, Recherches sur la répartition de la densité

J. Chim. Phys. 53, 218-221 (1956).

(17) L. Pauling and E.B. Wilson, Introduction to Quantum Mechanics.

McGraw-Hill, New York, 1935.