Trying to Understand, Making Bonds (2024)

Roald Hoffmann

In 2007, on the occasion of my 70th birthday, Bassam Shakhashiri organized a symposium for me at the Boston meeting of the American Chemical Society. The session was entitled “Roald Hoffmann at 70: A Craftsman of Understanding.” I began my talk with thanks to many. That section has been shifted to the end of this chapter.

Beginnings

I was born in a happy young Jewish family in unlucky times, 1937. In that war, most of us perished, 3800 of the 4000 Jews of Złoczów, now Zolochiv in Ukraine. Among those who were killed were my father, three of four grandparents, three aunts, and so on. I just want to show you three photos which relate to that time, one old and two recent. The last 15 months of the war we were hidden by a good Ukrainian man–Mikola Dyuk, the schoolteacher in the small village of Univ. The first year we were in an attic of the schoolhouse, the second year in a storeroom with no windows, maybe 6 × 10 feet, on the ground floor.

Here are two photos from 2006, when my sister, my son, and I visited Univ. Here is the attic in which we were hidden, with its one window (Figure 1-1).

The storeroom, a passageway, another ground floor room are gone, rebuilt into a new classroom of Univ's school.

It's a chemistry classroom (Figure 1-2). Such is fate. Under the plank floor we dug a bunker to sit in if the police came to the house.

I was five and a half when we went in. And nearly seven when we went out. Here's a photo of me, a few months after we came out (Figure 1-3).

We survived. Some of us. Good people helped us, I tell their story. I am also the speaker for the dead—the three million Polish Jews who were killed do not have good stories to tell, or photos to show.

We built a new life, in refugee camps where I read of Marie Curie and George Washington Carver, and then came to America in 1949.

Then we lived happily ever after! Or, to put it another way—it was wonderful, the only unhappiness in our lives put there by ourselves and no one else. My mother said I must never say anything bad about America.

Moving toward Chemistry

I got to our science sideways, so to speak. By first going toward (under parental pressure) medicine, then edging away. Not having enough courage to go into the humanities—that wonderful opening up of the world to me at Columbia, which you can sense in my mention of my teachers at the end of this chapter. Even in myfirst two years in graduate school at Harvard I wasn't sure about chemistry—I sat in on courses in astronomy, public policy, archaeology. Going to the Soviet Union for a year was a way to avoid commitment.

When I came back in 1961, I knew chemistry was my science. I did my Ph.D. work in one productive year.

You know what I've done, with more than a little help from my friends. I want to talk here about some lines that seem to emerge, ever so faintly, from more than 45 years of scientific activity.

The Rhetoric of Pedagogy

So the chairman of the department (Harold Scheraga) said more or less this: “We thought that after teaching physical chemistry lab, you might want to try an introductory chemistry course.” Jim Burlitch and I did as we were told, and rebuilt an honors course that had withered. We were lucky; in that course were Cornell's brightest students. And I learned how to explain to them thermodynamics without those partial derivatives.

I've taught introductory chemistry every year since; I finished teaching at Cornell with two such courses. The pedagogical imperative entered my research. I was in the business of explaining anyway—in time I realized that most of the difference between teaching an introductory course and writing my technical papers was in the audience—the strategies of communication were different on the surface, but underneath similar.

As a result of teaching, my research improved. I wanted people to understand—students, researchers. So I tried, using colloquial language, using strategies of optimal redundancy, many pictures, an occasional poke of colorful language to wake people up. No mysteries (beyond those of nature) were to be created. And if one could communicate to the reader (so much easier in a lecture) that I cared that they understand, I was home free. They learned.

I've always written my papers not for my colleagues, but for the 3rd year graduate student.

Calculations and Explanations

My research career is intimately tied to the computer age; there are all those tables in my very first theoretical chemistry paper, with Lipscomb. An IBM 604 diagonalized that matrix for C₂B₁₀H₁₂. The sound of the key punch is deeply embedded in my consciousness. As is FORTRAN. But from the beginning, there was this love-hate relationship I had with calculations. For Lipscomb emphasized symmetry and structure, and the work with Woodward taught me the tremendous power of a qualitative argument.

I could sense that the qualitative arguments, based on perturbation theory, were appreciated, used, and cited by people. I think I understood this firmly for the first time (outside of the context of the work I did with RBW) in my initial independent studies at Cornell—on the way a methylene approaches an ethylene, and in the explanation (through bond coupling) of the large splitting of the lone pair orbitals of pyrazine or diazabicyclooctane.

The tension is exacerbated in our times, as simulation crowds out model-building. I've written of this elsewhere.1

I was lucky; I became an explainer in an age that needed them. Now how much of explaining is telling just-so stories, you will have to decide. The stories I've told—of orbitals, of two and four electron two-orbital interactions, of through-bond coupling, the isolobal analogy, of a fragment MO analysis, of crystal orbital overlap populations, of how bands run, and bonds and bands in general—are pretty good ones; they have a way of holding on.

I'm very proud of teaching chemists (together with Jeremy Burdett and Mike Whangbo) the connection between the language and phenomena of solid state physics, of extended periodic systems and the molecular orbital theory of discrete molecules. As for motion in the other direction, convincing physicists that there is value in chemical intuition on bonding, well, the best I can say is that I haven't given up trying …

The Graphical Imagination, or Why Orbitals Matter

So organic and inorganic chemists, with no artistic training, became in the 20th century masters of simple 3-dimensional communication. The context was molecular structure; I am thinking of drawing a chair cyclohexane, benzene-Cr(CO)_3, or the organometallic molecules shown in (Figure 1-4).

All along, people were drawing orbitals and using them. Leslie Orgel's work in inorganic chemistry in the sixties in exemplary, as is Howard Zimmerman's stereo-electronic reasoning in the organic realm. So there was nothing novel in what we did (now that is a recurring theme, and true), only a persistent, pedagogically informed application of orbitals and interaction diagrams to one real chemical case after another, to a multitude of bonding puzzles, to build frameworks of understanding of some generality, in a language accessible to that 3rd year graduate student.

The net result of hundreds of interaction diagrams of the kind you see here Figure 1-5 is the grafting of electronic information in a graphical code onto an already existing stereochemical imagination. So, just as molecules could be seen as structures made up of a framework of definite three-dimensionality, with functionality (donors, acceptors, chromophores) geometrically dispersed on that framework, the same molecules could be furnished, so to speak, with orbitals, and those would explain and control reactivity, color, etc.

The necessary orbital theory was within reach of an intelligent graduate student; driven by my teaching instincts (and maybe my deflected art historical interests) we created a pictorial perturbation theory that seemed to be psychologically just right for the sophistication of the community. Michael Dewar couldn't understand how people didn't take to his numerical perturbation theory for organic chemistry—after all, it had everything in it. It did. But it was not taught nor used, just because it was not graphic. People like pictures. Chemists live off them.

Building Bridges, Making Bonds

I believe leitmotifs emerge, they are not programmed. If someone asked me, “Roald Hilelovich, what is your philosophy of research?” I'd answer—as I answer all your mail, some by hand because my mother, the strong force in my life, said you have to answer mail—I'd answer, “I don't work on important problems, I work on what the world gives me. I am an amoeba.” Now this is said in part for effect, I am, after all, a writer. But here is what I mean—the world is complex and particular, the bead of dew on that blade of grass, and no other blade, no other dew, no other observer than me. And the world is connected—the drop's water is my water, the grass blade's biochemistry has much overlap with mine, the chlorophyll is green whether in that blade or in R. B. Woodward's synthesis. There are differences—the weed respires in a way different from me. The differences are telling.

I have patience. And like a figurative omnivorous amoeba I will look at, listen to, taste anything in this world—organic or inorganic, heavy metal or Bach, Korea or Brazil. I will transform the riches of the world not as an amoeba, but as a human being. Which means I will try to understand the disparate chemical aspects of a rich universe (and not just the chemistry), try to see relationships between them. My faith—not a religious one, but certainly a spiritual one—is that the connections are there, that our humanity lies in seeking such connections.

I've been lucky, within theoretical chemistry. A method came my way—extended Hückel theory, but more broadly qualitative molecular orbitals and the perturbation theory-based way of following their interactions. That method could be used on any molecule under the sun. The fragment molecular orbital analysis attached to this methodology took molecules apart, and put the pieces together, on paper. This permitted connections to be made—one could see similarities, while at the same time the differences emerged in a way consilient with chemical thinking, with variations in electronegativity and donor and acceptor character.

I used that gift which came my way to make connections. So my Nobel Lecture is not a looking back to the story I am immensely proud of, the complex of orbital symmetry ideas developed with R. B. Woodward. Instead, the lecture sets out the isolobal analogy, under the title of “Building Bridges between Inorganic and Organic Chemistry.”

In time I came back to my love for the word and for art, and slowly began to build connections between chemistry and the makers of the spirit—artists, writers, actors, performers, religious thinkers. I could do this only by entering their world, on their terms—the world of struggling to get a poem published, or a play produced. I did this; it remains hard, but tremendously satisfying.

Quiet Subversion, and Fun

By this I mean many things, inter alia my gentle campaign to relax the constraints of the ossified format of a scientific article, or of journals for that matter. I am very proud of having convinced the nonpareil editor of Angewandte Chemie, Peter Gölitz, to publish essays. The Journal of the American Chemical Society and Physical Review wouldn't dream of doing it, right? In an article on silver fluorides, we were allowed to insert a commentary by a sage protagonist, Neil Bartlett, into the text. We could write a discussion on valence bond and MO theory as a dramatic trialogue.

Fun: I think science is fun, as does Bassam, even as it is as serious as a search after reliable knowledge, knowledge that can hurt or heal, must be. The gatekeepers won't let you crack jokes in a paper, but one can be playful in the heart of science. A recent paper on squeezing CC bonds we did traipsed through a jungle gym of organic structures in the service of torturing a nice normal CC bond. The work Roger Alder, Charlie Wilcox, and I once did on square planar carbon was I think, in its own way, compassionate. What could be done, electronically, to give comfort to this unhappy structure?

In the “Entertaining Science” cabaret I run monthly in New York City, we juxtapose science with performance—music, dance, theater, magic. The audience makes every connection we want, and then some. They laugh, with affection, at everything we do. Even I, an awfully serious character (not really), can come off as Harry Gray.

Emotional Engagement

You can't write poetry without it, nor without personal exposure. But I'm speaking of other things as well. It seems to be given to me to be faced with ethical problems. I did not evade the painful consideration of E. J. Corey's claim of having told R. B. Woodward of the MO argument for control of electrocyclic reactions. Nor have I avoided thinking about the actions of Peter Debye in prewar Germany. And in returning to Złoczów, the place where I was born, in 2006, we, the survivors and their children, were drawn into the agony of remembering, and forgiving. There are times I wish I had been spared; but then I reflect, and I am thankful to the world for giving me the opportunity to face them.

I think I've been a good teacher because I've allowed the emotional into what passes between me and my students. So I've been able to communicate to students that the subject is ever new to me, and that I care that they learn, and I care about them as human beings.

We have come full circle. There is a time to thank: For the existential act of living, day by day; the wonder of connections out there to be seen; the microcosm of chemistry; the power of art and the word; the great reach of explaining and teaching with feeling. For being able to try to do these things, ultimately to teach and to retain the desire to be taught, for this I am grateful.

So, at 70, let me give thanks:

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