Roald Hoffmann talks to Alison Stoddart about chemical bonding and his new playground of high pressure chemistry

Roald Hoffmann

Roald Hoffmann is professor of theoretical chemistry at Cornell University, US. Roald shared the 1981 Nobel Prize in chemistry for the development of theories concerning the course of chemical reactions. His current interests include high pressure chemistry, chemical bonding, poetry and philosophy.

 

What prompted you to become a chemist?
As a child, I read two books while I was in a refugee camp - the biography of Marie Curie by her daughter and the story of a black American agricultural chemist, George Washington Carver. These accounts formed a saintly image of chemistry. So the science interest was there early on, but there was pressure on me to become a doctor. I decided not to follow this path, but only in graduate school did I commit to chemistry. I liked chemistry; it fitted my abilities and interests. Somehow, I have been intuitively able to latch on to the chemical essence of a question.

What were your thought processes behind the Woodward-Hoffmann rules?
I had just done calculations on boron hydrides with William Lipscomb and Martin Gouterman. These gave me some respect for experiment and for complexity outside of planar  electron systems. I also developed a method to calculate almost any molecule, in particular organic molecules, so I was ready when Robert Burns Woodward approached me. Woodward knew that the stereochemistry of electrocyclic reactions presented an important problem. But I was young; I only realised that we had explained something worthwhile when I saw the reaction of the community. From this work, I learned to form explanations instead of just doing calculations.

Why did the Woodward-Hoffmann rules have such a big impact?

They formed a connection between organic and theoretical chemistry. That meeting point had been prepared for but the rules cemented things. Molecular Orbital theory was perceived as useful by organic chemists and not too difficult to learn. Organic chemists were given a pictorial language of where electrons were in molecules through pictures of orbitals and orbital interactions. This connected to the geometric structural language, prevalent for describing structures, adding an electronic dimension to the stereochemical pictorial language which was already there. When we drew ethylene in three dimensions and added the * orbital, it wasn't that far a stretch.

What remains to be learnt about bonding?
We have to move on from calculating simple energy surfaces and doing dynamics on them and develop a qualitative notion for how to describe molecules colliding on a surface and doing reactions. What's missing is a qualitative intuition for turning a potential energy surface into a feeling for the entropy and energy parts of an activation energy. Barry Carpenter at Cardiff is doing pioneering work on this; it's one thing I would do if I were starting over again.

Why are you interested in high pressure chemistry?
The high pressure realm is a playground for bonding ideas. It's so much fun to see carbon dioxide turn into quartz at high pressure - that is, in terms of its bonding properties. When the pressure is removed, carbon dioxide gas bubbles off. The high pressure community needs a valued high pressure synthesis of a desired material (other than diamond). The problem is that after you reach a metastable structure, the return to the ambient pressure form usually does not encounter large barriers because the reactions are symmetry-allowed. I would love to design a reaction that would face high barriers, but it's tough.  

Also, I have a wish to convince the physics community, which has a big stake in the high pressure research, that a chemist's intuition of what is stable or unstable has some validity and utility.

What advice would you give to young researchers?

Whether you are young or old, looking for connections is essential. But you need to be good at one thing (in my case it was theoretical chemistry); that gives you psychological strength. The danger then is to get stuck in that field, the area you're good in, and follow your intellectual fathers. You have to look outside the field and look for connections. So my advice would be to specialise, but don't forget the forest for the trees. Look for the connections between things - that's what makes us human.

What would you have been if you hadn't been a scientist?
I could easily have gone off into the arts or humanities. I may have become an art historian. I have no regrets - I have somehow been able to carve out a land of my own between chemistry, poetry and philosophy. Some of the science informs my poetry, and perhaps my writing in science has improved because of my love of literature. Two metaphors which are important to me are bridges and connections. My Nobel lecture was called 'Building bridges between inorganic and organic chemistry' - it was about the isolobal analogy. I think I have helped to build bridges between sciences, the arts and humanities.

What achievement are you most proud of?
It's the combination of making connections between different parts of science and being a teacher. Teachers wake up the minds of people - rather than teaching facts, they empower young people to make use of the abilities within them. I am proud of being a good teacher and teaching in many ways.