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MARGINALIA

That’s Interesting

Curiosity drives discovery. But what, exactly, makes us curious?

Roald Hoffmann

Food for Thought

2011-09MargHoffmannFB.jpgClick to Enlarge ImageAbnormality, like novelty, can evoke interest. But any given anomaly may be viewed as interesting or not, depending on the cognitive setting of the observation. The observer first has to know what is normal in order to care about what is not. Tetraamminelithium (Li(NH3)4), pictured at right, is a bronze-colored metallic liquid (this combination of properties is interesting by itself) that crystallizes into a metallic solid at 89 degrees Kelvin (–184 degrees Celsius). That is a low temperature, but so what? Other elements and molecules solidify at lower temperatures: molecular hydrogen (H2) at 14 degrees Kelvin and helium not at all. But none of these other atoms or molecules is a metal. Li(NH3)4, a liquid and a metal, has a freezing point nearly 150 degrees lower than that of any other liquid metal. How am I to think about the way intermolecular forces (responsible for solidification of all compounds) interweave with the free motion of electrons that is the hallmark of being a metal, to make Li(NH3)4 melt at such a low temperature?

2011-09MargHoffmannFC.jpgClick to Enlarge ImageWhen we make new observations, we compare them to the way we think the world works. And that’s something we are constantly revising: In one’s mind is a dynamic, shifting conception of how the pieces of the world fit in, now and in times past. With some parts of this world, one accumulates a lot of experience, as I have with chemistry. I’ve been at it long enough to form an intuition. But most subjects, from Venezuelan literature to the ecosystem of the Kazakh steppe, I know less well. And I assess an observation differently when it pertains to a field that I do know. A new touring-ski wax will not get my attention, but when a graduate student of mine brought to a research-group meeting the structures of two related organometallic molecules, illustrated to the right, I sat up. Each of these molecules, synthesized by Kanazawa University’s Kiyoshi Isobe and his collaborators, has a rectangle of sulfurs, bonded to iridium atoms in one case, to rhodium atoms in the other. Rhodium appears directly above iridium on the periodic table, and the two elements are expected to have similar properties: What happens for rhodium, in the way of molecular geometry, should happen for iridium. But the rectangle is turned one way in one molecule, the other way in the other. Now that was interesting! Enough so that it formed part of the Ph.D. work of Anne Poduska, the student who found the structure of these molecules in the literature.

Of course one also has to learn discrimination. Many of the world’s anomalies are unimportant, little zags where a number of variables combine to make something appear more abnormal than it really is. Others are just experimental mistakes. But there are anomalies worth worrying about. The molecule with an unexplainable feature sits there, staring you in the face with its peculiarity. “Understand me if you can,” it says. “Make me if you can.” The challenge is there, put forth quietly as I sit at my desk. In response, I will build an explanation. For that is my métier. And the next issue of a journal will provide me with a new puzzle.

Anne and I readily admit that coming up with an explanation for our perplexing dance of rectangles did not shake up the world. And yet, and yet—everything is connected to everything else. I believe that by solving a thousand such bonding puzzles, each of tiny significance, and keeping one’s eyes open for the inevitable connections between those little pieces of the world, ultimately one will have the world, at least the chemical part of it.

Incidentally, making those connections is inherently interesting, for it brings together two or more parts of our universe, parts that previously stood apart. Here is a personal example: Many inorganic molecules could be thought of as being built from fragments such as ML3, ML4 and ML5 (where M represents a transition metal atom and L an associated ligand, such as CO, PH3 or Cl). The Lego blocks of organic chemistry could be seen as CH3, CH2 and CH. I once realized (based on the foundational work of others) that, despite the traditional separation of organic and inorganic chemistry, the electrons in both sets of building blocks moved in orbitals that resembled each other. Here was a mapping that allowed one to see like features in the structures and reactions of both organic and inorganic molecules. Making that connection was one of the most satisfying experiences of my life. Which brings me to the joy of creation.




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