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MARGINALIA

One Shocked Chemist

Molecular surprises are sometimes right in front of us, if only we’d do the math

Roald Hoffmann

Graphane versus Benzene

Xiao-Dong found that over a wide pressure range, all three graphanes were more stable thermodynamically than benzene, per CH. When he showed me his computational results, I immediately said, as all research advisors do, “There’s something wrong with your calculations.” When he demonstrated to me that his calculations were correct, I sat back for a while and closed my eyes.

As a chemist, I have an intuition honed by 50 years of practice. It’s a pressure = 1 atmosphere intuition, of course. Under Neil Ashcroft’s tutelage, I have learned recently of the strange world of high pressure.

But a CH hydrocarbon that is more stable than benzene at ambient pressure? Benzene is the prototypical aromatic molecule, the emblem of chemistry. The six-carbon ring is a conserved entity in the chemical universe—one can run a multitude of substitutions on the ring, replacing one hydrogen after another. So from benzene, one can go (and chemists already did in the 19th century) to chlorobenzene (C6H5Cl), to toluene (C6H5CH3), to nitrobenzene (C6H5NO2), to the explosive trinitrotoluene (TNT, C6H2(CH3)(NO2)3), to aspirin, mescaline, novocaine, estrone… any of a host of substituted benzenes. All the time, in biological systems as well as the laboratory, the hexagon of carbons remains intact, a testimony to its stability.

Benzene is such a good thing that the literature is in fact full of mostly imagined “aromaticities,” supposed harbingers of stability that exist only in the paper-writers’ hype-addled minds. But the stability of the parent C6H6 system is sacrosanct. It’s hard to argue with it.

Except that benzene is unstable relative to graphane. Our calculations were showing this. And I should have known it.








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