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One Shocked Chemist

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

Roald Hoffmann

Squeezing Benzene

Let me tell you a story here of another molecular surprise that recently came my way. In the process, I was led to reflect again on the relation between stability and existence in chemistry (as I wrote about in “Unstable,” American Scientist November–December 1987).

2011-03MargHoffmannFB.jpgClick to Enlarge ImageXiao-Dong Wen, a talented postdoc in the small group that Neil Ashcroft and I have at Cornell University, has been studying benzene under pressure. People have been squeezing benzene between diamond anvils for some years, eventually obtaining some amorphous polymeric material, not terribly well characterized, in which carbon-carbon bonds have formed. We have been (and still are) looking theoretically for a hypothetical metallic benzene, perhaps one in which the scales of mobility of electrons, protons and carbons are separated.

At a certain high pressure, Xiao-Dong found that benzene rearranged spontaneously to not one, but a family of regular two-dimensional polymers of unchanged composition, CH, the same as benzene, C6H6, or (CH)6. But the polymers Xiao-Dong found, unlike benzene, are not aromatic, meaning that they are not especially stabilized (the way that benzene is) by a hexagon of alternating single and double bonds between the carbon atoms. The 2D networks are shown in the third figure; they would today be called graphanes.

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