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The Story of O

Roald Hoffmann

Everything in oxygen chemistry seemed more or less in place: Up there, in the stratosphere, there were oxygen atoms, O2 molecules and ozone, O3, as well as ions derived from these and a bit of active OH, all in a dance of creation and destruction.

Meanwhile, within our bodies, normal O2 served us well. There was even a place, under enzyme supervision, for the somewhat nasty relatives hydrogen peroxide (H2O2 and its deprotonated form, O22-) and superoxide (O2- and its protonated alter ego, HOO), whose chemistry may generate the harmful hydroxyl radical OH. Of course, there's also water everywhere. And here and there singlet dioxygen, a more reactive and excited state of normal diatomic oxygen.

A nice, neatly compartmentalized world: ozone for atmospheric chemists, but not biologists, who had plenty of more complicated molecules to worry about.

So they thought.

This complacent state has now changed—dramatically so—with a series of remarkable discoveries. There is new evidence for the occurrence of ozone in living cells. Hydrogen peroxide is being made by molecules thought incapable of doing so. Even a metastable laboratory curiosity, the unusual HOOOH molecule (which sounds like a holler; call it dihydrogen-trioxide), may be in living systems. Meanwhile, we are still puzzling out the state of oxygen in high-temperature superconductors. And the menagerie of alternative forms of elemental oxygen continues to expand—there are strong theoretical arguments for the existence of a cyclic ozone isomer, and there may even be ways of stabilizing it.

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