I should OCCO: Spectral structure

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  • Published: Aug 1, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: I should OCCO: Spectral structure

Old but elusive

Photoelectron imaging spectroscopy has been used to characterise a simple-seeming molecule that has remained elusive since its structure was first mooted in 1913, ethylenedione, OCCO. (Credit: Sanov et al/Angewandte Chemie/Wiley)

Photoelectron imaging spectroscopy has been used to characterise a seemingly simple molecule that has remained elusive since its structure was first mooted in 1913, ethylenedione, OCCO.

The Chemical Abstracts Service recently touched another milestone announcing the registration of its 100 millionth chemical entity; at the time of writing that number had risen by more than 1 million. Among those tens of millions of chemicals, there is one, with CAS Registry Number 4363-38-6, ethylenedione, perhaps more formally ethane-1,2-dione, but also known on occasion as dicarbon dioxide, dimeric carbon monoxide dimeric carbonous oxide, and dimeric carbon(II) oxide. It seems like such a simple chemical, two carbons, two oxygens connected in the order O=C=C=O by double bonds. One might imagine that it has been well studied and passed down from generation to generation of chemical students via fusty and musty textbooks dating back to the time of Friedrich Wöhler and his nineteenth century contemporaries.

But, no. OCCO has been an elusive organic. First proposed a little over a century ago it has been something of an enigma. It has also had some infamy as a purported active component of the "wonder drug" Glyoxylide, which in the 1940s was claimed to cure everything from fatigue to cancer. Although Glyoxylide turned out to be nothing but water,  the myth about it as a "lost" cancer remedy continues to be perpetuated by conspiracy theorists on the internet.

Scientists at the University of Arizona, Tucson, USA, have finally plucked OCCO from a neglected flask labelled "Glyoxal" (ethane-1,2-dione, OCHCHO) sitting in the storage fridge in Andrei Sanov's laboratory. Now, Sanov and a pair of his students have published the first definitive observation and spectroscopic characterization of OCCO. They point out that this seemingly simple compound may well be the fleeting intermediate in a  range of chemical reactions.

Rather than attempting to manipulate neutral species to synthesize OCCO, the team used a gas-phase reaction to produce a negatively charged species of OCCO and then used photoelectron imaging spectroscopy to analyse the product. The discovery has important implications for our understanding of radical chemistry and possibly processes in industrial chemistry and atmospheric chemistry alike. Sanov explains that ethylenedione represents an elegant fundamental puzzle for chemists. Its structure is easy to draw, of course, and sophisticated theoretical predictions suggest it ought to exist as a standalone, albeit transient, compound. Unfortunately, no previous effort has offered anything but inconclusive experimental evidence as to its existence.

Simples

"We are not talking about some complex compound here," Sanov explains. "This is a small molecule with only four atoms and an 'obvious' structure. Shouldn't modern science be able to tackle it?" he asks.

OCCO is unstable, it is a highly reactive diradical that naturally disintegrates into two carbon monoxide (CO) molecules within half a nanosecond or thereabouts. "Radicals and diradicals are all around us," Sanov adds. They are keen to react and frequently do in their efforts to use their 'underemployed' electrons. From the spectroscopist's point of view, the properties of OCCO suggest no reason why it should not be detectable. "And yet, it had never been observed, neither as a substance nor as a transient species, despite a century-long history of attempts," Sanov says.

Andrew Dixon was the student who spotted the unwanted flask chilling in the fridge."We started with a general interest in diradical systems and as part of those experiments, we decided to purchase some glyoxal, a precursor that is widely used in industrial applications but had not been explored as a potential synthesis molecule because its commercially high water content makes it very difficult to work with," Dixon explains. "Once we had bought it, we looked at it and I remember thinking, 'Oh man, this has 60 percent water. Let's figure this out some other day.'"

The day will come

That day came and Dixon having become aware of the potential for "molecular sieves" to strip the water from solutions went back to the fridge. Ultimately, the glyoxal without its originally high water content could succumb to spectroscopy and gave them a nice signal. The team tried to identify the molecule that they were seeing in their spectra, and a search pulled up "ethylenedione", which seemed to be an orphan compound classified as "hypothetical" by Wikipedia. It had been theorised but no one seemed to have made it. "That's when we noticed it was something new," Dixon adds.

"We tried to rule out every other possible solution," explains co-author Tian Xue, "to make sure it wasn't some other anion that could pose as OCCO." By blasting their sample with laser light as it passes through their mass spectrometer they were able to obtain a snapshot "portrait", a photoelectron image, of the elusive species.

OCCO's precursor molecule, glyoxal, has an important part to play in atmospheric chemistry leading Dixon to speculate that perhaps it is OCCO itself that plays a role and so might be a new component to add to atmospheric models. "Given that glyoxal is a known pollutant and by-product of combustion processes, whether man-made or natural, and given that OCCO seems to be a trivial molecule to create in our methodology, it is possible that it too could result from such processes, which, if true, could make it an unknown player in the atmosphere," Dixon adds. "And if you don't know there is a species that you should be accounting for, your model will never be completely correct."

"The next logical step concerning specifically OCCO is to determine its lifetime (known only theoretically so far) using time-resolved spectroscopy and to study its decomposition dynamics," Sanov told SpectroscopyNOW. "More broadly, OCCO is one of many important diradical species/reactive intermediates and its characterization paves the way to a better understanding of the structure and reactivity of this important class of compounds," he adds. "The synergy between the experiment and theory will eventually enable accurate predictive modelling of many environmental and biological processes."

Related Links

Angew Chem 2015, online: "Spectroscopy of Ethylenedione"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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