Xenon doubled up with water

Creating a water molecule with two noble gas atoms interpolated into its structure sounds an improbable feat, but a international team of researchers now claim to have trapped just such an exotic compound in xenon.

Since Neil Bartlett synthesised the first noble gas compound, xenon hexafluoroplatinate, in 1962, chemists have created hundreds of molecules and ions combining supposedly unreactive noble gases with other chemical elements. The latest addition to the list, HXeOXeH, is the smallest known neutral molecule holding two noble gas atoms, says Leonid Khriachtchev at the University of Helsinki, Finland, who led the research.

Khriachtchev’s team used ultraviolet light to decompose water and nitrous oxide molecules held in a matrix of solid, transparent xenon at 9K. Oxygen and hydrogen atoms diffused slowly through the xenon when it was heated to around 40K, forming species such as HXeO radicals and HXeOXeH hydrides. The molecules couldn’t be isolated from their xenon surround, but were detected by infrared spectroscopy. 

Nonetheless, says Lester Andrews, who works on spectroscopy of inaccessible molecular species at the University of Virginia, US, the low temperature matrix approach may show which noble gas compounds can be made experimentally, paving the way for preparing larger samples by other means. ’The first time anyone sees these things, the deck has to be stacked in their favour. You’ve got to start with solid xenon to have a chance,’ he says.

Computer calculations suggest a small energy barrier prevents HXeOXeH breaking up into xenon atoms and water, a far more stable arrangement. It’s unlikely the molecule would survive outside its cold xenon housing, though - which means it can’t contribute to any useful chemistry or applications as yet, points out Gary Schrobilgen, some of whose research at McMaster University, Canada, focuses on the chemical synthesis of noble-gas compounds, and their use as intermediates to aid chemical reactions.

Compounds such as HXeOXeH, and HXeCCXeH (discovered by Khriachtchev’s team three years ago), have whetted some researcher’s appetites for making high-energy polymers incorporating noble gas atoms. None have ever been prepared, but (Xe-CC)n is stable according to computational predictions, and Khriachtchev hopes (Xe-O)n chains may be a target. He also speculates that learning more about the way xenon combines with oxygen and hydrogen might shed more light on the so-called ’missing xenon’ problem - where the Earth’s atmosphere appears to hold far less xenon than would be expected by comparison with other planets in the solar system. It may be contained inside the Earth’s crust, allied with other elements under high pressure conditions.

’There may be nothing practical ever to come out of this research - but it shows what can be done and helps us understand the bonding of xenon, which we’re learning is more reactive than we’d thought,’ says Andrews.

Richard Van Noorden

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