Dubious X-rays

Last year, Mike Haley (University of Oregon) teamed up with Roland Boese (Essen), and Jay Siegel and Kim Baldridge (University of California San Diego) to report the synthesis and X-ray structure of a molecule with a curiously short carbon-carbon double bond, 7 picometres less than that in ethene. (Chem. Commun., 1998, 1137)
     The molecule, prepared by Haley, 3-ethynylcyclopropene, contains a highly strained carbon ring with an adjacent triple bond. Not only was it surprising that Boese measured the double bond at just 125.5 pm, but when Siegel and Baldridge used fast computational quantum mechanics to simulate it, the same bond was predicted to be longer, at about 128 pm. 'The team worked hard to resolve even this small discrepancy,' says Siegel. Boese checked the X-ray and Baldridge and Siegel checked their theoretical data but it stubbornly stuck.
     'Although the discrepancy may seem small, the magnitude of the difference is well beyond the expected uncertainties of the two approaches, explains Paul Schleyer (University of Georgia, Athens), 'Something had gone wrong, either the theory was inadequate or the experiments were inaccurate.'
     Theoretical chemist Schleyer, working with Henry Schaefer III, could not let this discrepancy lie. They reinvestigated the detailed structure to the highest practicable computational levels with an accuracy of 0.8 pm for related gas phase molecules. They expected essentially exact bond lengths, but, the discrepancy got even worse when they bumped up the theory 'level' with the double bond being 4.1 pm longer than the X-ray result. An expert crystallographic referee re-examined the X-ray data but found no errors. 'Geometry changes between the solid state and the gas phase seem unlikely,' says Schleyer, 'The only conclusion is unsatisfying: there is no plausible explanation at this time.'
     Siegel agrees that the discrepancy is annoying but adds that Schleyer's study is not too different from his and that the overriding message is that cyclopropene derivatives do have short double bonds. He has not ruled out geometry differences between the solid and gas phases.
     The next step will be an experimental reinvestigation using neutron diffraction and gas phase methods. 'If the 125.5 pm X-ray length stands up to scrutiny then it may be necessary to reduce the confidence in the quantitative performance of current chemical theory,' says Schleyer. Siegel is a bit more reticent, 'This discrepancy may be the pea beneath the princess' mattress,' he says, 'but I doubt it is the horn at the walls of Jericho!'

Best yet catalyst

A highly efficient catalyst system for hydrogenating aromatic compounds to make cleaner diesel fuels has been developed by French chemists. The fully catalyst recyclable operates at room temperature and atmospheric pressure.
     Alain Roucoux and his colleagues at ENS Chimie Rennes have developed a new liquid-liquid system based on rhodium metal particles just a few nanometres in diameter suspended in water to form a colloid. The second phase is the aromatic reactant and its products so no solvent is needed.
     The system efficiently hydrogenates benzene and its derivatives to their cyclohexane equivalents. 'This is a new reaction system of industrial importance,' says Roucoux.
     Previous efforts to develop biphasic catalysts, which allow the catalyst and product to be separated easily, require more drastic temperatures and pressures. This system appears to be the most efficient for converting benzene to cyclohexane and toluene, cumene, anisole and phenol to their hydrogenated forms in very mild conditions. (Chem. Commun., 1999, 535) Toothpaste tube
Sodium and potassium transport channels in cell membranes are shaped like a toothpaste tube according to the X-ray structure of a transmembrane potassium channel protein. A model being developed by UK researchers to explain these ion channels' selectivity could help in understanding cell ionic equilibria in diseases such as muscular dystrophy and why ion transport fails, as happens in mercury and thallium poisoning.
     Peter Cragg and his colleagues at the University of Brighton and Jon Steed's team at King's College London believe that the sodium selective channel works in a similar fashion to the potassium channel. They have modelled the transport process using calixarene molecules, replicating the sodium selectivity of the natural membrane. Sodium ions pass through the model compound in dissolved form along its length, however, potassium ions are too large and are excluded. 'Crystallographic structures of sodium channels have yet to be determined so the mechanism is poorly understood,' explains Cragg, 'Our work assumes that the sodium selectivity filter will be analogous to that of potassium and the model reflects that structure.'

(Chem. Commun., 1999, 553)

 

Buckybells
Wolfgang Kr�tschmer one of the scientists who first isolated and identified buckminsterfullerene has come up with a way to couple two of the carbon soccerballs together using oxygen and sulphur for the first time. The resulting bucky dumbbells could form the building blocks for novel fullerene materials with interesting optical and electronic properties or novel polymeric materials.
     Kr�tschmer and his team at the Max Planck Institute for Nuclear Physics in Heidelberg and colleagues at Heidelberg University and the Research Centre in Karlsruhe built on earlier research in which fullerenes had been joined by ethereal linkages. They started with C₁₂₀O and thermolysed it in a one-to-one mixture with elemental sulphur. Various spectroscopic techniques confirmed the structure. (Chem. Commun., 1999, 465)