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Model 1 of two hypothetical species containing He chemically bound to O.
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Model 2 of two hypothetical species containing He chemically bound to O
Images by The Polish Journal of Chemistry /
University of Warsaw
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Considered to be the smallest, most chemically inert and least polarizable of the 117 known chemical elements, helium has been challenging chemists for generations.
Unfortunately, no-one has managed to confirm experimentally the existence of either
HeBeO [considered an example of ‘weak interactions’ rather than a
genuine chemical bond (Frenking et al. 1986)] or HHeF [with a lifetime
in the femto–picosecond range (Wong 2000 and Gerber et al. 2001)], two important species previously predicted, nor a number of others. The two new molecular species to bind helium to oxygen, predicted using theory,
CsFHeO and NMe4FHeO are derivatives of a metastable [F-
HeO] anion first theorized in 2005 by a group from taiwan led by Prof.
Hu. The scientist responsible for performing these new quantum chemical calculations is
Dr Wojciech Grochala from ICM and the faculty of chemistry, the University of
Warsaw. Speaking of his results, Dr Grochala said, "The molecules are not as peculiar as they might appear at first light; the idea is to preserve the metastable
character of the fragile [F– HeO] entity by attaching it to a weakly coordinating cation (such as
Cs+ or
NMe4+) to achieve electric neutrality. the resulting species exhibit a
He–O bond with an electronic dissociation energy on the singlet potential energy surface (PES) as large as half an
eV for the tetramethylammonium derivative." Unfortunately, the kinetic stability of the molecules in question is limited by a crossing of the singlet–triplet
PESs and additionally by facile decomposition along the bending channel, both factors considerably limiting their lifetime.
The implication for a real world search for these molecules is that they should besought at a temperature of a few kelvin at most.
Commenting on how this could be achieved, Dr Grochala said, "The synthesis of both species might begin with the unusual hypofluorites,
CsOF and NMe4 of, embedded in an ultracold helium droplet.
Laser excitation of the O-F chemical bond should allow for insertion of a helium atom into the bond and for spectroscopic observation of the short– living molecules.
Of course, such experiments are quite challenging but this is what makes modern chemistry so much fun".
He added, "Despite various difficulties I am really excited about the predictions".
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