Helium happily shares electrons to create dianions

Posted by: Tarun Kumar
Helium, the most parsimonious element that
invariably sits with its arms tightly folded and
refuses to participate in chemistry, turns out to
be surprisingly generous when it is in the right
environment, willing to donate not just one but
two electrons to neighbouring species.
Researchers from Austria and the UK made the
surprising discovery by generating for the first
time isolated dianions – which are inherently
unstable and therefore rare – in nanodroplets of
helium. The work opens the way to creating other
dianions and also to the wider study of helium as
an unlikely electron donor.
‘Dianions are important building blocks in
chemistry but are often unstable and difficult to
make in isolation because of the strong Coulomb
repulsion between the two electrons: bringing an
electron to an anion has a very high energy
barrier,’ explains Jan Verlet of Durham University
in the UK, who was not involved in the research.
The team, led by Andrew Ellis from the University
of Leicester in the UK and Paul Scheier at the
University of Innsbruck, suspended clusters of the
fullerenes C or C in nanoscale droplets of
liquid helium, held at just 0.4 degrees above
absolute zero, and bombarded the nanodroplets
with electrons.
At the specific energy level of the incoming
electron beam, mass spectra showed a clear
signal for the presence of dianions of both C
and C clusters, as long as there were more
than five fullerene molecules within the clusters.
Intriguingly, it appears that two
electrons are transferred
simultaneously from helium to the fullerene
clusters. The researchers suggest the incoming
electron beam causes an electron within the
helium to be bounced into a higher orbital. This
effectively ‘loosens’ the atomic structure, allowing
another electron to be accepted, albeit highly
weakly bound, to create He .
When this anion meets a fullerene cluster, it
apparently transfers two electrons in a single
step. ‘It seems that electrons, despite having like
charges, have a tendency to act as a pair under
certain conditions,’ says Ellis.
This is the first time that dianions have been seen
in superfluid liquid helium. Ellis says: ‘This finding
may challenge our understanding of how
electrons are transferred in chemical reactions
and shows that new and surprising chemistry is
possible in superfluid liquid helium.’ In the future
it should be possible to stabilise a range of
species with solvents before generating their
dianionic form in this way, allowing detailed
chemistry and spectroscopy, he suggests.
Verlet comments: ‘The idea that the helium anion
can be an electron donor provides an exciting new
handle to generate exotic molecular species at
cryogenic temperatures, which in turn provides
the ideal environment to study these new species
in unprecedented detail.’
REFERENCES
A Mauracher et al, Angew. Chem., Int. Ed.,
2014, DOI: 10.1002/anie.201408172