Mass Spectrometry Evidence
for Tetraoxygen
Scientists in Italy have
discovered a new form of oxygen. In
addition to the two well-known forms -
ozone and the oxygen molecules in air -
there is evidence for a third in which
oxygen atoms are grouped in fours.
The oxygen molecules that we breathe
(denoted O2) consist of two oxygen atoms.
This, the most stable form of oxygen,
makes up about one-fifth of air. Ozone is
a more reactive form and comprises three
oxygen atoms (O3) produced in the
atmosphere in small quantities when
sunlight splits O2 into its component
atoms, which can combine with O2.
Scientists at the University of Rome have
obtained evidence of O4. The existence of
such a molecule has been predicted since
the 1920s but searches for it have yielded
only tentative sightings. To prove
conclusively that they had identified O4,
Cacace's team used mass spectrometry. The
researchers combined O2 molecules and
positively charged O2 ions to produce O4
ions, which are identifiable by being four
times as massive as oxygen atoms. They
then added an electron to each O4 ion,
transforming it to a neutral molecule.
After a short interval, the team stripped
an electron from each O4 molecule so that
they could detect them again as ions. The
team reasoned that if the neutral
molecules were sufficiently stable, they
would show up when re-ionized.
What O4 looks like is still a mystery.
Earlier theoretical calculations suggested
two possibilities: a rhombus-shaped
molecule with an atom at each corner, or a
triangle of atoms with the fourth in the
centre. But neither of these options fits
the researchers' results very well.
Instead, it is believed that O4 is
probably composed of two dumbbell-like O2
molecules that are loosely bound
together.
The interest in new oxygen allotropes is
not purely theoretical. Liquefied oxygen
is used as a rocket fuel as it reacts
energetically with fuels such as hydrogen.
As the O4 allotrope packs a lot of oxygen
into a small space, it might prove to be a
better fuel source.
The
full article appears in volume 40 of Angewandte
Chemie International Edition pp. 4062 -
4065.