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.