Dimers of oxygen molecules in the pores of Ca6A zeolite
Abstract
Dimers of O
2
molecules are formed in the pores of Ca
6
A zeolite at temperatures lower than 100 K. The O
4
molecule is diamagnetic and located on the Ca
2+
ions. No isotope exchange takes place between
16
O
2
and
18
O
2
via O
4
molecules, suggesting that O
4
is either a linear, chemically bound molecule or a van der Waals molecule. The heat of formation of adsorbed O
4
is only 2.1 kJ mol
-1
from adsorbed O
2
molecules, and is estimated to be 26 kJ mole
-1
from gaseous O
2
. Semi-quantitative studies show that Ca
2+
markedly weakens the van der Waals force between O
2
molecules. These results suggest that the O
4
molecule can exist as a linear molecule on Ca
2+
in Ca
6
A zeolite, and is a van der Waals species in free space.
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