D2d isomer: D2d isomer: Carbon tetroxide or Oxygen carbonate (in its C2v isomer) is a highly unstable oxide of carbon with formula CO4.

It was proposed as an intermediate in the O-atom exchange between carbon dioxide (CO2) and oxygen (O2) at high temperatures.

[1] The C2v isomer, which is 138 kJ mol−1 more stable than the D2d isomer, was first detected in electron-irradiated ices of carbon dioxide via infrared spectroscopy.

[2] The isovalent carbon tetrasulfide CS4 is also known from inert gas matrix.

It has D2d symmetry with the same atomic arrangement as CO4 (D2d).