A nitrogen compound with oxidation number +4. It is an odd electron compound with an unpaired electron, and is dark reddish brown in color. It is in equilibrium with the colorless dimer dinitrogen tetroxide, N2O4. The proportion of NO2 is 0.01% at -11 oC, and it increases gradually to 15.9% at its boiling point (21.2 oC), and becomes 100% at 140 oC.
N2O4 can be generated by the pyrolysis of lead nitrate as follows.