Dinitrogen trioxide

^•
NO + ^•
NO
₂ ⇌ N
₂O
₃

Dinitrogen trioxide is only isolable at low temperatures (i.e., in the liquid and solid phases). In liquid and solid states, it has a deep blue color. At higher temperatures the equilibrium favors the constituent gases, with K_D = 193 kPa (25°C).

This compound is sometimes called "nitrogen trioxide", but this name properly refers to another compound, the (uncharged) nitrate radical •NO₃.

Structure and bonding

The major isomer of dinitrogen trioxide molecule contains an N–N bond. One of the numerous resonant structures of the molecule of dinitrogen trioxide is O=N−NO₂, which can be described as a nitroso group −N=O attached to a nitro group −NO₂ by a single bond between the two nitrogen atoms. Similar to nitronium nitrate, this molecule can also co-exist in equilibrium with an ionic gas called nitrosonium nitrite ([NO]⁺[NO₂]^–)

Typically, N–N bonds are similar in length to that in hydrazine (145 pm). Dinitrogen trioxide, however, has an unusually long N–N bond at 186 pm. Some other nitrogen oxides also possess long N–N bonds, including dinitrogen tetroxide (175 pm). The N₂O₃ molecule is planar and exhibits C_s symmetry. The dimensions displayed on the picture below come from microwave spectroscopy of low-temperature, gaseous N₂O₃:

ON-NO₂ is considered as the "anhydride" of the unstable nitrous acid (HNO₂), and produces it when mixed with water, although an alternative structure might be anticipated for the true anhydride of nitrous acid (i.e., O=N−O−N=O). That isomer can be produced from the reaction of tetrabutylammonium nitrite and triflic anhydride in dichloromethane solution at −30°C.