Dinitrogen trioxide

Dinitrogen trioxide (also known as nitrous anhydride) is the inorganic compound with the formula N₂O₃. It is a nitrogen oxide. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21°C (−6°F):^[4]

^•
NO + ^•
NO
₂ ⇌ N
₂O
₃

Dinitrogen trioxide

Dinitrogen trioxide resonance hybrid
Dinitrogen trioxide is blue
Names
IUPAC name N-Oxonitramide[1]
Other names Nitrous anhydride Nitrogen sesquioxide
Identifiers
CAS Number	10544-73-7 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29799 Y
ChemSpider	55446 Y
ECHA InfoCard	100.031.013
EC Number	234-128-5
PubChem CID	61526
UNII	16E0524PXI Y
UN number	2421
CompTox Dashboard (EPA)	DTXSID7065120
InChI InChI=1S/N2O3/c3-1-2(4)5 Y Key: LZDSILRDTDCIQT-UHFFFAOYSA-N Y InChI=1/N2O3/c3-1-2(4)5 Key: LZDSILRDTDCIQT-UHFFFAOYAC
SMILES [O-][N+](=O)N=O
Properties
Chemical formula	N2O3
Molar mass	76.011 g·mol−1
Appearance	Deep blue liquid
Density	1.447 g/cm3, liquid 1.783 g/cm3, gas
Melting point	−100.7[2] °C (−149.3 °F; 172.5 K)
Boiling point	3.5 °C (38.3 °F; 276.6 K) (dissociates[2])
Solubility in water	reacts to form nitrous acid
Solubility	soluble in ether
Magnetic susceptibility (χ)	−16.0·10−6 cm3/mol
Structure
Molecular shape	planar, Cs
Dipole moment	2.122 D
Thermochemistry
Heat capacity (C)	65.3 J/(mol·K)
Std molar entropy (S⦵298)	314.63 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	91.20 kJ/mol
Hazards
GHS labelling:[3]
Pictograms
Signal word	Danger
Hazard statements	H270, H310+H330, H314
Precautionary statements	P220, P244, P260, P262, P264, P270, P271, P280, P284, P301+P330+P331, P302+P350, P303+P361+P353, P304+P340, P305+P351+P338, P310, P320, P321, P322, P361, P363, P370+P376, P403, P403+P233, P405, P410+P403, P501
NFPA 704 (fire diamond)	4 0 2 OX
Flash point	Non-flammable
Related compounds
Related compounds	Nitrogen oxide Nitrous acid Nitrous oxide Nitric oxide Nitrogen dioxide Dinitrogen tetroxide Dinitrogen pentoxide Nitrogen trioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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.^[2] At higher temperatures the equilibrium favors the constituent gases, with K_D = 193 kPa (25°C).^{[5][clarification needed]}

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

Structure and bonding

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. This isomer 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). This isomer can be produced from the reaction of tetrabutylammonium nitrite and triflic anhydride in dichloromethane solution at −30°C.^[6]

If the nitrous acid is not then used up quickly, it decomposes into nitric oxide and nitric acid. Nitrite salts are sometimes produced by adding N₂O₃ to water solutions of bases:

N₂O₃ + 2 NaOH → 2 NaNO₂ + H₂O

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:^[4]

Similar to nitronium nitrate, this molecule can also co-exist in equilibrium with an ionic gas called nitrosonium nitrite ([NO]⁺[NO2]^–) ^[7]