Phase Relationships in the N 2 O 3 /N 2 O 4 System and Crystal Structures of N 2 O 3

Mixtures of N 2 O 3 and N 2 O 4 have been prepared from measured volumes of NO and O 2 . Phase relationships in the system N 2 O 3 /N 2 O 4 , which depend on the composition of the samples and on the temperature, were studied by the Guinier technique and differential thermal analysis. Single crystals of phases A and B of N 2 O 3 were grown in situ on a diffractometer and studied at temperatures between −107 and −170°C by X‐ray diffraction. The structure analysis of A‐N 2 O 3 (tetragonal, space group I 4 1 / a , T = −170°C, a = 1625.57 (16), c = 880.49(13) pm, Z = 32, R 1 = 0.051 for 1030 unique reflections) is hampered by twinning and additional disorder of one of two crystallographically independent molecules. B‐N 2 O 3 is nicely ordered with one molecule in the asymmetric unit (orthorhombic, space group P 2 1 2 1 2 1 , T = −160°C, a = 506.86(4), b = 647.96(5), c = 863.26(6) pm, Z = 4, R 1 = 0.023 for 1352 unique reflections). The most interesting features of the N 2 O 3 molecule are its planarity and the extraordinarily long NN bond (189.0(1) pm).