Weakly bound oxide of nitrogen formed from an equilibrium reaction between nitric oxide and either nitrogen dioxide or dinitrogen tetroxide, q.q.v. Occurs in the solid, aqueous, and gaseous phases. The asymmetric isomer is more stable and predominates over the symmetric form. Initial observations: L. J. Gay-Lussac, Ann. Chim. Phys. 1, 394 (1816); P. L. Dulong, ibid. 2, 317 (1816). Prepn: A. W. Shaw, A. J. Vosper, J. Chem. Soc. A 1970, 2193 DOI. Crystal structure: T. B. Reed, W. N. Lipscomb, Acta Crystallogr. 6, 781 (1953) DOI. Ab initio spectral and structural studies: X. Wang et al., J. Mol. Struct. 403, 245 (1997) DOI. Electronic spectrum: A. W. Shaw, A. J. Vosper, J. Chem. Soc. Dalton Trans. 1972, 961 DOI. Interconversion studies and spectral characterization: E. M. Nour et al., J. Phys. Chem. 87, 1113 (1983) DOI. Hydration and dissociation equilibrium studies: A. J. Vosper, J. Chem. Soc. Dalton Trans. 1976, 135 DOI. Role in endogenous nitrosation: S. Basu et al., Nat. Chem. Biol. 3, 785 (2007) DOI PubMed. Review: I. R. Beattie, Prog. Inorg. Chem. 5, 1-26 (1963) DOI.