Detection and Structure of HOON: Microwave Spectroscopy Reveals an O–O Bond Exceeding 1.9 Å

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Science  13 Dec 2013:
Vol. 342, Issue 6164, pp. 1354-1357
DOI: 10.1126/science.1244180

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A Glimpse of HOON

Bonds between two oxygen atoms are relatively weak, as manifested in the sometimes explosive reactivity of O2 and various peroxides. Thus, although nitrous acid (HONO) can be rearranged on paper to an isomer with an O-O rather than N-O bond, nitrosyl-O-hydroxide (HOON) has been considered too unstable to be observed. Crabtree et al. (p. 1354) used microwave spectroscopy to detect HOON formation in a dilute gaseous mixture of NO and OH in neon. Isotopic substitutions enabled determination of its structure, which included an unusually long O-O bond.


Nitric oxide (NO) reacts with hydroxyl radicals (OH) in the gas phase to produce nitrous acid, HONO, but essentially nothing is known about the isomeric nitrosyl-O-hydroxide (HOON), owing to its perceived instability. We report the detection of gas-phase HOON in a supersonic molecular beam by Fourier transform microwave spectroscopy and a precise determination of its molecular structure by further spectroscopic analysis of its 2H, 15N, and 18O isotopologs. HOON contains the longest O–O bond in any known molecule (1.9149 ± 0.0005 Å) and appears surprisingly stable, with an abundance roughly 3% that of HONO in our experiments.

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