Matrix-isolated (NO)2 has been obtained by rapid deposition of NO–N2 mixtures on an optical window at 15°K, by limited diffusion of matrix-isolated NO at low temperatures, and by photochemical production of O atoms in an N2O environment. Controlled low-temperature studies allowing limited molecular diffusion indicate spectral growth patterns due to three different species. Experiments carried out with 15NO and N18O isotopic samples along with the photochemical production of (NO)2 suggests that cis and trans O=N=N=O and possibly another unstable cis form are produced.

The matrix-isolated infrared spectra of cis- and trans-nitrous acid molecules have been obtained in argon at low temperatures by the reaction of H atoms with NO2. The spectra, which include 15N- and 18O-substituted molecules, are not complicated by the stabilization of the acid in the presence of large concentrations of NO, NO2, and H2O. All six fundamentals of the cis and three of the trans isomers have been identified, and found generally to confirm the assignments of a previous study where some uncertainty did exist. The normal coordinate calculation performed for the cis molecule predicted the average frequencies of the four 15N- and 18O-substituted species with an error of 0.60 cm−1 or 0.068%.

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