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Theautoxidation of NO? was studied in glass-like matricesof 2-methylbutane at 110 K and in a 8:3 v/v mixture of 2,2-dimethylbutaneand n-pentane （rigisolve） at 80–90 K, by lettinggaseous NO? diffuse into these solvents that weresaturated with O2. In 2-methyllbutane, we observed a redcompound. However, in rigisolve at 85–90 K, a bright yellowcolor appears that turns red when the sample is warmed by 10–20K. The new yellow compound is a precursor of the red one and alsodiamagnetic. The UV–vis spectrum of the yellow compound containsa band which resembles that present in ONOO–. Becausethe red and yellow intermediates are not paramagnetic, we postulatethat ON–O–O? is in close contactwith NO?, or with another ON–O–O?. Diffusion of gaseous O2 into rigisolvesaturated with NO? does not produce a color; however,a weak EPR signal （g = 2.010） is observed. This signalmost likely indicates the presence of ONOO?. Thesefindings complement our earlier observation of a red color at lowtemperatures and the presence of ONOO? in the gasphase （Galliker, B.; Kissner, R.; Nauser, T.; Koppenol, W. H. Chem. Eur. J.2009, 15, 6161–6168）,and they indicate that the termolecular autoxidation of nitrogen monoxideproceeds via the intermediate ONOO? and not viaN2O2. A new, yellow, EPR-silentintermediate in the autoxidation of NO? has beenfound below 90 K.
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