Conflicting Observations Resolved by a Far IR and UV/Vis Study of the NO 3 Radical

Codeposition of NO and O 2 diluted in Ne at 6 K yield a ON⋅⋅⋅OO complex that exhibit strong UV absorption. This complex is converted into NO 3 radicals by UV irradiation, and is regenerated by radiation of visible light (see spectra). magnified image By codeposition of NO/Ne and O 2 /Ne mixtures at 6 K, weakly bound complexes between O 2 and NO are formed. They exhibit a strong, structured charge transfer UV band at λ max =275 nm. The UV band disappears during UV irradiation of the neon matrix, while the visible spectrum of the NO 3 radical appears. Simultaneously, the fundamental ν 4 of the NO 3 radical in the X^~ 2 A 2 ′ ground state is observed in infrared absorption for the first time at 365.6 cm −1 . Its 14/15 N and 16/18 O isotopic shifts reveal strong couplings between the two e′‐type modes of NO 3 , which are both active in a pseudo‐Jahn–Teller interaction with the excited B^~ 2 E′ electronic state. The dispute on the vibrational fundamentals of the NO 3 radical is now concluded by the unambiguous assignment of combination bands associated with the fundamental ν 4 . Taking into account the observed isotopic shifts and estimated anharmonicities for ν 4 and the most intense IR band of NO 3 at 1492 cm −1 (ν 3 +ν 4 ), the frequency of the so far not observed fundamental ν 3 is estimated to be 1100±10 cm −1 . A tentative assignment of the vibronic levels in the IR spectrum in the range from 1000 to 3000 cm −1 is given.