Conflicting Observations Resolved by a Far IR and UV/Vis Study of the NO3 Radical

Abstract

Codeposition of NO and O₂ diluted in Ne at 6 K yield a ON⋅⋅⋅OO complex that exhibit strong UV absorption. This complex is converted into NO₃ radicals by UV irradiation, and is regenerated by radiation of visible light (see spectra). By codeposition of NO/Ne and O₂/Ne mixtures at 6 K, weakly bound complexes between O₂ 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 NO₃ radical appears. Simultaneously, the fundamental ν₄ of the NO₃ radical in the ${\mathop X\limits^\sim }$ ²A₂′ ground state is observed in infrared absorption for the first time at 365.6 cm⁻¹. Its ^14/15N and ^16/18O isotopic shifts reveal strong couplings between the two e′‐type modes of NO₃, which are both active in a pseudo‐Jahn–Teller interaction with the excited ${\mathop B\limits^\sim }$ ²E′ electronic state. The dispute on the vibrational fundamentals of the NO₃ radical is now concluded by the unambiguous assignment of combination bands associated with the fundamental ν₄. Taking into account the observed isotopic shifts and estimated anharmonicities for ν₄ and the most intense IR band of NO₃ at 1492 cm⁻¹ (ν₃+ν₄), the frequency of the so far not observed fundamental ν₃ 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.