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

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 bond at lambda(max) = 275 nm. The UV bond disappears during UV irradiation of the neon matrix, while the visible spectrum of the NO3 radical appears. Simultaneously, the fundamental nu(4) of the NO3 radical in the (X) over tilde (2)A(2)' ground state is observed in infrared absorption for the first time at 365.6 cm(-1). Its N-14/15 and O-16/18 isotopic shifts reveal strong couplings between the two e'-type modes of NO3, which are both active in a pseudo-John-Teller interaction with the excited (B) over tilde E-2' electronic state. The dispute on the vibrational fundamentals of the NO3 radical is now concluded by the unambiguous assignment of combination bonds associated with the fundamental nu(4), Taking into account the observed isotopic shifts and estimated anhormonicities for nu(4) and the most intense IR bond of NO3 at 1492 cm(-1) (nu(3)+nu(4)), the frequency of the so far not observed fundamental v, is estimated to be 1100 +/- 10 cm(-1). A tentative assignment of the vibronic levels in the IR spectrum in the range from 7000 to 3000 cm(-1) is given.