A Discharge Flow-Photoionization Mass Spectrometric Study of the NO3(2A2') Radical: Photoionization Spectrum, Adiabatic Ionization Energy, and Ground State Symmetry

The photoion efficiency spectrum of the nitrate radical (NO3) was measured over the region λ = 90-104 nm by using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. NO3 was generated by the reaction of fluorine atoms with nitric acid. A value of 12.57 ± 0.03 eV was obtained for the adiabatic ionization energy (IE) of NO3 from photoion thresholds, corresponding to the NO3+(1A1′) ← NO3(2A2′) transition. These direct ionization measurements are the first to be reported for the NO3 radical. Relative energetics and optimized geometries for ground state NO3 and several states of NO3+ were determined in both D3h and C2v symmetries using multiconfiguration self-consistent-field calculations. The results of the present study strongly suggest that the neutral ground state of NO3 has D3h symmetry. This conclusion is based on the following observations: (1) the experimental photoion threshold exhibits a large, steep initial step with no evidence of significant structure, indicating that the neutral and cation must share the same symmetry, and (2) the theoretical evidence is unambiguous that the cation symmetry is D3h. A value of ΔfH°298(NO3+) and the proton affinity of NO3 are also derived. A brief comparison is made of the ionization energies of NO, NO2, and NO3, and some new results on the dissociative ionization of HNO3 are discussed.