Vibrational assignment and vibronic interaction for NO3 in the ground electronic state

• Published in: Journal of molecular spectroscopy 2015-04, Vol.310, p.99-104
• Main Author: Hirota, Eizi
• Format: Article
• Language: English
• Subjects: High-resolution infrared spectroscopy; Nitrate radical; Vibrational assignment; Vibration–rotation interaction; Vibronic interaction
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/j.jms.2014.12.010

Transitions observed for 14NO3 and assigned based on Assignment I. [Display omitted] •Vibrational assignment for NO3 was established based on spectroscopic data.•An alternative proposed by an ab initio calculation was shown to be unacceptable.•High-symmetry molecular systems like NO3 are big challenges for MO theory.•New vibronic interaction model proposed in the ground electronic state.•This model accounts for vibration–rotation effects and anomalous progression. Two important problems exist for the NO3 free radical. One is the frequency of the degenerate N–O stretching mode ν3. It has been assigned to a band at 1492cm−1 (Assignment I), whereas Stanton calculated it by an ab initio MO method to be around 1000cm−1 (Assignment II). The second concerns an anomalous ν4 progression, which appeared in the photoelectron spectra of the NO3 anion and was accounted for by Herzberg–Teller (H–T) mechanism, but the interaction parameter derived was too large. The present study critically examines Assignment II and the H–T vibronic interaction model against the results of high-resolution infrared (IR) spectroscopy supplemented with dispersed fluorescence (DF), and concludes Assignment I to be correct and the H–T mechanism to be complemented by a new vibronic interaction model, based upon the observations: (1) Stanton’s ab initio MO ν3 appeared in neither IR nor DF spectra, (2) only one A–E type subband was present in the Z−ν4 hot band (Z denotes the upper state of the 1492cm−1 band), at variance with the two predicted by Assignment II, (3) the ℓ-type doubling constant and the first-order Coriolis coupling constant derived for the Z state by assuming Assignment II were not acceptable, and (4) anomalous features expected from the H–T vibronic interaction model for the ν4 fundamental state were not observed at all. Infrared spectroscopic results on a few 2E′ degenerate states indicated that the first-order Coriolis coupling constant and the effective spin–orbit interaction constant were closely correlated, suggesting that the unpaired electron azimuthal motion was affected much by that of the degenerate vibrational mode. This sort of vibronic interaction has been well known for linear polyatomic free radicals in 2Σ electronic states with a bending mode singly excited. A similar vibronic interaction should be present also in symmetric-top free radicals, where a degenerate vibrational mode is singly excited. However, few examples have been reported and the NO3 radical is pe