Infrared spectroscopy of 2ν₄ and ν₃ + 2ν₄ bands of the NO₃ radical

The 2ν₄ band spectrum of ¹⁴NO₃ observed with infrared diode laser spectroscopy was analyzed with the Fourier transform (FT) spectrum in the 760 cm⁻¹ region, including the Coriolis interaction between v₄ = 2 and v₂ = 1. The vibrational frequencies of v₄ = 2, l = 0, and l = ±2 have been determined to be 752.4033(86) and 771.7941(81) cm⁻¹, respectively. By considering the anharmonic interaction among the 2ν₄, ν₃, ν₄, ν₂, and ν₂ + 3ν₄ states, a relation among the cubic anharmonic constants was obtained as 0.452 Φ₄₄₄ζ_2,4 − 0.271 Φ₃₄₄ζ_2,3 = 127.7 cm⁻¹. The ratio of transition moments μ(ν₂)/μ(2ν₄) was determined to be 0.3 from the perturbation analysis. The second strongest infrared band of ¹⁴NO₃, ν₃ + 2ν₄, observed around 1927 cm⁻¹ has been analyzed with the hot band ν₃ + 2ν₄ − ν₄ by including the Coriolis interaction with the v₂ = 1, v₄ = 3 state. Similarly, the same band of ¹⁵NO₃ was analyzed to give the band origin of 1897.9325(6) cm⁻¹. The isotope shift 28.2 cm⁻¹ for the ν₃ + 2ν₄ vibrational frequency is consistent with a predicted value of 27.3 cm⁻¹. Although there are two A' states in v₃ = 1, v₄ = 2, only one A₂' state has been assigned in the hot band, indicating that the other band has weak intensity. This fact and the strong intensity of the ν₃ + 2ν₄ band (l₃ = ±1, l₄ = 0) are understood as the effect of vibronic interaction. The first-order Coriolis coupling constant ζ of ν₃ + 2ν₄, l₃ = 1, l₄ = 0 is similar to those of the ν₄ and ν₃ + ν₄ states, and it is concluded that the vibrational Coriolis coupling constant is nearly zero and the observed constants of ζ = −0.19 (¹⁴NO₃) and ζ = −0.15 (¹⁵NO₃) also originate from the effect of vibronic interaction.