Infrared spectroscopy of the ν1 + ν4 and 3ν4 bands of the nitrate radical

Abstract High-resolution Fourier transform infrared spectra of the ν 1  +  ν 4 and 3 ν 4 bands of 14 NO 3 were observed in the 1414 and 1174 cm −1 regions, respectively, and the corresponding ones of 15 NO 3 in the 1407 and 1159 cm −1 regions, respectively, and analyzed as E ′- A 2 ′ bands. The rotational constants of the upper states of 14 NO 3 are determined to be 0.457584 and 0.46089 cm −1 for ν 1  +  ν 4 and 3 ν 4 , respectively, consistent with the vibrational assignment. Effective Coriolis coupling constants of the ground electronic state are partly explained by vibronic interaction from the B 2 E ′ state, and a large change (37% decrease) in the value of the ν 1  +  ν 4 state compared with that of the ν 4 state is attributed to a mixing with the ν 3  +  ν 4 state (1492 cm −1 ) through vibrational anharmonicity.