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

High-resolution Fourier transform infrared spectra of the ν1 + ν4 and 3ν4 bands of 14NO3 were observed in the 1414 and 1174 cm−1 regions, respectively, and the corresponding ones of 15NO3 in the 1407 and 1159 cm−1 regions, respectively, and analyzed as E′-A2′ bands. The rotational constants of the upper states of 14NO3 are determined to be 0.457584 and 0.46089 cm−1 for ν1 + ν4 and 3ν4, respectively, consistent with the vibrational assignment. Effective Coriolis coupling constants of the ground electronic state are partly explained by vibronic interaction from the B2E′ state, and a large change (37% decrease) in the value of the ν1 + ν4 state compared with that of the ν4 state is attributed to a mixing with the ν3 + ν4 state (1492 cm−1) through vibrational anharmonicity.