The microwave spectrum of the dihydronitrosyl radical, H2NO (2B1)

The microwave rotational spectrum of the H2NO radical in the X 2B1 ground vibronic state was observed by using a source-modulation spectrometer and a 1 m long free-space cell. The H2NO radical was generated in the cell by a reaction of hydroxylamine (NH2OH) with the products of 2450 MHz discharge in CF4. The spectral lines of H2NO were observed for the a-type R-branch transitions of N=1 left-arrow 0 to N=6 left-arrow 5, most of which showed resolved hyperfine structures due to the hydrogen and nitrogen nuclei. The 19 molecular constants including the hyperfine coupling constants of both the nuclei were precisely determined by the least-squares fit of 129 observed lines. The observed hyperfine structure shows that the H2NO radical in the ground electronic state is essentially planar with C2v symmetry. The nitrogen hyperfine coupling constants determined indicate that an unpaired electron occupies the pπ orbital perpendicular to the molecular plane, and the electronic wave function in the ground electronic state belongs to 2B1 symmetry. However, the small but negative inertial defect (Δ=-0.0051 uA2 ) suggests a possibility that H2NO may have a double-minimum potential function for the out-of-plane vibration