The microwave spectrum of the NH2 radical: The hyperfine structure of the ²B₁ ground electronic state

The pure rotational spectrum of the NH₂ radical in its X̃²B₁ ground electronic state was measured in the frequency region of 230-470 GHz by microwave spectroscopy. The radical was generated in a free space cell by dc-glow discharge of NH₃. Seventy nine fine and hyperfine components of four rotational transitions were measured, and were analyzed by least squares methods. The hyperfine coupling constants for both the nitrogen and hydrogen nuclei were redetermined with higher precision than those of the previous analysis for microwave optical double resonance (MODR) data. The nuclear spin-rotation constants for the hydrogen nucleus as well as the nitrogen nucleus were found to be anomalously large. The anomalous constant of C_aa(H) is interpreted by the same reason for the anomalous large value for C_aa(N) originating from the large A constant and the relatively low-lying à ²A₁ electronic state.