The microwave spectrum of the NHD radical in the ground electronic state, ²A″

The microwave spectrum of the monodeuterated species of NH₂, NHD(X̃ ²A″), was observed in the millimeter- and submillimeter-wave region. The NHD radical was produced by a dc-glow discharge through a NH₃ and D₂ mixture at around 220 K. Ten rotational transitions with both b- and a-type selection rules were observed in the 177-517 GHz region. The most important rotational transitions for astronomical observations, 1₀₁-0₀₀ and 1₁₀-1₀₁, were included and their detailed fine and hyperfine components were measured with high precision. The hyperfine interaction of the nitrogen, hydrogen, and deuterium nuclei, which are similar in magnitude, caused complicated hyperfine patterns and made their assignment relatively difficult. A least-squares analysis of the measured frequencies resulted in the determination of 40 molecular constants including nuclear spin-rotation coupling constants of N, H, D and, furthermore, nonaxial components of magnetic dipolar terms of H and D. The necessity of a higher K-dependent term of the dipole-dipole interaction constant for the hydrogen nucleus relative to the principal axis system reflects the effective geometrical change of the rotational levels.