The rotational spectrum of $^{15}$ND in its ground electronic $X^{3}Sigma^{-}$ state has been observed for the first time. Forty-three hyperfine-structure components belonging to the ground and v = 1 vibrational states have been recorded with a frequency-modulation millimeter-/submillimeter-wave spectrometer. These new measurements, together with the ones available for the other isotopologues NH, ND, and $^{15}$NH, have been simultaneously analysed using the Dunham model to represent the ro-vibrational, fine, and hyperfine energy contributions. The least-squares fit of more than 1500 transitions yielded an extensive set of isotopically independent $U_{lm}$ parameters plus 13 Born--Oppenheimer Breakdown coefficients $Delta_{lm}$. As an alternative approach, we performed a Dunham analysis in terms of the most abundant isotopologue coefficients $Y_{lm}$ and some isotopically dependent Born--Oppenheimer Breakdown constants $delta_{lm}$ [R. J. Le Roy, J. Mol. Spectrosc. 194, 189 (1999)]. The two fits provide results of equivalent quality. The Born--Oppenheimer equilibrium bond distance for the imidogen radical has been calculated [$r_e^{BO}$ =103.606721(13) pm] and zero point energies have been derived for all the isotopologues.
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