The rotational spectrum of $^{15}$ND. Isotopic-independent Dunham-type
  analysis of the imidogen radical
  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.
