  The ion-molecule reactions $\text{D}_2^++\text{NH}_3$ and
$\text{D}_2^++\text{ND}_3$ are studied at low collision energies
($E_{\text{coll}}$ from zero to $\sim k_\textrm{B}\cdot 50\,\text{K}$), with
the $\text{D}_2^+$ ions in the ground rovibrational state and for different
rotational temperatures of the ammonia molecules, using the Rydberg-Stark
merged-beam approach. Two different rotational temperatures
($\sim\,15\,\text{K}$ and $\sim\,40\,\text{K}$), measured by (2+1)
resonance-enhanced multiphoton-ionization spectroscopy, are obtained by using a
seeded supersonic expansion in He and a pure ammonia expansion, respectively.
The experimental data reveal a strong enhancement of the rate coefficients at
the lowest collision energies caused by the charge-dipole interaction.
Calculations based on a rotationally adiabatic capture model accurately
reproduce the observed kinetic-energy dependence of the rate coefficients. The
rate coefficients increase with increasing rotational temperature of the
ammonia molecules, which contradicts the expectation that rotational excitation
should average the dipoles out. Moreover, these reactions exhibit a pronounced
inverse kinetic isotope effect. The difference is caused by
nuclear-spin-statistical factors, and the smaller rotational constants and
tunneling splittings in $\text{ND}_3$.
