  Much of the dense gas in molecular clouds has a filamentary structure but the
detailed structure and evolution of this gas is poorly known. We have observed
54 cores in infrared dark clouds (IRDCs) using N$_2$H$^+$ (1-0) and (3-2) to
determine the kinematics of the densest material, where stars will form. We
also observed N$_2$D$^+$ (3-2) towards 29 of the brightest peaks to analyse the
level of deuteration which is an excellent probe of the quiescent of the early
stages of star formation. There were 13 detections of N$_2$D$^+$ (3-2). This is
one of the largest samples of IRDCs yet observed in these species. The
deuteration ratio in these sources ranges between 0.003 and 0.14. For most of
the sources the material traced by N$_2$D$^+$ and N$_2$H$^+$ (3-2) still has
significant turbulent motions, however three objects show subthermal N$_2$D$^+$
velocity dispersion. Surprisingly the presence or absence of an embedded 70
$\mu$m source shows no correlation with the detection of N$_2$D$^+$ (3-2), nor
does it correlate with any change in velocity dispersion or excitation
temperature. Comparison with recent models of deuteration suggest evolutionary
time-scales of these regions of several free-fall times or less.
