On the frequency of N

Context. Dynamical studies of prestellar cores search for small velocity differences between different tracers. The highest radiation frequency precision is therefore required for each of these species. Aims. We want to adjust the frequency of the first three rotational transitions of N2H+ and N2D+ and extrapolate to the next three transitions. Methods. We compare N2H+ and N2D+ to NH3, the frequency of which is more accurately known and which has the advantage of being spatially coexistent with N2H+ and N2D+ in dark cloud cores. With lines among the narrowest, and the N2H+ and NH3 emitting region among the largest, L18