Abundance trends (normalised to the maximum values) predicted by the Aikawa et al. (2011) models for dyazenilium and chemically related species. The N₂H⁺ abundance decreases toward the cloud centre following the N₂ freeze-out (green curve) onto dust mantels at high gas density. The concomitant steeper drops of free electron (grey curve) and CO (brown curve) give rise to the N₂H⁺ abundance maxima at radii of ~10⁴ and ~10³ astronomical units. N₂D⁺ shows similar behaviour (blue curve), but it features a much stronger inner peak owing to the strong deuterium fractionation existing in the highly-CO depleted central region.