Steady-state abundance ratios of the N-hydrides as a function of the gas-phase elemental abundance ratio of C/O for three different values of the gas-phase elemental abundance of sulphur (from left to right: [S]_tot = 8.0 × 10^-8, 8.0   ×   10^-7, 8.0   ×   10^-6). C/O values include 0.3, 0.4, 0.5, 0.6, 0.67, 0.7, 0.75, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5. In all panels, the vertical dashed lines locate C/O = 0.67 (Flower et al. 2006 value) and 0.8 (our best model value). The abundances derived from Herschel/HIFI observations towards IRAS 16293-2422 are represented by the hatched boxes, considering an o/p ratio of NH₂~ 2, as predicted by our model.