Steady-state abundances of NH, NH₂, and NH₃ as a function of the C/O gas-phase elemental abundance ratio for three different values of 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 each panel, the solid line refers to the model predictions and the vertical dashed lines locate C/O = 0.67 (Flower et al. 2006 value) and 0.8 (our best model value). For NH₂ and NH₃, the ortho and para abundances are shown separately (dashed and dotted lines respectively). The observed abundances towards IRAS 16293-2422 are represented by the hatched boxes, considering an o/p ratio of NH₂~ 2, as predicted by our model.