Modelled D/H abundance ratio for CH (black diamonds), HCO⁺ (blue triangles) and C₂H (red circles) as a function of temperature computed for a time of t = 5 × 10⁵ years, where the horizontal grey shaded regions mark the dispersion in the CD/CH abundance ratio derived toward IRAS 16293−2422. The solid, dashed, and dotted curves represent simulations run for OPRs of H₂ = 10⁻³, 0.1, and 3.0, respectively.