Modelled ratio between singly deuterated methanol (CH₂ DOH) and non-deuterated methanol (CH₃OH) for several models at four different time steps ranging from 10⁵ yr to 10⁶ yr. The best-fit time is t = 3 × 10⁵ yr. We show two models with slow thermal hopping (E_d/E_b = 0.55; blue lines), two models with slow thermal hopping and tunnelling diffusion of H and D (red lines), and two models with fast thermal hopping (E_d/E_b = 0.2; orange lines). The solid lines indicate models with addition and abstraction reactions, while the dashed lines indicate models with only addition reactions. The black line shows the observed ratio (errors as grey-shaded areas).