Calculated fractional abundances of C₂H₅CN with respect to C₂H₃CN as a function of the CRIR. In each panel, the different symbols indicate the abundances calculated at T = 150 K for different minimum dust temperatures and the horizontal black line shows the observed ratio. Uncertainties (1σ) are shown in gray. The [C₂H₅CN]/[C₂H₃CN] ratios obtained for models N3-CR500 ad N3-CR1000 (<10⁻²) are not displayed.