Chemical abundance ratios of HCN, H¹³CN and HC¹⁵N as function of time, as derived from our shock models for several different physical parameters. The time = 0 yr corresponds to the advent of the shock. The values of the model parameters, namely Hydrogen particle density, shock velocity and cosmic ray ionization rate (in unit of the standard Galactic value of ζ₀ = 3 × 10⁻¹⁷ s⁻¹) are given at the top of each panel.