Galactocentric gradients of fractional abundances with respect to H₂, X_mol. The plot shows the trends for HCN (a), HCO⁺(b), c-C₃H₂ (c), H¹³CO⁺ (d), HCO (e), SO (f), H₂CO (g), and CH₃OH (h) as a function of the galactocentric radius (R_GC). The light blue data represent the abundances calculated using a constant gas-to-dust ratio (γ = 100), while the dark blue data illustrate those estimated using a non-constant gas-to-dust ratio (γ = γ(R_GC) from Giannetti et al. 2017). For both datasets, the linear regression results are shown as the light blue and dark blue lines, respectively. The 1σ error bars over the slope of the gradients are displayed for each fit. The upper-limit values are represented with triangles. The gradients of the elemental abundances of carbon (C and ¹³C), nitrogen (N), oxygen (O), and sulfur (S), as reported by Méndez-Delgado et al. (2022), are plotted as dashed lines. The product of the parent elements of the species is represented, in each subplot, by a dash–dotted black line. All elemental trends are plotted with a vertical offset to align with the starting point of the molecular gradients derived using γ(R_GC), to facilitate comparison of their slopes with the linear fit of the molecular abundances. In the upper-right side of each subplot, the Pearson correlation coefficient, ρ, is shown (estimated only for the abundances estimated using the non-constant gas-to-dust ratio).