Scheme of the chemical network that connects the main molecular species investigated in this work for the WM (see Sect. 5.1). The prefix g- indicates molecules in icy mantles on dust grains. The arrows are colour-coded depending on the involved reactant: H (blue), H₂ (red), free electrons (green), H⁺ (orange), other species (black; the reactant is indicated on the side). Dashed arrows refer to transitions from solid to gas phase, or vice versa. For the reactions forming and destroying HCNH⁺, their relative contribution (in percentage) is also indicated.