Models of thermal chemistry for the relative abundances N(CO⁺)/N(HCO⁺) and N(HOC⁺)/N(HCO⁺) with default rates to the left, and deviations in the middle and right panels. In all panels, results for HOC⁺ are shown in green and for CO⁺ in orange. In all cases, N(HCO⁺) is taken to be the observed value N(HCO⁺) = 3 × 10⁻⁹ N(H₂). Left panel: baseline models with the observed upper limit for N(CO⁺)/N(HCO⁺) from this work and the mean N(HOC⁺)/N(HCO⁺) ratio from Gerin et al. (2019) are illustrated schematically as horizontal dashed lines. Middle panel: the solid lines represent the case that the rate constant of H₂ + HOC⁺ → HOC⁺ +H₂ interconversion is taken as 10⁻¹¹cm³ s⁻¹. The dashed-dotted lines represent model results when the rate constant for the reaction H₂ + CO⁺ → HOC⁺ + H is set to 0. Right panel: the solid lines show models in which the reaction of C⁺ and H₂O does not form HOC⁺ and the dashed-dotted lines show model results when CH⁺ is as abundant as OH, X(CH⁺) = 10⁻⁷.