2026ApJS..282...47K

The physical and chemical conditions within a protoplanetary disk play a crucial role in determining its chemical composition, which is subsequently inherited by any forming planets. To probe these conditions, high-resolution molecular line observations, coupled with modeling, are essential. In this study, we investigate the chemistry of the nearby, massive, and relatively line-rich protoplanetary disk around HD 163296 using high-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA) across Bands 3, 4, 6, and 7. We constrain the disk-averaged and radial distributions of column density and excitation temperature for the detected molecules using the new retrieval code DRive. The disk chemistry is modelled using the astrochemical code PEGASIS, with variations in the initial elemental C/O ratio. Our modelling, informed by molecular observations of HCO⁺, DCO⁺, HCN, DCN, CS, HC₃N, H₂CO, CH₃OH, HNCO, and NH₂CHO, allows us to place strong constraints on the C/O ratio, with a best-fit value of 1.1 that is broadly consistent with previous estimates. We present the highest-resolution DCO⁺ emission map of this disk to date, revealing triple-ringed chemical substructures that closely align with the dust continuum rings. Additionally, our results provide the first and most stringent upper limits on the column densities of NH₂CHO and HNCO in this protoplanetary disk, measured at <7 × 10¹¹ cm^–2 and <1 × 10¹¹ cm^–2, respectively. Our chemical models suggest that NH₂CHO and HNCO predominantly form on grain surfaces within the disk. However, physicochemical desorption mechanisms are inefficient at releasing these species into detectable gas-phase abundances, yet they remain promising targets for future ALMA observations.