QUIJOTE discovery of the cation radicals HC₅N⁺ and HC₇N⁺

We present the discovery with the QUIJOTE line survey of the cations HC 5 N + and HC 7 N + in the direction of TMC-1. Seven lines with half-integer quantum numbers from J = 25/2–23/2 to 37/2–35/2 have been assigned to HC 5 N + and eight lines from J = 55/2–53/2 to 71/2–69/2 to HC 7 N + . Both species have inverted 2 Π ground electronic states with very good estimates for their B 0 and A SO constants based on optical observations. The lines with the lowest J of HC 5 N + exhibit multiple components due to the hyperfine structure introduced by the H and N nuclei. However, these different components collapse for the higher J . No hyperfine structure is found for any of the lines of HC 7 N + . The derived effective rotational and distortion constants for HC 5 N + are B eff = 1336.662 ± 0.001 MHz and D eff = 27.4 ± 2.6 Hz, while for HC 7 N + they are B eff = 567.85036 ± 0.00037 MHz and D eff = 4.01 ± 0.19 Hz. From the observed intensities, we derived T rot = 5.5 ± 0.5 K and N = (9.9 ± 1.0) × 10 10 cm −2 for HC 5 N + , while we obtained T rot = 8.5 ± 0.5 K and N = (2.3 ± 0.2) × 10 10 cm −2 for HC 7 N + . The HC 5 N/HC 5 N + , C 5 N/HC 5 N + , C 5 N − /HC 5 N + , HC 7 N/HC 7 N + , HC 5 N + /HC 7 N + , and C 7 N − /HC 7 N + abundance ratios are 670 ± 80, 4.8 ± 0.8, 1.2 ± 0.2, 1000 ± 150, 4.2 ± 0.5, and 2.2 ± 0.2, respectively. We have run chemical modelling calculations to investigate the formation and destruction of these new cations. We find that these species are mainly formed through the reactions of H 2 and the cations C 5 N + and C 7 N + , and by the reactions of H + with HC 5 N and HC 7 N, while they are mostly destroyed through a reaction with H 2 and a dissociative recombination with electrons. Based on the underestimation of the abundances of HC 5 N + and HC 7 N + by the chemical model by a factor ∼20, we suggest that the rate coefficients currently assumed for the reactions of these cations with H 2 could be too high by the same factor, something that will be worth investigating.