Discovery of the acetyl cation, CH₃CO⁺, in space and in the laboratory

Using the Yebes 40m and IRAM 30m radiotelescopes, we detected two series of harmonically related lines in space that can be fitted to a symmetric rotor. The lines have been seen towards the cold dense cores TMC-1, L483, L1527, and L1544. High level of theory ab initio calculations indicate that the best possible candidate is the acetyl cation, CH₃CO⁺, which is the most stable product resulting from the protonation of ketene. We have produced this species in the laboratory and observed its rotational transitions J_u = 10 up to J_u = 27. Hence, we report the discovery of CH₃CO⁺ in space based on our observations, theoretical calculations, and laboratory experiments. The derived rotational and distortion constants allow us to predict the spectrum of CH₃CO⁺ with high accuracy up to 500 GHz. We derive an abundance ratio N(H₂CCO)/N(CH₃CO⁺)~44. The high abundance of the protonated form of H₂CCO is due to the high proton affinity of the neutral species. The other isomer, H₂CCOH⁺, is found to be 178.9 kJ mol^-1 above CH₃CO⁺. The observed intensity ratio between the K=0 and K=1 lines, ~2.2, strongly suggests that the A and E symmetry states have suffered interconversion processes due to collisions with H and/or H₂, or during their formation through the reaction of ${\text{H}}_3^{+}$ with H₂CCO.