| Abstract
| - Using the Yebes 40 m and IRAM 30 m 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, CH 3CO +, 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 Ju = 10 up to Ju = 27. Hence, we report the discovery of CH 3CO + in space based on our observations, theoretical calculations, and laboratory experiments. The derived rotational and distortion constants allow us to predict the spectrum of CH 3CO + with high accuracy up to 500 GHz. We derive an abundance ratio N(H 2CCO)/ N(CH 3CO +) ∼ 44. The high abundance of the protonated form of H 2CCO is due to the high proton affinity of the neutral species. The other isomer, H 2CCOH +, is found to be 178.9 kJ mol −1 above CH 3CO +. 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 H 2, or during their formation through the reaction of H 3+ with H 2CCO.
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