The reaction of β-propiolactam in the superacidic systems HF/MF 5 (M = Sb, As) led to the formation of monoprotonated 3-aminopropanoyl fluoride in the form of [C(O)F(CH 2 ) 2 NH 3 ][SbF 6 ] and [C(O)F(CH 2 ) 2 NH 3 ][AsF 6 ]. In the presence of traces of water, the diprotonated species β-alanine [C(OH) 2 (CH 2 ) 2 NH 3 ][AsF 6 ] 2 was synthesized for the first time. All salts were characterized by low-temperature infrared and Raman spectroscopy. Additionally, single-crystal X-ray analyses were conducted in the case of [C(O)F(CH 2 ) 2 NH 3 ][SbF 6 ] and [C(OH) 2 (CH 2 ) 2 NH 3 ][AsF 6 ] 2 . By using SO 2 instead of HF as the solvent, the salt [C(OH) 2 (CH 2 ) 2 NHSO][SbF 6] 2 was obtained, and single-crystal X-ray analysis of this salt containing a thionylimide moiety was conducted. For the formation of these open-chain compounds, an acyl cationic species as intermediate is assumed, which is formed from N-protonated βpropiolactam. Quantum chemical calculations at the B3LYP/ aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory were carried out to gain a better understanding of the formation and the structural properties of protonated β-propiolactam.

These cations were prepared in superacidic media starting from β‐propiolactam under the reaction conditions described on their labels. In the lower part of the picture is a Petri dish containing a mold culture representing penicillinium, which carries the β‐propiolactam motif as an important component. The magnifying glass highlights the ring structure, and the scissors show where cleavage occurs in superacids. More information can be found in the Research Article by A. J. Kornath et al. (DOI: 10.1002/chem.202104086).