The molecular structure and the conformational composition of fumaryl fluoride were determined by low‐temperature vibrational spectroscopy and single‐crystal X‐ray structure analysis. Three planar rotational isomers, trans‐trans‐, cis‐cis‐ and cis‐trans‐fumaryl fluoride were identified. C4H2F2O2 crystallizes in the monoclinic space group P21/c with four formula units per unit cell. Besides, Lewis acid‐base adducts between fumaryl fluoride and arsenic pentafluoride were synthesized. These adducts, which contain O–As bonding interactions, were found to crystallize as the monoadducts trans‐cis‐C4H2F2O2·AsF5 and cis‐trans‐C4H2F2O2·AsF5. Moreover, the diadduct trans‐trans‐C4H2F2O2·2 AsF5 was determined by X‐ray crystallography. The experimental data are discussed together with quantum chemical calculations of trans‐trans‐, cis‐cis‐, and cis‐trans‐fumaryl fluoride.

Croconic acid reacts in superacidic solutions HF/MF5 (M = As, Sb) to yield its corresponding salts [H3O5C5][MF6] and [(H3O5C5)H(H3O5C5)][MF6]3·2HF (M = As, Sb). The degree of protonation is strongly dependent on the stoichiometric ratio of the Lewis acid regarding croconic acid. Monoprotonated salts were characterized by vibrational spectroscopy and in the case of [H3O5C5][AsF6] (1) by a single‐crystal X‐ray structure analysis. [H3O5C5][AsF6] crystallizes in the monoclinic space group P21/c with four formula units per unit cell. The sesquiprotonated species of croconic acid [(H3O5C5)H(H3O5C5)][SbF6]3·2HF (4) was also characterized by single‐crystal X‐ray structure analysis. It crystallizes in the triclinic space group P1 with one formula unit per unit cell. The vibrational spectra of the monoprotonated salts were compared to quantum chemical calculations of the [H3O5C5]+·3HF cation and experimental data reported for croconic acid.

The reaction of fumaryl fluoride with the superacidic solutions XF/MF5 (X=H, D; M=As, Sb) results in the formation of the monoprotonated and diprotonated species, dependent on the stoichiometric ratio of the Lewis acid to fumaryl fluoride. The salts [C4H3F2O2]+[MF6]− (M=As, Sb) and [C4H2X2F2O2]2+([MF6]−)2 (X=H, D; M=As, Sb) are the first examples with a protonated acyl fluoride moiety. They were characterized by low‐temperature vibrational spectroscopy. Low‐temperature NMR spectroscopy and single‐crystal X‐ray structure analyses were carried out for [C4H3F2O2]+[SbF6]− as well as for [C4H4F2O2]2+([MF6]−)2 (M=As, Sb). The experimental results are discussed together with quantum chemical calculations of the cations [C4H4F2O2 ⋅ 2 HF]2+ and [C4H3F2O2 ⋅ HF]+ at the B3LYP/aug‐cc‐pVTZ level of theory. In addition, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated in order to investigate the electron distribution and the charge‐related properties of the diprotonated species. The C−F bond lengths in the protonated dication are considerably reduced on account of the +R effect.

The syntheses of [OC(COX) ][MF ] and [(COX) ][MF ] ⋅2 HF were carried out in superacidic media XF/MF (M=As, Sb; X=H, D). The degree of protonation is highly dependent on the stoichiometric ratio of the Lewis acid with regard to squaric acid. The salts of diprotonated squaric acid were characterized by Raman spectroscopy and, in the case of [(COH) ][MF ] ⋅2 HF (M=As, Sb), by single-crystal X-ray structure analyses. [(COH) ][AsF ] crystallizes in the monoclinic space group P2 /n with two formula units per unit cell. Analysis of the vibrational spectra was achieved with the support of quantum chemical calculations of the cation [(COH) ] ⋅4 HF on the PBE1PBE/6-311G++(3df,3pd) level of theory. Furthermore, a salt of monoprotonated squaric acid, [OC(COH) ][AsF ], was characterized by a single-crystal X-ray structure analysis. It crystallizes in the monoclinic space group P2 /n with four formula units per unit cell. The protonation of squaric acid leads to a change of the carbon skeleton, which is discussed for the entire series, starting with the dianion of squaric acid and ending with the tetrahydroxy dication.

γ-Butyrolactone and γ-butyrolactam were reacted in the superacidic systems XF/MF 5 (X = H, D; M = As, Sb). Salts of the monoprotonated species of γ-butyrolactone were obtained in terms of [(CH 2) 3 OCOH] + [AsF 6 ] À , [(CH 2) 3 OCOH] + [SbF 6 ] À and [(CH 2) 3 OCOD] + [AsF 6 ] À and the analogous lactam salts in terms of [(CH 2) 3 NHCOH] + [AsF 6 ] À , [(CH 2) 3 NHCOH] + [SbF 6 ] À and [(CH 2) 3 NDCOD] + [AsF 6 ] À. The salts were characterized by low temperature Raman and infrared spectroscopy and for both protonated hexafluoridoarsenates, [(CH 2) 3 OCOH] + [AsF 6 ] À and [(CH 2) 3 NHCOH] + [AsF 6 ] À , single-crystal X-ray structure analyses were conducted. In addition to the experimental results, quantum chemical calculations were performed on the B3LYP/ aug-cc-pVTZ level of theory. As in both crystal structures C•••F contacts were observed, the nature of these contacts is discussed with Mapped Electrostatic Potential as a rate of strength.