The crystal structures of methyl prop-2-ynoate, dimethyl fumarate and their protonated species

Methyl prop-2-ynoate (C₄H₄O₂) was investigated in the binary superacidic system HF/MF₅ (M = Sb or As) and dimethyl fumarate (C₆H₈O₄) in the superacidic system HF/SbF₅, as well as HF/BF₃. The starting materials methyl prop-2-ynoate and dimethyl fumarate were crystallized, the former for the first time. The protonated species of these esters, namely, (1-methoxyprop-2-yn-1-ylidene)oxidanium hexafluoroarsenate, C₄H₅O₂⁺ AsF₆^-, 1,4-dimethoxy-4-oxobut-2-en-1-ylidene]oxidanium tetrafluoroborate bis(hydrogen fluoride), C₆H₉O₄⁺ BF₄^- 2HF, and hemi{[1,4-dimethoxy-4-oxidaniumylidenebut-2-en-1-ylidene]oxidanium} undecafluorodiantimonate, 0.5C₆H₁₀O₄²⁺ Sb₂F₁₁^-, were characterized by single-crystal X-ray diffraction and Raman spectroscopy. The protonated species were recrystallized from anhydrous hydrogen fluoride. In the solid state of the monoprotonated species of methyl prop-2-ynoate and the diprotonated species of dimethyl fumarate, strong intramolecular O-H...F hydrogen bonds build a three-dimensional network. The monoprotonated species of dimethyl fumarate builds chains by strong O-H...O hydrogen bonds between the cations.