Zweidimensionales Eis und H₅O₂ ⁺-Ionen. Zur Bildung und Struktur tiefschmelzender molekularer und ionischer Hemi-bis Hexahydrate halogenierter Essigsäuren CCl_nF_3-nCOOH (n = 1 bis 3) [1] / Two-Dimensional Ice and H₅O₂ ⁺ Ions. On Formation and Structure of Low-Melting Molecular and Ionic Hemi-through Hexahydrates of Haloacetic Acids CCl_nF_3-nCOOH (n = 1 to 3) [1]

Abstract

Abstract Hydrate formation of haloacetic acids CCl n F 3-n COOH (n = 1, 2, 3) has been studied by DTA and temperature-dependent X-ray powder diffraction, and evidence obtained for five phases, all melting at temperatures below ambient. The hydrates have been confirmed and further characterized by their crystal structures at -150 °C. Three lower hydrates, CCl 2 FCOOH· 0.5H 2 O (space group P2 1 /c and Z = 8 formula unites per unit cell), CClF 2 COOHH 2 O (P2 1 /c, Z = 4), and CClF 2 COOH ·4H 2 O (P1̄, Z = 2), have molecular structures with the acid and water molecules hydrogen-bonded in two-dimensional arrays. The structures of the remaining hydrates, CCl 2 FCOOH·6H 2 O and CCl 3 COOH · 6H 2O (similar, but not isotypic, both P1̄ and Z = 2), are ionic, as (H 5 O 2 + )(CX 3 COO - ) · 4H 2 O , and three-dimensional. The neutral water molecules are hydrogen-bonded in ice-like layers rare in crystal chemistry. Also, CCl 2 FCOOH is established only as the second acid of which a lower hydrate is molecular and a higher one ionic.