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Hydrate formation o f haloacetic acids CCl"F3_"COOH (n = 1, 2, 3) has been studied by D TA and tem perature-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 -1 5 0 °C. Three lower hydrates, CC12F C O O H 0.5H -> 0 (space group P2,/c and Z = 8 formula unites per unit cell), CC1F2C 0 0 H H 20 (P2,/c, Z = 4), and CC1F2C 0 0 H -4 H 20 (P I, Z = 2), have molecular structures with the acid and water m olecules hydrogen-bonded in two-dimensional arrays. The structures of the remaining hydrates, CCl2F C O O H -6H 20 and CC13C 0 0 H -6 H 20 (sim i lar, but not isotypic, both P 1 and Z = 2), are ionic, as (H s0 2+)(C X 3C 0 Ö ) -4 H 20 , and threedimensional. The neutral water molecules are hydrogen-bonded in ice-like layers rare in crystal chemistry. A lso, CCl2FCOOH is established only as the second acid o f which a lower hydrate is molecular and a higher one ionic. 0 9 3 2 -0 7 7 6 /9 6 /0 4 0 0 -0 5 3 6 $0 6 .0 0 © 1996 V erlag der Z eitschrift für N aturforschung. A ll rights reserved. D

Professor Hanskarl Miiller-Buschbaum zum 65. Geburtstag gewidmet Inhaltsiibersicht. Tiefschmelzendes Selensaure-Tetrahydrat wur-c = 6,378(3) A fur die tetragonale Form. Beide Formen sind de erstmals durch Kristallstrukturbestimmung untersucht. Es gemail3 (H,02),Se0, Oxoniumsalze. Die Ionen sind durch Waswurden zwei Formen festgestellt, eine orthorhombische rnit der serstoffbrucken dreidimensional vernetzt. Die Briicken inner-Raumgruppe P n p a und Z = 4 sowie eine tetragonale rnit der halb der Diaquahydrogen-Kationen haben 0 . . . 0-Abstande Raumgruppe P42,c und Z = 2. Die Gitterkonstanten bei von 2,422(5) und 2,425(4) A . Die tetragonale Form ist isotyp -150°C betragen a = 6,130(3), b = 12,776(6) und zum Tetrahydrat der Schwefelsaure. c = 9,299(5) fur die orthorhombische sowie a = 7,676(4) und Crystal Structures of Acid Hydrates and Oxonium Salts. 36 [I] Selenic Acid Tetrahydrate. Ionic as (H,O,),SeO, in an Orthorhombic as well as a Tetragonal FormAbstract. Low-melting selenic acid tetrahydrate has been studied for the first time by crystal structure analysis. Two forms have been observed, an orthorhombic one with space group Pnma and Z = 4 and a tetragonal one with space group P42,c and 2 = 2. The lattice parametersoat -150 "C are a = 6.1 30(3), b = 12.776(6) and c = 9.299(5) A for the orthorhombic and a = 7.676(4) and c = 6.378(3) a for the tetragonal form. Both forms are oxonium salts, corresponding to (H50J2SeO4. The ions are hydrogen-bonded into a three-dimensional array. The bonds within the diaquahydrfgen cations have 0 . . O distances of 2.422(5) and 2.425(4) A. The tetragonal form is isotypic to the tetrahydrate of sulfuric acid.

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