supplementary materials

The radiation-induced free radical chemistry of dicarboxylic acids and their salts has received attention for several decades. The Rochelle salt, is of particular interest as it as it exhibits a ferroelectric phase between 255 and 297 K, where the structure is monoclinic, space group P2₁; outside this temperature range the compound is paraelectric and presents orthorhombic phases in space group P2₁2₁2. The nature of the radicals formed in Rochelle salt is currently investigated in order to understand the mechanisms producing changes in the ferroelectric properties of this compound upon irradiation (Suzuki, 1974; Treeck, van & Windsch, 1977). For the analysis of the electron magnetic resonance data, precise knowledge of the low-temperature orthorhombic form is necessary. Structural data for the high-temperature orthorhombic form were first provided by Beevers & Hughes (1941). Iwata et al. (1989) carried out a neutron diffraction study for both orthorhombic forms; more accurate X-ray diffraction studies were later presented by Solans et al. (1997), who concluded that differences between the two P2₁2₁2 states are "small but significant". None of these structures are, however, available in the Cambridge Structural Database (Version 5.29 of November 2007; Allen, 2002). A high-precision redetermination of Rochelle salt at low temperature has therefore been executed.

The molecular structure of (I) is shown in Fig. 1. The crystal packing arrangement, illustrated in Fig. 2, is very similar to those found in the P2₁2₁2 structures of other salts of tartaric acid in which Na⁺ is replaced by Li⁺ and/or K⁺ by NH₄⁺ or Tl⁺ [Li⁺/K⁺: Ottenz et al., 1998; Li⁺/NH₄⁺: Hinazumi & Mitsui, 1972; Li⁺/Tl⁺: Kay, 1978; Na⁺/NH₄⁺ (II): Kuroda & Mason, 1981; Brożek & Stadnicka, 1994; Suzuki et al., 1996a] as well as in salts where K⁺ has been only partly replaced by NH₄⁺ (Suzuki et al., 1996a; Suzuki et al., 1996b). The pure sodium (Ambady & Kartha, 1968) or potassium salts (Boese et al., 1995) on the other hand, have completely different structures.

Hydrogen bonds are listed in Table 1, the most unusual feature is the almost symmetric four-center interaction involving H31W.

When K⁺ is replaced by NH₄⁺ [as, for instance, in II] the four shortest K2···O contacts are converted into hydrogen bonds, while only the two K1···O4 interactions are transformed into short hydrogen bonds, the K1···O1W and K1···O2W contacts being replaced by a three-center hydrogen bond.