Redetermination of triethylammonium chloride in the space group P31c.

The structure of triethylammonium chloride has been reported four times to date (Hendricks, 1928; Genet, 1965; James et al., 1985; Ilyukhin, 2000). All four structure determinations were made in the space group P63mc (No. 186) and showed `propeller'-like disorder of the cation (Fig. 1) caused by a crystallographic mirror plane. Two closely related models were used in the later re®nements. In the ®rst, both independent C atoms occupy general positions [12 (d): x, y, z; Genet, 1965; James et al., 1985; Fig. 1(a)]. In the second, the methylene C atom lies on a general position, while the methyl C atom lies on the mirror plane [6 (c): x, x, z; Ilyukhin, 2000; Fig. 1(b)]. Such disorder is a common feature of the Et3NH + cation. A total of 379 structures containing the triethylammonium cation are reported in the Cambridge Structural Database (CSD, Version 5.25; Allen, 2002). Of these, 126 structures are disordered (33.2%) and 76 structures (20.1%) possess the disordered cation. These ®gures are noticeably higher than the statistical appearance of disorder in the CSD (18.2%; Allen, 2002). The same type of disorder was observed previously for some other trialkylammonium derivatives with approximate C3v symmetry, namely silatranes (Zaitseva et al., 1996) and germatranes (Karlov et al., 2001). These two structures were re®ned in the space group Pnma. However, re®nements in the lower-symmetry space group Pna21 retain the `propeller'-like disorder, with occupancy ratios ' 0.5:0.5. On the contrary, the re®nement of [NHEt3][Sn(acac)Cl4] in the lower-symmetry group led to an ordered cation, but was not found to be convincing (Korte et al., 1988). Against this background, we present here a further redetermination of the structure of triethylammonium chloride, (I) (Fig. 2). A new data set for (I) was collected on a Bruker SMART CCD diffractometer at 120 K. The systematic absences were consistent with the space groups P31c (No. 159) and P63mc. Comparison of the |Fo(hkl)| and |Fo(hkl)| values points to P63mc, since their equality holds in P63mc but not in P31c. However, the mean value of |E 2 ÿ 1| (0.678) was lower than expected for non-centrosymmetric crystals (Herbst-Irmer & Sheldrick, 1998).