Tetra-n-butylammonium bromide: a redetermination at 150 K addressing the merohedral twinning

The redetermined, low temperature (150 K), structure of tetra-n-butylammonium bromide, (C4H9)4N+·Br−, has been found to be merohedrally twinned via twin law −1 0 0, 0 − 1 0, 1 0 1. The structure was previously determined, with low precision, no inclusion of H atoms and only the bromide ion refined with anisotropic displacement parameters, by Wang et al. (1995 ▶). Mol. Cryst. Liq. Cryst. Sci. Tech. A, 264, 115–129. The redetermined structure has considerably improved precision in all geometrical parameters, has all non-H atoms refined anisotropically, H atoms included, and is isomorphous with the iodide analogue. The structure is otherwise routine, with the shortest cation to anion contacts being between the bromide anion and the CH atoms close to the ammonium nitrogen centre at a distance of ca. 2.98–3.11 Å. Each anion makes eight such contacts to four different anions. The n-butyl chains are fully extended, adopting an all-anti conformation with approximate S 4 point symmetry.


Related literature
The structure was previously determined by Wang et al. (1995). For the uses of tetra-n-alkylammonium salts and the isomorphous structure of tetra-n-butyl ammonium iodide, see: Prukała et al. (2007). For a related stucture, see: McMullan & Jeffrey (1959). For the conformation of n-butyl chains, see: Alder et al. (1990). For details of the Cambridge Structural Database, see: Fletcher et al. (1996); Allen (2002).

Experimental
Crystal data C 16 H 36 N + ÁBr À M r = 322.37 Monoclinic, C2=c a = 13.9773 (9) Å b = 13.8623 (9) Å c = 20.0450 (14)  of the Cambridge Structural Database (version 5.32 + 3 updates, Fletcher, et al., 1996, Allen, 2002) revealed just one reported structure of this compound with an R1 of 0.098 that had clearly been problematic (Wang et al., 1995). This earlier determination had only the bromide ion refined anisotropically and did not include hydrogen atoms in the model. The authors ruled out dynamic disorder as the cause of the difficulties and concluded that static disorder was the causeof the poor residual.
The crystals of (I) formed readily by vapour diffusion of diethyl ether into an acetonitrile solution. The data collection set-up was trouble free. After data reduction the structure did not solve readily with SHELXS (Sheldrick, 2008a); only the bromide, the nitrogen and two n-butyl chains being evident. When the structure failed to develop, the coordinates from the published structure were used as a starting point (Wang et al., 1995), but the R1 was ca. 35% for an isotropic model with all non-H atoms in the model. Twinning was suspected and confirmed by the TWINROTMAT routine in PLATON (Spek, 2009). Application of the merohedral twin law -1 0 0, 0 -1 0, 1 0 1, led to a reduction in R1 to ca. 5.0% at the same, isotropic, stage of refinement. Anisotropic refinement, and addition of H atoms, led to a good final R1 <3% with no adverse indicators.
The ratio of major to minor twin components is 60.69: 39.31 (7)% The structure is isomorphous with that of the iodide analogue described in detail recently (Prukała et al., 2007). The n-butyl chains are fully extended adopting an all-anti conformation with approximate S 4 point symmetry (Alder et al., 1990).
The bromide anion resides in a pocket between four cations, making four pairs of weak C-H···Br contacts in the range 2.98-3.11 Å to methylene hydrogens located one or two carbon atoms from the nitrogen cationic centre. The structures of the chloride and fluoride analogues have not been determined to date, although the unit cell of the hydrate of the chloride has been reported (McMullan & Jeffrey, 1959).

Experimental
The title compound (I) was used as received and crystallized from an acetonitrile solution via vapour diffusion with diethylether to give colourless blocks.

Refinement
H atoms were included in a riding model with constrained bond lengths: for CH 2 = 0.99 and CH 3 = 0.98 Å with U iso (H) = 1.2 Ueq(CH 2 ) and =1.5Ueq (CH 3 Fig. 1. The asymmetric unit in the structure of (I) with displacement ellipsoids drawn at the 50% probability level.