The low-temperature phase of diiso­propyl­ammonium bromide

The title compound, C₆H₁₆N⁺·Br^-, was refined against room-temperature data in space group P2₁ as a racemic twin by Kociok-Köhn, Lungwitz & Filippou [Acta Cryst. (1996). C52, 2309-2311]. At low temperature, we found a different phase, which is characterized by a different cell (twice as big as the cell at room temperature) and a different space group (P2₁/n). Surprisingly, the cell parameters obtained at low temperature can be transformed into those measured at ambient temperature. Even the coordinates can be transformed and the structure can be refined in the small cell. However, some warning signs (e.g. a low |E² - 1| value, apparent twinning and peaks in the electron-density map) point to the correct cell and space group.

S1. Comment

Phase transition is a phenomenon which does not occur very often when data are collected at low temperature, but which is noticed from time to time. Since data collection is nowadays performed almost routinely at low temperature, a new phase of an already known crystal structure might be encountered. The title compound, diisopropylammonium bromide, (I), has already been described by Kociok-Köhn et al. (1996) in space group P2₁ as a racemic twin based on data collected at room temperature. At low temperature, however, we found a different phase, which is characterized by a different cell (twice as big as the cell at room temperature) and a different space group (P2₁/n).

We noticed on the first frames that the cell determination was not straightforward, because there were many weak, but nevertheless observable, reflections. Including these weak reflections in the cell determination yielded a monoclinic cell with the cell parameters given in the Crystal data (see Experimental). The weak reflections belonged to the class h+l = 2n+1 (for all hkl). Whereas the mean intensity and the mean intensity-to-sigma ratio for all reflections were 130.8 and 22.7, respectively, these values were 19.4 and 4.8, for the h+l = 2n+1 reflections. Thus, a B-centred lattice is pretended. Since these reflections are rather weak, they can be easily overlooked on a diffractometer equipped with a point counter, the result of which would be a halved cell.

The reason for the weakness of these reflections is that the structure contains one fairly heavy atom, i.e. Br, on a pseudo-special position. The coordinates of the four Br atoms in the cell (Table 1) show that Br1 and Br2, as well as Br3 and Br4, are related by an approximate translation of (x + 1/2, y, z + 1/2). However, the y coordinates are not exactly equal, but are related by a mirror plane. Nevertheless, the differences between the y coordinates of related atoms are rather small: 0.0213, which corresponds to 0.1697 Å.

If the weak reflections are overlooked, the resulting cell parameters would be a = 7.822, b = 7.973, c = 7.916 Å, α = 90.00, β = 116.59, γ = 90.00° and V = 441.46 ų. The same cell parameters can be obtained by applying the matrix (-0.5,0,-0.5/0,-1,0/-0.5,0, 1/2) to the correct cell parameters. The resulting space group is P2₁. However, the structure appears as the average of two mirror-related images. It might be surprising that the structure could be solved at all and refined in a halved cell. The reason for this is that the two images nearly overlap when the cell is halved (Fig. 2). Whereas the atoms overlap exactly in x and z, the differences between the y coordinates are very small (Table 2). The Br atoms fall so close to each other that they cannot be resolved as a result of the resolution of the data. On the other hand, an electron-density difference map reveals that the highest peaks show up exactly where the atoms of the second molecule would be (Fig. 3). The heights of the peaks, however, are rather small. Thus, they can be easily overlooked. Refinement against the data collected at room temperature by Kociok-Köhn et al. (1996) does not show this pattern in the electron-density difference map.

Refinement in the small cell (including the TWIN -1 0 0 0 - 1 0 0 0 - 1 and BASF instruction) using unit weights yields the results listed in Table 3. The explanation for the slightly better R values could be that the omitted reflections were all very weak and, as a result, less accurately determined. Nevertheless, the s.u.'s of the coordinates are significantly better in the correct space group. Furthermore, refinement in P2₁ resulted in many correlations between the refined parameters. These correlations vanished when the correct space group was used. Further warning signs are the low |E²-1| value and the pretended twinned crystal structure. The geometric parameters of the structure in the false cell look normal, but the two N—C bonds are significantly different, although they should be more or less equal (Table 4).

After having realised that the title compound had been found in a different phase at room temperature, we checked our crystals at room temperature and ended up with the same results as Kociok-Köhn et al. (1996). Upon cooling, the crystals underwent a phase transition with doubling of the cell. When we determined the cell again after warming the (previously cooled) crystal to room temperature, we still found the cell of the low-temperature phase. Thus, the phase transition is irreversible.

The importance of the weak reflections during structure refinement has already been stressed by Watkin (1994). The present case is an instructive example of the importance of weak reflections for cell determination. Furthermore, we presume that errors of this type can be more easily avoided when the data are collected on an area detector.

S2. Experimental

The title compound was obtained by stirring a solution of 5 mmol 5-bromopent-1-ene in MeOH (10 ml) in the presence of t-Pr₂NH (5 mmol) at ambient temperature. Colourless crystals of [t-Pr₂NH₂]Br were grown by storing this solution at room temperature for 2 d.

S3. Refinement

All H atoms were located by difference Fourier synthesis. They were refined with fixed individual displacement parameters [U_iso(H) = 1.5U_eq(C) or 1.2U_eq(N)], using a riding model, with N—H distances of 0.92 Å, methyl C—H distances of 0.98 Å and tertiary C—H distances of 1.0 Å.

Computing details

Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 1991).

Figures

	Fig. 1. Perspective view of the title compound with the atom-numbering scheme. Displacement ellipsoids are shown at the 50% probability level.
	Fig. 2. Picture of the unit cell when the correct is halved by applying the matrix (-0.5,0,-0.5/0,-1,0/-0.5,0, 1/2) to the cell parameters and the matrix (-1,0,-1/0,-1,0/-1,0,1) to the coordinates.
	Fig. 3. The difference electron-density map obtained when the structure is refined in the small cell. The peaks heights are (e Å^-3): Q1 = 0.76, Q2 = 0.60, Q3 = 0.57, Q4 = 0.55, Q6 = 0.49.

;

Crystal data

C6H16N^+·Br^−	F(000) = 376
Mr = 182.11	Dx = 1.370 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 9603 reflections
a = 8.2712 (13) Å	θ = 3.6–25°
b = 7.9726 (12) Å	µ = 4.58 mm^−1
c = 13.390 (2) Å	T = 173 K
β = 90.762 (12)°	Needle, brown
V = 882.9 (2) Å^3	0.46 × 0.14 × 0.12 mm
Z = 4

Data collection

Stoe IPDS-II two-circle diffractometer	1538 independent reflections
Radiation source: fine-focus sealed tube	1299 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.044
ω scans	θmax = 25.0°, θmin = 3.9°
Absorption correction: empirical (using intensity measurements) (MULABS; Spek, 1990; Blessing, 1995)	h = −9→9
Tmin = 0.227, Tmax = 0.610	k = −9→9
5310 measured reflections	l = −15→14

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.044	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.114	H-atom parameters constrained
S = 1.12	w = 1/[σ^2(Fo^2) + (0.0563P)^2 + 1.4082P] where P = (Fo^2 + 2Fc^2)/3
1538 reflections	(Δ/σ)max = 0.001
73 parameters	Δρmax = 0.57 e Å^−3
0 restraints	Δρmin = −0.56 e Å^−3

Special details

Experimental. ;

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Br1	0.75616 (6)	0.76063 (5)	0.88940 (3)	0.0355 (2)
N1	0.6767 (4)	0.6636 (4)	0.6542 (3)	0.0245 (7)
H1A	0.6970	0.5512	0.6452	0.029*
H1B	0.6997	0.6879	0.7200	0.029*
C1	0.7945 (5)	0.7613 (5)	0.5904 (3)	0.0271 (9)
H1	0.7814	0.8837	0.6046	0.033*
C11	0.7601 (6)	0.7309 (7)	0.4798 (3)	0.0382 (11)
H11A	0.6496	0.7668	0.4634	0.057*
H11B	0.7717	0.6111	0.4651	0.057*
H11C	0.8368	0.7952	0.4399	0.057*
C12	0.9657 (6)	0.7081 (7)	0.6213 (4)	0.0391 (11)
H12A	0.9829	0.7312	0.6925	0.059*
H12B	1.0445	0.7712	0.5822	0.059*
H12C	0.9793	0.5878	0.6089	0.059*
C2	0.4973 (5)	0.6923 (6)	0.6366 (3)	0.0277 (9)
H2	0.4689	0.6615	0.5661	0.033*
C21	0.4557 (6)	0.8752 (6)	0.6535 (4)	0.0425 (12)
H21A	0.5154	0.9451	0.6064	0.064*
H21B	0.4855	0.9070	0.7220	0.064*
H21C	0.3393	0.8919	0.6430	0.064*
C22	0.4082 (6)	0.5767 (7)	0.7067 (4)	0.0388 (11)
H22A	0.4382	0.4602	0.6928	0.058*
H22B	0.2914	0.5906	0.6967	0.058*
H22C	0.4373	0.6045	0.7759	0.058*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0400 (3)	0.0320 (3)	0.0343 (3)	−0.0015 (2)	−0.00237 (19)	−0.00176 (19)
N1	0.0243 (17)	0.0251 (17)	0.0243 (18)	−0.0002 (14)	0.0015 (14)	−0.0001 (13)
C1	0.028 (2)	0.023 (2)	0.030 (2)	−0.0019 (16)	0.0075 (17)	0.0021 (16)
C11	0.032 (2)	0.058 (3)	0.025 (2)	0.005 (2)	−0.0013 (18)	0.007 (2)
C12	0.024 (2)	0.053 (3)	0.040 (2)	−0.004 (2)	0.0067 (19)	−0.001 (2)
C2	0.020 (2)	0.030 (2)	0.032 (2)	0.0024 (16)	−0.0059 (17)	−0.0063 (17)
C21	0.030 (2)	0.033 (3)	0.064 (3)	0.005 (2)	−0.002 (2)	−0.001 (2)
C22	0.026 (2)	0.046 (3)	0.044 (3)	−0.002 (2)	0.004 (2)	0.000 (2)

Geometric parameters (Å, º)

N1—C2	1.517 (5)	C12—H12B	0.9800
N1—C1	1.519 (5)	C12—H12C	0.9800
N1—H1A	0.9200	C2—C22	1.514 (7)
N1—H1B	0.9200	C2—C21	1.516 (7)
C1—C11	1.523 (6)	C2—H2	1.0000
C1—C12	1.529 (6)	C21—H21A	0.9800
C1—H1	1.0000	C21—H21B	0.9800
C11—H11A	0.9800	C21—H21C	0.9800
C11—H11B	0.9800	C22—H22A	0.9800
C11—H11C	0.9800	C22—H22B	0.9800
C12—H12A	0.9800	C22—H22C	0.9800
C2—N1—C1	118.0 (3)	C1—C12—H12C	109.5
C2—N1—H1A	107.8	H12A—C12—H12C	109.5
C1—N1—H1A	107.8	H12B—C12—H12C	109.5
C2—N1—H1B	107.8	C22—C2—C21	112.3 (4)
C1—N1—H1B	107.8	C22—C2—N1	107.2 (4)
H1A—N1—H1B	107.2	C21—C2—N1	110.2 (4)
N1—C1—C11	110.6 (3)	C22—C2—H2	109.0
N1—C1—C12	107.7 (3)	C21—C2—H2	109.0
C11—C1—C12	112.3 (4)	N1—C2—H2	109.0
N1—C1—H1	108.7	C2—C21—H21A	109.5
C11—C1—H1	108.7	C2—C21—H21B	109.5
C12—C1—H1	108.7	H21A—C21—H21B	109.5
C1—C11—H11A	109.5	C2—C21—H21C	109.5
C1—C11—H11B	109.5	H21A—C21—H21C	109.5
H11A—C11—H11B	109.5	H21B—C21—H21C	109.5
C1—C11—H11C	109.5	C2—C22—H22A	109.5
H11A—C11—H11C	109.5	C2—C22—H22B	109.5
H11B—C11—H11C	109.5	H22A—C22—H22B	109.5
C1—C12—H12A	109.5	C2—C22—H22C	109.5
C1—C12—H12B	109.5	H22A—C22—H22C	109.5
H12A—C12—H12B	109.5	H22B—C22—H22C	109.5
C2—N1—C1—C11	−56.0 (5)	C1—N1—C2—C22	178.1 (4)
C2—N1—C1—C12	−179.1 (4)	C1—N1—C2—C21	−59.3 (5)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1B···Br1	0.92	2.38	3.301 (3)	178
N1—H1A···Br1^i	0.92	2.40	3.314 (4)	176

Symmetry code: (i) −x+3/2, y−1/2, −z+3/2.