1-Allyl-2-aminopyridin-1-ium bromide

In the cation of the title salt, C8H11N2 +·Br−, the dihedral angle between the planes of the pyridinium ring and the allyl group is 79.4 (3)°. In the crystal, N—H⋯Br and weak C—H⋯Br hydrogen bonds link the cations and anions, forming chains of alternating R 2 1(7) and R 4 2(8) rings, which run parallel to the c-axis direction. The crystal studied was an inversion twin with components in a 0.753 (12):0.247 (12) ratio.

In the cation of the title salt, C 8 H 11 N 2 + ÁBr À , the dihedral angle between the planes of the pyridinium ring and the allyl group is 79.4 (3) . In the crystal, N-HÁ Á ÁBr and weak C-HÁ Á ÁBr hydrogen bonds link the cations and anions, forming chains of alternating R 2 1 (7) and R 4 2 (8) rings, which run parallel to the caxis direction. The crystal studied was an inversion twin with components in a 0.753 (12):0.247 (12) ratio.

Data collection
Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: DENZO; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97. As part of our studies on pyridinum salts (Seethalakshmi et al., 2006a(Seethalakshmi et al., ,b,c, 2007(Seethalakshmi et al., , 2013, we report herein the crystal structure of the title compound, (I). The asymmetric unit of (I) is shown in Fig. 1. The dihedral angle between the planes of the pyridinium ring and allyl group (C6/C7/C8) is 79.4 (3)°. The corresponding bond lengths and angles of the cation in (I) are comparable with those of related structures reported earlier (Seethalakshmi et al., 2006a(Seethalakshmi et al., ,b,c, 2007(Seethalakshmi et al., , 2013. In the crystal (Fig. 2) the amino group acts as a donor for two different bromide anions (Table 1). These intermolecular N-H···Br hydrogen bonds link the cations via bromide anions into one-dimensional chains which run parallel to the c axis. In addition, weak intermolecular C-H···Br interactions are observed between C6 (via H6A and H6B) and two bromide anions. The N2-H2A···Br1 i and C6-H6B···Br1 i interactions combine to generate a R 1 2 (7) ring (Bernstein et al., 1995) and two N-H···O hydrogen bonds and two C-H···Br interactions combine to form a R 2 4 (8) ring motif. These two ring motifs are arranged alternately and run parallel to the c axis (Fig. 3).

Experimental
A solution of 2-aminopyridine (1.175 g, 25 ml) and allyl bromide (1.51 g, 25 ml) in dry acetone (15 ml) was stirred for 44 h at room temperature (303 K). The solid that separated was filtered, washed with dry acetone and dried in vacuum to give the stable salt, which was recrystallized from an aqueous ethanol (80% v/v) solution (m.p. 419-421 K, yield 63%).

Refinement
The positions of amino H atoms were determined from a difference Fourier map and refined freely along with their isotropic displacement parameters. In the final round of refinement, the N-H bond lengths of amino group were restrained to 0.86 (2) Å. The remaining H atoms were placed in geometrically idealized positions (C-H = 0.95-0.99 Å), with U iso (H) = 1.2U eq (C) and were constrained to ride on their parent atoms. The crystal used is an inversion twin with components in the ratio 0.753 (12):0.247 (12)

Computing details
Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008    Arrangement of alternate R 1 2 (7) and R 2 4 (10) ring motifs in a one-dimensional chain.  (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0051 (7) Absolute structure: Flack (1983), 945 Friedel pairs Flack parameter: 0.247 (12) Special details Experimental. The minimum and maximum absorption values stated above are those calculated in SHELXL97 from the given crystal dimensions. The ratio of minimum to maximum apparent transmission was determined experimentally as 0.611792. 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq