Crystal structure of allylammonium hydrogen succinate at 100 K

The asymmetric unit of the title compound, C2H8N+·C4H5O4 −, consists of two allylammonium cations and two hydrogen succinate anions (Z′ = 2). One of the cations has a near-perfect syn-periplanar (cis) conformation with an N—C—C—C torsion angle of 0.4 (3)°, while the other is characterized by a gauche conformation and a torsion angle of 102.5 (3)°. Regarding the anions, three out of four carboxilic groups are twisted with respect to the central C–CH2–CH2–C group [dihedral angles = 24.4 (2), 31.2 (2) and 40.4 (2)°], the remaining one being instead almost coplanar, with a dihedral angle of 4.0 (2)°. In the crystal, there are two very short, near linear O—H⋯O hydrogen bonds between anions, with the H atoms shifted notably from the donor O towards the O⋯O midpoint. These O—H⋯O hydrogen bonds form helical chains along the [011] which are further linked to each other through N—H⋯O hydrogen bonds (involving all the available NH groups), forming layers lying parallel to (100).

The independent part of the unit cell of the title salt, (I), consists with two allyloammonium cations and two hydrogen succinate anions (Fig. 1). A geometry of amonium cations is normal (CSD; CONQUEST Version 1.16; Allen, 2002) and comparable with those found in other crystal structures which include this cation (Allen, 2002). The N11 cation has perfect syn-periplanar (cis) conformation with N11-C12-C13-C14 torsion angle of 0.4 (3)°, while N21 cataion is characterized by gauche conformation (the torsion angle N21-C22-C23-C24 amounts 102.5 (3)°). Three out of four carboxalic groups are twisted with respect to the central C-CH 2 -CH 2 -C group; the remaining one being rather co-planar.
In the crystal structure of (I), there are two linear or nearly linear O-H···O hydrogen bonds between the hydrogen succinate, which can be identified as a very strong interactions (Steiner, 2002). The O···O distances in these interactions are close to that observed for O-H···O hydrogen bonds formed between the monoanionic oxalate units in the structures of diethylammonium hydrogen oxalate (Ejsmont, 2007). These O-H···O hydrogen bonds forming helical chains along <011> direction. The allylammonium cations are linked to polianionic chains through the N-H···O hydrogen bonds (Table   2, Fig. 2).

S2. Experimental
Crystals of (I) were grown at room temperature by slow evaporation of an aqueous solution containing allylamine and succinatic acid in a 1:1 stoichiometric ratio.

S3. Refinement
The H atoms attached to atoms O and N were located in difference electron density maps and were freely refined with isotropic displacement factors [O-H = 1.08 (3)  libration, the ending C23═C24 bond appears significantly shorter that its corresponding C13═C14 one.

Figure 1
The molecular structure of (I), showing 50% displacement ellipsoids. Hydrogen bonds are shown as dotted lines.

Special details
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.