Benzylammonium heptanoate

The title 1:1 stoichiometric salt, C7H10N+·C7H13O2 −, is formed by proton transfer between heptanoic acid and benzylamine. This combination contrasts to the recently published 2:1 acid–amine adduct of cation, anion and neutral acid molecule from the same components [Wood & Clarke (2013 ▶). Acta Cryst. E69, o346–o347]. There are N—H⋯O hydrogen bonds of moderate strength in the structure [the most important graph-set motifs are R 2 4(8) and R 4 4(12)], as well as weak C—H⋯O interactions.


Related literature
For spectroscopic studies of acid-amine complexes, see: Kohler et al. (1981); Karlsson et al. (2000); Paivarinta et al. (2000); Smith et al. (2001Smith et al. ( , 2002. For recent diffraction studies of acid-amine complexes, see: Jefferson et al. (2011); Sun et al. (2011); Wood & Clarke (2012a,b, 2013. For the categorization of hydrogen bonds, see Gilli & Gilli (2009). For graph-set motifs, see Etter et al. (1990). H atoms treated by a mixture of independent and constrained refinement Á max = 0.32 e Å À3 Á min = À0.19 e Å À3 Table 1 Hydrogen-bond geometry (Å , ). These have been identified by a number of experimental methods, such as NMR and IR spectroscopy (e. g. Karlsson et al., 2000;Paivarinta et al. (2000); Smith et al. (2001Smith et al. ( , 2002). Unfortunately, the atomic details of the materials and hence the nature of the bonding that can be obtained from single-crystal diffraction has only been determined in a few cases. This is partly due to the challenges in growing crystals large enough to be suitable for single-crystal diffraction. Some crystals of sufficient size have been grown (e. g. Jefferson et al. (2011);Wood & Clarke, 2012a, 2012b. Of the stoichiometic combinations reported to date, the majority have been 1:1 complexes, though some 2:1 and even 3:1 examples have been reported by calorimetry and NMR spectroscopy, generally when the environment is acid-rich (e. g. Kohler et al., 1981;Sun et al., 2011), and very recently using single-crystal diffraction (Wood & Clarke, 2013).
Here we report growth of a suitable crystal for single-crystal X-ray diffraction of a 1:1 complex formed between heptanoic acid and benzylamine ( Figs. 1 and 2). This work follows a previous publication (Wood & Clarke, 2013) in which an acid-rich 2:1 complex formed between these two species is described. In the previous work, one acid molecule donates its proton to the amine group and one acid group retains its proton. The hydrogen bonding extends across both the ions and the neutral acid molecule.
As described below, the sample of the present study was grown by vapour phase condensation. Each preparation resulted in a batch of several crystals. The crystal used in this present study was taken from a different region of the same batch of samples as for the 2:1 combination (Wood & Clarke, 2013). We attribute the combination of compositions to the concentration gradients across the vapour streams in the preparation.

Experimental
Benzylamine and heptanoic acid (purities 99.7% and 99.8% respectively, determined by titration and gas chromatography) were purchased from Sigma Aldrich and used without further purification. A small volume of amine (approximately 1 ml) was placed into a small vial that was itself placed within a larger vial containing a similar volume of the acid, and left in an inert atmosphere. Extensive crystal growth was observed after a few weeks, particularly on a polypropylene surface included in the vial to encourage nucleation.

Refinement
All the hydrogens were discernible in the difference electron density map. Nevertheless, the hydrogens attached to the C atoms were situated in the idealized positions and refined under these constraints: C aryl -H aryl =0.95, C methyl -H methyl =0.98, C methylene -H methylene =0.99 Å. U eq (H aryl )=1.2U iso (C aryl ), U eq (H methylene )=1.2U iso (C methylene ), U eq (H methyl )=1.5U iso (C methyl ). The positional parameters of the hydrogens attached to N of the benzylammonium cation were freely refined while their U eq (H N )=1.5U iso (H N ).    A section from the hydrogen bond pattern in the title structure. The symmetry codes: i: 1+x, y, z; ii: 1-x, -y, -z; iii: -x, -y,z; iv: 2-x, -y, -z; v: -1+x, y, z. 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.