l-Histidinium p-toluenesulfonate

In the title salt, C6H10N3O2 +·C7H7O3S−, the imidazole ring makes a dihedral angle of 70.93 (12)° with the plane of the toluene ring. In the crystal, the ions are linked via N—H⋯O and weak C—H⋯O hydrogen bonds forming two-dimensional networks lying parallel to (001). These networks are linked via C—H⋯π interactions, forming a three-dimensional structure.

In the title salt, C 6 H 10 N 3 O 2 + ÁC 7 H 7 O 3 S À , the imidazole ring makes a dihedral angle of 70.93 (12) with the plane of the toluene ring. In the crystal, the ions are linked via N-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds forming two-dimensional networks lying parallel to (001). These networks are linked via C-HÁ Á Á interactions, forming a three-dimensional structure.
In the crystal, the ions are linked via N-H···O and weak C-H···O hydrogen bonds forming two-dimensional networks lying parallel to (001); see Table 1 and Fig. 2. These networks are linked via C-H···π interactions forming a threedimensional structure.

Experimental
L-histidine and 4-toluenesulfonic acid were mixed in an equimolar (1:1) ratio using distilled water as solvent and stirred for 1 h, giving a clear solution. The solution was filtered into a clean beaker and optimally closed and kept at room temperature for slow evaporation. After a period of 10 days, block-like colourless crystals suitable for X-ray diffraction analysis were obtained.

Refinement
The NH 3 H atoms were located in a difference Fourier map and refined freely. The NH H atoms and the C-bound H atoms were positioned geometrically and refined using a riding model: N-H = 0.86 Å, C-H = 0.93 and 0.98 Å for CH(aromatic) and CH(methine) H atoms, respectively, and 0.96 Å for CH 2 and CH 3 H atoms, with U iso (H) = 1.5U eq (Cmethyl) and = 1.2U eq (N,C) for other H atoms.

Figure 2
The crystal packing of the title compound viewed along the b axis. The hydrogen bonds are shown as dashed lines (see Table 1 for details; H-atoms not involved in hydrogen bonds have been omitted for clarity).  (7) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.