(E)-1-Methyl-4-[2-(2-naphthyl)vinyl]pyridinium iodide1

In the title compound, C18H16N+·I−, the cation is disordered over two orientations related by a 180° rotation about its long axis with occupancies of 0.554 (7) and 0.446 (7). Both disorder components exist in an E configuration. The dihedral angle between the pyridinium ring and the naphthalene ring system is 4.7 (6)° in the major disorder component and 1.6 (8)° in the minor component. In the crystal structure, centrosymmetrically related cations are stacked along the a axis, with significant π–π interactions between the pyridinium ring and the naphthalene ring system [centroid-centroid distance = 3.442 (9) Å]. The iodide ions are located between adjacent columns of cations. The cations are linked to the iodide ions by C—H⋯I interactions. Weak C—H⋯π interactions involving the methyl group are also observed.


Comment
In order to obtain second-order non-linear optical (NLO) single crystals, the main requirements should be the choice of molecules with large hyperpolarizability (β) and these molecules should align into a noncentrosymmetric space group in the crystal. Organic crystals with extensive conjugated π systems with large hyperpolarizability which exhibit NLO properties have been reported (Ogawa et al., 2008;Yang et al., 2007). Styryl pyridinium derivatives are considered to be good conjugated π-systems (Cheng et al., 1991a(Cheng et al., , 1991b. In our on-going research in searching for NLO materials (Chanawanno et al., 2008;Chantrapromma et al., 2006Chantrapromma et al., , 2007Chantrapromma et al., , 2008Chantrapromma et al., , 2009a, we have previously reported the crystal structure of (E)-1methyl-4-[2-(1-naphthyl)vinyl]pyridinium 4-bromobenzenesulfonate (Chantrapromma et al., 2009a). In order to study the effect of different positions of the subsituent group and anions, the title compound was synthesized by replacing the 1-naphthyl and 4-bromobenzenesulfonate in the (E)-1-methyl-4-[2-(1-naphthyl)vinyl]pyridinium 4-bromobenzenesulfonate with 2-naphthyl and iodide in the title compound. However it crystallized in the monoclinic centrosymmetric space group P2 1 /c and would not exhibit second-order nonlinear optical properties. Fig. 1 shows the asymmetric unit of the title compound which consists of a C 18 H 16 N + cation and a Ianion. The whole cation is disordered over two sites; the major component A and the minor component B (Fig. 1), with the refined site-occupancy ratio of 0.554 (7)/0.446 (7). The cation exists in the E configuration with respect to the C6═C7 double bond. The napthalenyl moiety is essentially planar in both disorder components as indicated by the interplanar angle between the two aromatic C8-C10/C15-C17 and C10-C15 rings [1.5 (8)° for the major component A and 3.2 (9)° for the minor compon-  (Allen et al., 1987) and are comparable to those observed in related structures (Chanawanno et al., 2008;Chantrapromma et al., 2006Chantrapromma et al., , 2007Chantrapromma et al., , 2008Chantrapromma et al., , 2009a. In the crystal packing (Fig. 2), centrosymmetrically related cations are stacked along the a axis, with significant π-π interactions between pyridinium ring and naphthalene ring system [centroid-centroid distance is 3.442 (9) Å]. The iodide ions are located between adjacent coloumns of cations. The cations are linked to the iodide ions by C-H···I weak interactions (Table 1). The crystal structure is further stabilized by C-H···π interactions involving the methyl group (Table 1; Cg1 and Cg2 are centroids of the C10A-C15A and C10B-C15B rings, respectively).
Yellow plate-shaped single crystals of the title compound suitable for X-ray structure determination were recrystallized from methanol by slow evaporation at room temperature over a few weeks (m.p. 557-558 K).

Refinement
The cation is disordered over two orientations with occupancies of 0.554 (7) and 0.446 (7). The same U ij parameters were used for atom pairs N1A/N1B and C15A/C16A and all disordered atoms were subjected to a rigid bond restraint. All H atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.93 Å (aromatic and CH) and 0.96 Å (CH 3 ). The U iso values were constrained to be 1.5U eq of the carrier atom for methyl H atoms and 1.2U eq for the remaining H atoms. A rotating group model was used for the methyl groups. The highest residual electron density peak is located at 0.69 Å from I1 and the deepest hole is located at 0.56 Å from I1. Fig. 1. The molecular structure of the title compound, with 50% probability displacement ellipsoids and the atom-numbering scheme. Open bonds show the minor disorder component.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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.