2-[(E)-2-(4-Ethoxyphenyl)ethenyl]-1-methylpyridinium iodide monohydrate

In the title compound, C16H18NO+·I−·H2O, the cation is essentially planar, with a dihedral angle of 3.13 (16)° between the pyridinium and benzene rings. The molecule adopts an E configuration with respect to the alkene double bond. In the crystal structure, the cations are packed in an anti-parallel manner through π–π interactions between adjacent pyridinium and benzene rings along the a axis, with centroid-to-centroid distances of 3.615 (2) and 3.630 (2) Å. Water molecules bind the iodide ions through O—H⋯I hydrogen bonds into layers. These layers link with the cations through weak C—H⋯O and C—H⋯I interactions.

In the title compound, C 16 H 18 NO + ÁI À ÁH 2 O, the cation is essentially planar, with a dihedral angle of 3.13 (16) between the pyridinium and benzene rings. The molecule adopts an E configuration with respect to the alkene double bond. In the crystal structure, the cations are packed in an anti-parallel manner throughinteractions between adjacent pyridinium and benzene rings along the a axis, with centroid-tocentroid distances of 3.615 (2) and 3.630 (2) Å . Water molecules bind the iodide ions through O-HÁ Á ÁI hydrogen bonds into layers. These layers link with the cations through weak C-HÁ Á ÁO and C-HÁ Á ÁI interactions.

Comment
The design of nonlinear optical (NLO) materials is of great interest due to the various applications of NLO materials. At molecular level, such compounds are likely to exhibit large values of molecular hyperpolarizability (β) and they have to have polarizable electrons (conjugated π system) spread over a large distance (Oudar & Chemla, 1977). We have been previously synthesized pyridinium and quinolinium derivatives to study their non-linear optical properties (Chantrapromma et al., , 2006Chantrapromma, Jindawong, Fun, Patil & Karalai, 2007;Jindawong et al., 2005). The single-crystal x-ray structural study of the title compound was undertaken in order to establish the structure and conformation of the various groups. However, the title compound crystallized in the centrosymmetric P1 triclinic space group and therefore does not exhibit non-linear optical properties (Williams, 1984).
The asymmetric unit of the title compound consists of the pyridinium cation, iodide anion and one water molecule ( Fig.   1). The water molecule forms an O1W-H1W1···I1 hydrogen bond to the iodide ion (Table 1). The cation is essentially planar and exist in E configuration with respect to the C6═C7 double bond [1.342 (5) Å]. The dihedral angle between the pyridinium and benzene rings is 3.13 (16)°. The ethenyl unit is also planar with respect to the two aromatic rings with the torsion angles C4-C5-C6-C7 = −1.1 (5)° and C6-C7-C8-C13 = 3.8 (6)°. The ethoxy substituent deviates only slightly from the benzene ring plane, with a C14-O1-C11-C10 torsion angle of 5.3 (5)°. Bond lengths and angles are in normal ranges (Allen et al., 1987) and the bond lengths and angles of the cation are comparable with those for closely related structures Zhang et al., 2000).
In the crystal structure, the cations are packed in an anti-parallel fashion through π···π interactions along the a axis with Cg 1 ···Cg 2 distances 3.615 (2) Å (symmetry code −x, 1 − y, 1 − z) and 3.630 (2) Å (symmetry code 1 − x,1 − y, 1 − z) where Cg 1 is the centroid of the C1-C5/N1 pyridinium ring Cg 2 is the centroid of the C8-C13 benzene ring. Water molecules bind to iodide ions by O-H···I hydrogen bonds forming layers. These layers are linked with the cations through weak C-H···O and C-H···I interactions, Table 1.
Yellow single crystals of the title compound suitable for x-ray structure determination were recrystalized from methanol by slow evaporation of the solvent at room temperature over several days (Mp. 481-483 K).

sup-2 Refinement
All H atoms were positioned geometrically and allowed to ride on their parent atoms, with O-H = 0.85 ° and C-H distances in the range 0.93-0.97 Å. 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 0.85 Å from I1 and the deepest hole is located at 0.63 Å from I1. Fig. 1. The structure of (I) showing 50% probability displacement ellipsoids and the atomnumbering scheme. The O-H···I hydrogen bond is drawn as a dashed line.