SYNTHESIS, GROWTH AND CHARACTERIZATION OF BIS(TETRAETHYLAMMONIUM) BIS(HYDROGEN L-TARTRATE) L-TARTARIC ACID MONOHYDRATE

Materials that exhibit highly nonlinear optical (NLO) responses are currently of great scientific and technical interest (Halvorson et al (1994), Marder et al (1997), Slepkov et al (2002), Bing Gu et al (2003)). In recent years, π-conjugated organic materials have attracted considerable interest because of their highly nonlinear optical properties and the fast response time of the nonlinearity (Gomes et al 1996). The crystal structures of bis(tetraethylammonium) tartrate bis (thiourea) dihydrate, (Mei et al 2002), Tetraethylammonium L-tartarate dihydrate (Rahman et al 2008), (Tetraethylammonium L-malate 1.36-hydrate) were reported. L-Tartaric acid was a well known organic NLO material (Martin Britto Dhas et al 2007). In the present work, bis (tetraethylammonium) bis (hydrogen L-tartrate) L-tartaric acid monohydrate was synthesised and grown by slow evaporation solution growth method for the first time and characterized.

HSE thanks the staff of the XRD Application LAB., CSEM, Neuchâ tel, for access to the X-ray diffraction equipment.

Comment
Our interest in the determination of the structure of the title compound is due to recent advances in organic non-linear optical (NLO) materials on account of their widespread potential industrial applications (Dega-Szafran et al., 2008;Bosshard et al., 1995). The majority of promising compounds constitute dipolar donor-π-acceptor molecules and these must be arranged non-centrosymmetrically to afford macroscopic structures capable of showing bulk quadratic NLO effects, such as frequency doubling [second-harmonic generation, SHG] (Coe et al., 2005). In particular, molecule-based NLO materials offer ultrafast response times, lower dielectric constants, better processability characteristics and enhanced nonresonant NLO responses relative to the traditional inorganic crystals. Previous work has shown that an inherent relationship exists between the structure of title material and its observed properties, although the SHG output was found to be rather weak when compared to KDP or urea.
The molecular structure of the title compound is illustrated in Fig. 1. The asymmetric unit is composed of two tetraethylammonium cations, two hydrogen L-tartrate anions, a molecule of L-tartaric acid and a water molecule. The various moieties are linked by O-H···O hydrogen bonds (Table 1).
In the crystal two-dimensional networks (Fig. 2) are formed via O-H···O hydrogen bonds and C-H···O interactions (Table 1) involving the water molecule, the hydrogen L-tartrate anions and the L-tartaric acid molecules. These layers stack along [001] are separated by tetraethylammonium cations, which are also involved in C-H···O interactions with the anions and the L-tartaric acid and water molecules ( Fig. 3 and Table 1). This arrangement is similar to that in the crystal structure of Tetra-ethylammonium hydrogen L-tartrate dihydrate, which has been reported on previously (Rahman et al., 2008).

Experimental
The title compound was synthesized using tetraethyl ammonium and L-tartaric acid in an equimolar ratio. The measured quantity of L-tartaric was dissolved in double distilled water until a saturated solution was obtained. Tetraethylammonium hydroxide (20% water) was then added slowly drop wise to the aqueous solution of L-tartaric acid. The mixture was stirred well at RT until a homogeneous solution was obtained. It was then stirred for 4 hrs at 350 K (oil bath) and then cooled to RT.
The cooled solution was then filtered and the filtrate covered using a thick parafilm sheet, in order to control the evaporation rate at RT in a constant temperature bath. Good quality single crystals of title compound were obtained after 1 month.

Refinement
In the final cycles of refinement, in the absence of significant anomalous scattering effects, 3324 Friedel pairs were merged and Δf " set to zero. The absolute configuration is referred to that of L-tartaric acid. The water H-atoms were located in difference electron-density maps and were refined with O-H distance restraints of 0.84 (2) Å, and U iso (H) = 1.5 × U eq (O).