A new nonlinear optical organic compound, namely, L-histidinium 2-nitrobenzoate (abbreviated as LH2NB (I); ([C6H10N3O2]+ [C7H4NO4]？)), was synthesized. The molecular structure of LH2NB (I) was elucidated using single crystal X-ray diffraction technique. The second harmonic generation (SHG) efficiency of this compound is about two times that of the standard potassium dihydrogen phosphate crystals. 1. Introduction Recently, considerable efforts are being made to design new noncentrosymmetric crystal structures by combining amino acids with various interesting organic and inorganic matrices to produce compounds for nonlinear optical (NLO) applications. A number of L-histidine compounds exhibiting the NLO behaviour, namely, L-histidine acetate [1], L-histidine chloride monohydrate [2], L-histidine tetrafluoroborate [3], L-histidine hydrochloride monohydrate [4], L-histidine hydrofluoride dihydrate [5], L-histidine bromide [6], and L-histidinium trichloroacetate [7], were reported earlier. The crystal growth and characterization of L-histidinium trifluoroacetate and L-histidine nitrate were reported from this laboratory [8, 9], recently. In this paper, another new compound possessing the NLO property, namely, L-histidinium 2-nitrobenzoate [LH2NB, (I)] is reported. To our knowledge, (I) is the first reported compound of an amino acid with 2-nitrobenzoic acid. The details regarding the preparation, crystal structure, hydrogen bonding, and SHG efficiency of the title compound are discussed. 2. Experimental Procedures 2.1. Synthesis and Crystallization of LH2NB The starting compounds, namely, L-histidine (Loba Chemie, 99%) and 2-nitrobenzoic acid (Alfa Aesar, 95%) were used without further purification. L-histidine and 2-nitrobenzoic acid were mixed in the stoichiometric ratio, in 1？:？1 proportions and dissolved in distilled water. The resultant mixture was stirred continuously to obtain a homogeneous solution, filtered and kept undisturbed for crystallization to take place. Good quality single crystals of the title compound were obtained after about a week’s time. The chemical structure of the compound is shown in Figure 1. Figure 1: The chemical structure of (I). 2.2. Crystal Structure Determination Three-dimensional intensity data for a crystal of (I) were collected on a Bruker SMART APEX CCD area-detector diffractometer using graphite-monochromated MoKα radiation (0.71073？？). The crystal structure was solved by direct methods using SHELXS-97 [10]. Full-matrix least-squares refinement and subsequent Fourier synthesis procedures were performed by using
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