Synthesis , crystal structure and spectroscopy studies of a new non-centrosymmetric organic cation nitrate

2532 | P a g e J u l y 3 1 , 2 0 1 4 Synthesis, crystal structure and spectroscopy studies of a new non-centrosymmetric organic cation nitrate H.Saadouni, D. E. Janzen, M.Rzaigui and W.Smirani Sta* a-Laboratoire de chimie des matériaux, Faculté des sciences de Bizerte, 7021, Zarzouna, Tunisia bDepartment of Chemistry and Biochemistry, St Catherine University, St Paul, Minnesota, 55105, USA *Corresponding author: wajda_sta@yahoo.fr ABSTRACT


INTRODUCTION
Nonlinear optical materials have attracted much attention because of their potential applications in optoelectronic technologies [1] and [2]. These applications depend upon the various properties of the materials, such as transparency, birefringence, refractive index, dielectric constant, thermal, photochemical and chemical stability. Inorganic crystals have excellent mechanical and thermal properties but possess relatively low optical nonlinearities because of the lack of πelectron delocalization. Organic crystals have large nonlinear susceptibilities compared to inorganic crystals. However, these crystals have certain limitations such as increased optical absorption, narrow transparency window and poor mechanical and thermal stability. Combining the high optical nonlinearity and chemical flexibility of organics with thermal stability and excellent transmittance of inorganics, semiorganic materials have been proposed and are attracting a great deal of attention in the field of nonlinear optics. Interestingly, the simple inorganic acid nitric acid can be used to access nitrate salts with organic cations. Among crystals of organic cations with nitrate anions, the most interesting for us are those that are non-centrosymmetric because of their non-linear optical properties. Several examples of nitrate salts exhibiting second harmonic generation have been reported by Kaminskii et al. [3]. In an attempt to find new materials suitable for nonlinear optical applications, we have prepared and report here the characterization of a new mixed organicinorganic non-centrosymmetric nitrate.

Material Preparation
Crystals of m-anisidinium nitrate were obtained by mixing, in stoichiometric ratio, a solution of nitric acid (1 M) with an ethanolic solution of m-anisidine. The mixture was stirred for 2 hours. The obtained solution was slowly evaporated at room temperature. After one week of evaporation at 293 K, X-ray quality yellow crystals appeared in the solution.

Physical measurement
X-ray diffraction measurements were made on a Rigaku XtaLAB mini diffractometer [4]. A crystal was placed on the tip of a Mitigen micromount and X-ray intensity data were measured at low temperature (173(2) K) with graphite monochromated Mo K radiation ( = 0.71075 Å). Preliminary sets of cell constants were calculated from reflections harvested from one set of 12 images with frame times of 30 s. A randomly oriented region of reciprocal space was surveyed to the extent of one sphere and to a resolution of 0.77 Å . Three major sections of frames were collected with 1.0 steps in  at three different settings. The intensity data were corrected for absorption and decay [4]. Final cell constants were calculated from the xyz centroids of 1820 strong reflections from the actual data collection after integration [4]. The structure was solved and refined using SHELXL-97 [5]. A direct-methods solution was calculated that provided most of the non-hydrogen atoms from the E-map. Full-matrix least-squares/difference Fourier cycles were performed that located the remaining nonhydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. Most of the hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement with the exception of H1A-C. H1A, H1B, and H1C were located in the electron density map, their x,y,z positions were refined, and their thermal parameters were treated as riding. The final refinement cycles led to R1= 0.033 and wR2= 0.088. The absolute structure was deduced based on Flack parameter [6], -0.2(10), using 973 Friedel pairs. As the intensity of high angle reflections were weak, the ambiguity in the Flack parameter is not surprising.
The infrared spectrum of solid C7H10NO.NO3 was recorded on a Nicolet IR200 FT-IR Spectrometer at ambient temperature.
The UV-Vis absorption (water solution) and optical diffuse reflectance spectra (solid) were measured at room temperature with a Perkin Elmer Lambda 11 UV/Vis spectrophotometer in the range of 200-800 nm.
Solution 13 C and 1 H NMR spectra were measure with a JEOL 400MHz NMR in D2O. A small amount of CH3CN was added as an internal reference (reference shifts of 1.94ppm for 1 H, 118.69 ppm for 13 C)

X-ray diffraction analysis
Crystal data summary of intensity data collection and structure refinement are reported in Table 1. The final atomic coordinates and Beq or Biso are given in Table 2. J u l y 31, 2 0 1 4 The asymmetric unit of the title compound, C7H10NO.NO3 (Figure 1), contains one nitrate anion and one m-anisidinium cation.

Figure 1. ORTEP view of C7H10NO.NO3, showing 50% probability ellipsoids
The structure consists of a protonated m-anisidine cation with a nitrate anion. No solvent or disorder was present in the structure. The cation is quite planar with a mean deviation from a least-squares plane formed by atoms N1, O1, C1-C7 of 0.0255 Å. All hydrogens except H1A-C were refined as riding atoms. H1A, H1B, and H1C were located in the electron density map, their x,y,z positions were refined, and their thermal parameters were treated as riding. The cations are arranged within the unit cell in layers located at b = 0 and ½. Nitrates are in layers between the cations at b =1/4 and ¾ ( Figure 2). While the nearly planar cations are parallel with nearest neighbor cations (3.63 Å interplanar spacing) , no  interaction is present. J u l y 31, 2 0 1 4

Figure 2. Two dimensional (2D) layer composed of nitrates anions and ammonium cations with hydrogen bonds as dotted lines
Organic cation and nitrate anion have normal valence bond lengths and angles (Table 3), as observed in the structures of 2-methoxyanilinium nitrate [7]. The hydrogen atoms of the protonated amine (H1A, H1B, H1C) undergo hydrogen bonding interactions with oxygen atoms of three different nitrate anions. O1 of the methoxy group does not participate as a hydrogen bond acceptor (Table 4). All hydrogen bonding occurs in two-dimensional sheets (ac planes) in the regions where cation layers meet the nitrate layers ( Figure 3).

Infrared Spectroscopy
The infrared spectrum of the crystalline nitrate is shown in Figure 4. To assign the IR peaks to vibrational modes, we examined the modes and frequencies observed in similar compounds [8].
The planar NO3 free ion (D3h) has six vibrational degrees of freedom. They form four fundamental modes: antisymmetric stretching (3) at approximately 1330 cm -1 (doubly degenerate, both IR and Raman active), symmetric stretching (1) at 1040 cm -1 (Raman active), an out-of-plane bending (2) at 800 cm -1 (IR active) and an in-plane bending (4) at 730 cm -1 (doubly degenerate, both IR and Raman active) [9]. Two bands in the infrared spectrum are observed at 835 and 820 cm -1 as very weak and weak, respectively. They originate from the bending type of vibrations of nitrate anions. The splitting of this mode may be ascribed to a lowering of symmetry of the NO3 ion from D3h to C2v or Cs.
The remaining observed bands in the spectrum can be assigned to CH3, CH, NH3 + , and skeletal symmetric and asymmetric stretching and deformation modes [10]. A broad band extending from 3395 to 2609 cm -1 is observed in the IR spectrum. This broad band must be due to the symmetric and asymmetric stretching modes of NH3, CH3 and bending. Bands in the 1300-1000 cm -1 are attributed to the stretching of the organic groups C-C), C-N) and is characteristic of 1,3-substituted benzene rings. The shifting of the stretching and bending vibrations of the NH3 group from the free state value confirms the formation of hydrogen bonds of varying strengths in the crystal. Frequencies in the range 1469 -1305 cm -1 are attributed to s(CH3), as(CH3), s(CH) and as(CH). The presence of C=C stretching vibrations of the aromatic ring is consistent with the absorption bands at 1608 and 1480 cm −1 .

UV-Vis Absorption and Diffuse Reflectance
The UV-Vis absorption spectrum revealed two bands at 276nm and 288nm. These bands correspond respectively to the π →π* transition of the aromatic ring and n →π* transition of the nitrate NO3 ⁻ anion. The optical diffuse reflectance spectrum ( Figure 5) indicates an optical band gap of 4.35 eV, hence, t h e t i t l e c o m p o un d is a wide band-gap dielectric material. A similar high gap energy value was observed in N,N'-diphenylguanidinium nitrate at 3.9eV [10].    Figure 7 illustrates the 1 H NMR spectrum of C7H10NO.NO3. The singlet at  3.75 ppm is assigned to the methoxy OC(7)H3 protons. The triplet at 7.36 ppm, the two doublets of doublets of doublets at 6.97 ppm and at 6.93 ppm and the triplet at 6.89 ppm are attributed to H2, H4, H6 and H5, respectively. H2, H4 and H6 are meta magnetically coupled to each other. H4 is also ortho coupled to H5. Likewise, coupling is present between H6 and H5, where H6 is located at the ortho position relative to H5. The NH3 protons do not appear as they have likely exchanged with D of the D2O. J u l y 31, 2 0 1 4

Conclusion
A non-centrosymmetric material, C7H10NO.NO3, with Cc space group symmetry has been prepared and structurally characterized. The atomic arrangement of this nitrate consists of a three-dimensional network of R