A new semi-organic crystal

A new semi-organic crystal, with nonlinear optical (NLO) efficiency, namely, 2-Nitroanilinium nitrate, (C6H7NO2+.NO3-), was successfully grown by slow evaporation solution growth technique under ambient temperature. The crystal and molecular structures of the specimen are determined from single crystal X-ray diffraction analyses which shows that the compound crystallized in the orthorhombic non-centrosymmetric space group (Pmn21). The crystal packing is dominated by N–H•••O hydrogen bonds forming an infinite chain C22(6) motif running along the a-axis of the unit cell. A bifurcated N–H•••O hydrogen bond is observed between the cation and anion leading to ring R12(4) motifs. The combination of these primary motifs form a secondary ring R66(18) motifs. The intermolecular interactions of the compound was further studied through Hirshfeld surfaces. Further, the molecular structures are optimized by the quantum chemical methods. The optimized geometrical parameters of the present compounds and computed anharmonic fundamental frequencies are found in satisfactory agreement with the experimentally measured FT-IR and FT-Raman spectral data. The computed first order hyperpolarizability values suggest the possible nonlinear optical (NLO) activity of the molecule. In turn, it is assured through the experimental SHG test of the material. The Natural Bond Orbital (NBO) analyses are carried out to interpret hyperconjucative interaction and Intramolecular Charge Transfer (ICT). HOMO and LUMO energies and the corresponding band gap were also computed.
Keywords: Single Crystal XRD, FT-IR and FT- Raman, quantum chemical studies, HOMO, LUMO

1. Introduction
Nonlinear optics (NLO) is a new frontier area of science and technology which play a major role in the emerging technology of photonics. Nonlinear optical processes are provided the key functions of frequency conversion and optical switching 1-3. Efficient nonlinear optical (NLO) crystals are required for laser devices of optoelectronics, signal processing and instrumentation 4-7. High nonlinear optical materials have potential applications in signal transmission, data storage, laser printing, displays, fluorescence, photolithography, remote sensing, chemical and biological species detection, high resolution spectroscopy, medical diagnosis and underwater monitoring and communications 8-10. Though the NLO property, especially the SHG efficiency, is a material level phenomena, it arises only from the molecular level asymmetry, which possess higher molecular hyperpolarizability values. Hence, it is a meaningful effort to identify the molecules with high hyperpolarizability and to crystallize them in non-centrosymmetric lattice.
In this perspective, many organic and inorganic molecules were identified. Researchers around the world are attempting to crystallize such as organic, inorganic, semiorganic, organometallic and polymeric materials expecting good NLO activity. In which, the organic materials receive the maximum attention because they are found to exhibit very high NLO response over a wide frequency range due to the presence of active ? bonds. In addition to the above properties, these materials possess large structural diversity, inherent synthetic flexibility, large optical damage threshold for laser power, low frequency dispersion, ultrafast response, photo stability and permit to modify the chemical structures and properties to achieve the desired NLO properties 11-13. Though the organic materials possess good optical nonlinear optical property than the inorganic materials, they are thermally unstable and exhibit a low laser damage threshold 14-16. Semiorganic materials, which are formed through organic and inorganic hybrids, have higher damage threshold, higher chemical stability, wide transparency range, less deliquescence, an excellent nonlinear optical coefficient, low angular sensitivity and exceptional mechanical properties 17, 18.
Aniline, an organic compound contains both amine and carboxyl acid groups, is a well known structure extension molecule through its both donor and acceptor groups. It is an organic synthon known for development of supramolecular system through its classical hydrogen bonding network. Such kind of compounds play a very important role in designing organic materials for molecular electronics. The higher first order hyperpolarizabilities of nitro substituted aniline molecules have led to describe them as prototypes for second harmonic generation. The present work is attempted to synthesize a new semi-organic NLO material from 2-nitroanline (2NA), namely, 2-Nitroanilinium nitrate (2NANO) and crystallize it in non-centrosymmetric phase.
The single crystal XRD studies are used to determine the molecular structure and the crystal packing specificity of the molecule in the solid crystalline state. The strength of the intermolecular interactions and its influence in the shifting of wavenumbers are studied through spectral measurements. As quantum chemical calculations are excellent alternative methods in the design of NLO molecules and help to predict some properties of the new materials, such as molecular dipole moments, hyperpolarizabilities, etc., the molecular assembly is optimized by quantum chemical methods and the different vibrational modes are calculated. The bond analyses, band gap values and first order hyperpolarizability values are calculated by this analysis. Additionally, the chemical hardness, chemical potential and electronegativity are predicted by Frontier Molecular Orbital (FMO) studies.
2 Experimental
2.1 Preparation
Single crystals of the compound, 2NANO, were achieved through the slow evaporation solution growth technique at room temperature from an ethanolic solution of 2-nitroaniline and nitric acid with a 1:1 stoichiometric ratio. Within a week, good quality single crystals were formed due to the spontaneous nucleation. The quality of the crystals suitable for single crystal X-ray diffraction is achieved through recrystallization process.
2.2. Density Measurement
The density of the grown crystal was calculated through sink and swim method (flotation technique) with the liquid mixture of carbon tetrachloride and bromoform. Initially, carbon tetrachloride of 10 ml was taken in a test tube and a good quality of three dimensional crystal is placed on it. Due to the more density than the liquid, the crystal started to sink. After that, bromoform was added drop-by-drop with continuous agitation to get uniform density over the liquid. When the density of the crystal matches with the liquid, the crystal start to levitate on the middle of the test tube. Then, the density of the liquid was found with specific gravity bottle from the concept of relative density. Thus, the density of the crystal is determined to be 1.62 (1) Mg.m-3.

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2.3. Single crystal XRD analyses
The complete crystallographic work were performed with Bruker SMART APEX CCD area detector diffractometer (graphite- monochromated, MoK? = 0.71073 Å). The cell refinement and data reduction are completed by SAINT programme 27. The structure of the present compound was solved and refined with the SHELX-2014 programme 28. The Reliability index (R-factor) for F2 > 2?(F2) is found to be 3.75%, which confirms the convergence of the reliable structure. Crystallographic information, details of data collection and refinement statistics were given in Table 1. All the non-H atoms are refined anisotropically. All the H atoms attached to the carbon, they are positioned geometrically and refined using a riding model, with C?H = 0.93 Å and Uiso(H) = 1.2 Ueq(parent atom). The H atoms attached to the nitrogen , which are located from difference Fourier map and refined, isotropically.
2.4. Vibrational spectroscopic measurements
A Jasco FT-IR spectrometer, model 410 under a resolution of ~1 cm-1 with a scanning speed of 2 mm/sec was used for. IR spectral measurements. The samples are prepared using pellet technique and the spectra recorded over the range of 4000–400 cm-1. The FT-Raman spectrum of the compound is measured in the frequency range of 50-4000 cm-1 using a FT-Raman Spectrometer with the BRUKER RFS 27 module. The Nd: YAG Laser source has operated at 1064 nm with the resolution of 2 cm-1.
2.5 SHG powder test
As 2NANO was crystallized in a non-centrosymmetric space group and it has shown possible NLO activity in theoretical computation. A preliminary study of the powder SHG conversion efficiency has been carried out through Kurtz and Perry method 29. A Q-switched Nd:YAG laser beam of 1064 nm wavelength has used with an input power of 4.4 mJ, pulse width of 10 ns and the repetition rate of 10 Hz. The crystals of 2NANO are grounded to a uniform particle size of about 125–150 ?m and packed in capillaries of uniform bore and exposed to the laser radiation. Powder of potassium dihydrogen orthophosphate (KDP), with the same particle size, has used as the reference. The output from the sample monochromated is to collect only the second harmonic (? = 532 nm) wavelength eliminating the fundamental and the intensity is measured using a photomultiplier tube. Second harmonic signal of 5.58 mV has obtained.

2.6 Computational details
The molecular geometries and electronic structure of 2NANO are optimized theoretically by the 6-311++G(d,p) method on a Intel Core i5/ 3.20 GHz computer using Gaussian 09W 30 program package without any constraint on the geometry optimization 31. Initial geometry has taken from the single crystal X-ray studies and it is minimized (optimized) by Hartee?Fock (HF) method using the 6-311++G(d,p) basis set. Further, the molecular geometries also have been optimized by the Density Functional Theory (DFT) using the Becke three – parameters exchange functional (B3) 32 in combination with the Lee – Yang – Parr correlation functional (LYP) 33. It is proved to be a cost-effective method for the computation of optimized molecular structure and vibrational frequencies. Structural parameters of the optimized 2NANO are utilized in the vibrational frequency calculations at the same level to characterize all stationary points as minima. Then vibrationally averaged nuclear positions of the structure have been used for harmonic vibrational frequency calculations in IR and Raman frequencies. Finally, the computed molecular coordinates give thermodynamic properties by the principle of statistical mechanics. The computed spectra are assigned with highest degree of accuracy by combing GAUSSVIEW program 34 and symmetry considerations. There is always some ambiguity in defining internal coordination. However, the assigned coordinate matches well with the molecular vibrations seen through the GAUSSVIEW program. The natural Bonding orbitals (NBO) are computed using HF and DFT/B3LYP method with the 6-311++G(d,p) basis set to get more detailed information about the chemical bonds of the molecule. Also, the frontier molecular orbitals (FMO) are calculated and investigated by HF and DFT/B3LYP method with the same basis set.
3. Results and discussion
3.1. Molecular Geometry
The molecular structure of 2NANO consists of 2-nitroanilinium cation and a nitrate anion. It is crystallized in orthorhombic crystal system with non-centrosymmetric Pmn21 space group. All the atoms of the asymmetric unit, except one of the H atoms of the -NH3 group in cation and one of the O atoms of the nitrate anion, are situated in the mirror plane. Nitrogen site of the 2-nitroaniline is protonated which is confirmed by the elongated C–N bond distance. A characteristic feature of 2-nitroaniline, the twisting of the nitro group from the benzene plane, is not observed in the present structure due to the mirror plane symmetry which is reiterated from the planar torsional angles (Table 2). The planes of the cation (includes all non-hydrogen atom) and anions are oriented almost perpendicular to each other with the dihedral angle of 89.2(3)?. This perpendicular orientation of the ion moieties may responsible for the large first order hyperpolarizability of the molecule.
The optimized structures of 2NANO along with ORTEP diagram are shown in Fig. 1. The optimized bond lengths, bond angles and torsion angles of non-hydrogen atoms are listed in Table 2. From which, it is observed that there is a slight deviation in the calculated bond geometries are compared with the experimental values. Factually, the molecular optimization and their calculations are carried out on the chemical species in free gaseous state. However, the experimental results correspond to molecule in solid crystalline state.
The optimized bond length of C–C in benzene ring are falling in the range of 1.371–1.399 Å in HF and 1.382–1.413 Å in DFT, whereas the single crystal XRD shows that these bond distances are in the range of 1.367 (7) –1.400 (5) Å. Five of N-O bond distances of the ions are varying from 1.167–1.320 Å in HF and 1.199–1.384 Å in DFT levels. The experimental N–O bond distances are observed to be in the range of 1.208(5)–1.243(3) Å. The optimized C–N bond length of the nitro group is observed as 1.459 and 1.467 Å in HF and DFT levels respectively. The corresponding experimental bond length is 1.462 (5) Å. These results show that the theoretical result is in agreement with the experimental value. For C-N bond distance of substituted amino group is observed to be 1.395 and 1.399 Å at HF and DFT levels respectively, with the experimental value of 1.457(4) Å. The close proximity of amino N and nitrate O atoms in the theoretical and experimental work confirms the non-covalent interaction between the ions pairs.
3.2. Hydrogen Bonding features and crystal packing
The crystal structure of 2NANO is stabilized through a three dimensional hydrogen bonding network formed by N–H•••O and C–H•••O hydrogen bonds as listed in Table 3. The ions are aggregated parallel to the bc-plane of the unit cell. The N–H•••O intermolecular hydrogen bonds form two infinite chain C22(6) motifs running along the a-axis of the unit cell with an alternate chain C21(4) motifs. Further bifurcated hydrogen bonds are observed between cations and anions leading to ring R12(4) motifs. These two primary motifs are connected to form ring R66(18) motif stacked along the a-axis of the unit cell which are adjacented by ring R64(12) motif (Fig. 2). These ring and chain motifs has lead to the hydrophilic layers at y = 1/2 which are sandwiched between the two alternate hydrophobic layers at y = 1/4 and 3/4 (Fig. 3). Interestingly, an intermolecular C–H•••O interaction is observed in the crystal packing which supports the molecular assembly especially in the hydrophobic regions. This excessive hydrogen bonding features and the special position of the atoms are similar to the crystal of the structures of well known NLO materials KDP and ADP 35.
3.3 Hirshfeld surface analysis
In the present study, the identification of the intermolecular interactions by Hirshfeld surfaces and the corresponding finger print plot analysis in molecular crystals. The Hirshfeld surfaces are created and the corresponding graphs are plotted using the Crystal Explorer 3.1 program 36-38. Initially, the input for this analysis has been taken from the from single crystal XRD. The classical hydrogen bonding interactions are well reflected in the shape of dnorm surface and the corresponding fingerprint plots (Figs. 4 and 5). The Hirshfeld surfaces show almost similar proportions of O…H/H…O and N…H/H…N contacts which contributed in the crystal structure. The obtained plots are used to describe various intermolecular interactions including O???H, H???H, C???H, C???C and other contacts present in crystal structures. The fingerprint plot of 2NANO showed that the O???H/H???O interactions are contributed with the percentage of 29.1 % which are characterized by ‘spikes’ in the fingerprint plot. The interactions developed on oxygen atoms represent the closest contacts in this structure. The relative contributions for C•••N/N•••C contacts are 30.4%, which shows that the this intermolecular interaction provides the support for molecular assembly.
3.4. Mulliken charge analysis
Generally, Mulliken atomic charge calculation gives many clues about the packing of the molecules in the solid crystalline state and the charges have significant influence on dipole moment, polarizability, electronic structure and vibrational modes 39. The Mulliken charges of the 2NANO were calculated at HF and DFT/B3LYP levels and given in Table 4. The corresponding graphical plots are shown in Fig. 6. The charge distribution of 2-nitroaniline cation shows that the five carbon atoms of the ring (C1, C7, C12, C8 and C10) are negative while the remaining one carbon atom is positive. This is due to the influence of their substituent. Generally, carbon bonded with the electronegative atoms shows positive charge. The carbon C12 atom has more electronegativity than the others and this carbon atom is participating in intermolecular C–H•••O interaction in the crystal packing supporting the molecular aggregation especially in hydrophilic region. This proves the delocalization of charges. The nitrogen N2 (-0.535 e in HF and -0.471 e in B3LYP) atom has more electronegative in the cation moiety which is also due to the strong intermolecular interactions.
All the hydrogen atoms have electropositive charge, out of them hydrogen H15 (0.477 e in HF and 0.438 e in B3LYP) atom is more electropositive than the others. This hydrogen atom connects the ion pairs through the N–H•••O intermolecular hydrogen bond. Though the quantum chemical calculations prefer the lowest energy for the molecules to be in neutral-neutral pair than the cation-anion pair, this excessive positive nature of the hydrogen confirms the intermolecular interaction between the moieties through the N and O atoms. All oxygen atoms in the nitrate anion are electronegative, which are involved in bifurcated hydrogen bonds as acceptor forming ring R12(4) motifs.
3.5. Vibrational spectroscopy analysis
Vibrational spectroscopy gives valuable information regarding the molecular structure, symmetry, bond length, inter and intramolecular interactions, hydrogen bonding, crystalline field effect etc. The present study is to investigate the shifting of vibrational bands due to the hydrogen bonds in solid crystalline environments. The molecular structure of 2NANO has various functional groups such as –NH3+, -CH, -NO2, -C-N, -C-C-, disubstituted phenyl ring, etc. The functional groups are normally observed to undergo variations in their intensity and position due to their environments 40. A complete vibrational analysis of the 57 fundamental vibrational modes of 2NANO were carried out along with quantum chemical calculations and it is compared with the experimental infrared and Raman spectra. The computed vibrational wavenumbers, measured FT-IR and FT-Raman bands are listed in Table 5 and the corresponding spectra are presented in Figs 7 and 8.
3.5.1 Vibrations of the -NH3+ group
Normally the -NH3+ stretching vibrational modes are occurring in the range of 3300-2600 cm-1. The asymmetric and symmetric bending modes are expected to appear in the band of 1625–1560 and 1550–1505 cm?1, respectively 41. In the title compound, the -NH3+ asymmetric and symmetric stretching vibrations are appeared as medium band at 3482 and 3353 cm-1 in IR. In quantum chemical calculations, these stretching bands are splitting into -NH2 vibration of the NH3+ species which is due to the deviation one of the hydrogen atoms from the N site. This hydrogen atom is located between two electronegative atoms and result with the very strong intensity peak for the N?H…O band in theoretical computation.
The scissoring vibration of the amine group appears in the range at 1630–1610 cm-1. In the present case, the bands are observed as medium and strong bands at 1621 and 1581 cm-1 in IR and 1616 and 1579 cm-1 in Raman, respectively. These corresponding vibrations are calculated at 1829/1742 cm-1 at HF level and 1657/1563 cm-1 in DFT/B3LYP method. The slight deviation between the results are due to the mixing of C=C stretching vibrations. The NH2 twisting vibration is observed as medium band at 1166 cm-1 in IR spectrum and weak band at 1155 cm-1 in Raman spectrum. In the lower wavenumber region, most of the bands are assigned to the NH3+ rocking and twisting vibrations which are in good agreement with the theoretical results.
The effect of hydrogen bonding has important role in the absorption spectra which may cause the downshifting of stretching mode of vibrations and up shifting of deformation modes. The bond length for strong, normal and weak hydrogen bonds are 2.4–2.7 Å, 2.7 – 2.9 Å and above 2.9 Å, respectively 42. The present compound is stabilized by the strong N–H•••O intermolecular hydrogen bonds in the solid crystalline environment. This is replicated in the shifting of -NH3+ bands in the vibrational spectra. The broad peak centered at 2922 cm-1 in IR and strong peak at 3079 cm-1 in Raman is assigned to the N–H•••O intermolecular hydrogen bond vibration. This same vibration is appeared at 3333 and 3107 cm-1 in HF and DFT/B3LYP methods, respectively.
3.5.2 C-H and C=C vibrations
Most of the aromatic compounds have several peaks in the region of 3100-3000 cm-1 and they are arising due to the stretching vibrations of the ring C-H bonds 43. A shoulder peak observed at 3164 cm-1 in IR is assigned to the symmetric C-H stretching vibrations. For, the C-H in plane bending vibrations, the peaks are identified as weak bands at 1077 and 1114 cm-1 in IR and medium bands at 1118 and 1170 cm-1 in Raman. Generally, the C-H out-of-plane bending vibrations are observed in the region at 1100–675 cm-1 44. In the present case, the medium peak appeared at 1037 cm-1 in IR and strong peak at 1039 cm-1 in Raman are assigned to the C-H out-of-plane vibrations. The C=C stretching vibration is observed as a medium band at 1621 cm-1 in IR and 1616 cm-1 in Raman, are in agreement with theoretical values.
3.5.3 Vibrations of nitro (-NO2) and nitrate (NO3-) groups
The free state of the nitrate anion has D3h symmetry and its normal modes of vibrations are A1′ (?1), A2″(?2) and E’ (?3 and ?4). In which, ?1 is Raman active, ?2 is IR active, ?3 and ?4 are both IR and Raman active. Normally, the symmetric stretching vibration ?1 occurs at 1049 cm-1 and symmetric bending (?2) at 530 cm-1, while ?3 and ?4 asymmetric stretching and bending modes are occurring at 1355 and 690 cm-1 45, 46.
Here, the asymmetric NO3- stretching vibration is observed as medium peak at 1758 cm-1 in IR spectrum and Raman inactive. The same stretching vibration is predicted at 1929 cm-1 at HF level and 1733 cm-1 in DFT/B3LYP methods. The asymmetric stretching vibration of NO2 is observed at 1581 cm-1 in IR and 1579 cm-1 in Raman and calculated at 1742 cm-1 at HF level and 1563 cm-1 in DFT/B3LYP methods. The band observed at 1343 cm-1 in IR and 1342 cm-1 in Raman are assigned to the symmetric NO3 stretching vibration. The deformation modes of these groups (rocking, wagging, scissoring and twisting) have contributed to several normal modes in the lower frequency region.
3.6 Natural Bond Orbital (NBO) analysis
The NBO analysis gives the information about the interactions in both filled and virtual orbital spaces that could enhance the intra and intermolecular interactions. The second order Fock matrix is carried out to evaluate the donor–acceptor interactions in the NBO analysis 47-49. This interaction is a loss of occupancy from the localized NBO of the idealized Lewis structure into an empty non?Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i ? j is estimated as
E(2) = qi
where qi is the donor orbital occupancy, ?i and ?j are the orbital energies and F(i,j) is an off diagonal NBO Fock matrix element.
NBO analysis of the molecules have been carried out by the HF and DFT/B3LYP methods with 6-311++G(d,p) basis set, in order to explain the intramolecular interaction, re-hybridization and delocalization of electron density within the molecules. The most significant element of the NBO analysis is to study the charge transfer between lone pairs of proton acceptor and anti-bonds of proton donor. In present molecular structure, intramolecular charge transfers (ICT) are observed around the C–C bonds of the aromatic ring. The C-C interactions between bonding and anti-bonding have large stabilization energy due to the hyper conjugative interactions of ? to ?*. This intramolecular hyperconjugative interactions between (C2-C10) and (C3-C4), (C6-C8) and (N13-O15) lead to stabilization energies of 46.56, 31.25 and 31.87 kJ/mol at HF level and 22.25, 23.31 and 16.96 J/mol in DFT/B3LYP methods, respectively. The complete NBO energies are listed in Table S1.
3.7 Nonlinear Optical studies
The Non-linear optical (NLO) materials are studied for the frequency shifting, optical modulation, optical switching and optical memory in the technologies, like telecommunications, signal processing and optical interconnections 50.
The quantum chemical calculation is an effective method to investigate the NLO properties of an organic molecule which may get crystallize in non-centric lattice. Based on the finite–field approach, the NLO parameters such as electronic dipole moment (?), polarizability (?) and the first order hyperpolarizability (?0) of the present compound are calculated by HF and DFT/B3LYP methods (Table 6). The dipole moment (?), polarizability (?), mean polarizability (??) and the average value of the first order hyperpolarizability ( ) are calculated using the following equation,
? =
? = ( + + )
?? = ( – )2 + ( – )2 +( – )2 + + +
2 2 2
The dipole moment and first order hyper polarizability are calculated as 6.732 / 6.345 Debye and 2.436×10-30 / 10.121×10-30 esu by HF / DFT methods, respectively. The first order hyperpolarizability values computed at HF and DFT/B3LYP methods are found to be 3 and 12 times greater than the urea, respectively (? and ?0 of urea are 1.613 Debye and 0.808×10-30esu). The first order hyper polarizability of the present molecule is appreciably matches with the 2-nitroaniline and 4-methoxy-2-nitroaniline 51. The existence of large first order hyperpolarizability value is due to more number of intramolecular charge transfers around the aromatic ring. It results the electron cloud movement through ? conjugated frame work in donor and acceptor level. The larger dipole moment of the compound is due to the delocalization of oxygen lone pair of electrons into the nitrate group. This high degree of electronic charge delocalization has occurred along the charge transfer axis by the low band gaps. This result shows that the 2NANO is a good candidate for NLO application and it was confirmed by the experimental SHG results.
3.8 Frontier Molecular Orbitals (FMO) analysis
The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) are called as frontier molecular orbitals. This orbital’s (HOMO and LUMO) find out the way in which the molecule interacts with other species. The frontier orbital gap facilitates to characterize the chemical reactivity , kinetic stability , chemical hardness, electro negativity and chemical potential of the molecule 52. Energy difference between HOMO and LUMO is called as frontier energy gap which is perceived to be an important for the stability of the structure. The HOMO and LUMO energies of the compound are calculated and listed in Table 7. A molecule with a small frontier orbital gap is more polarizable and is usually related with a high chemical reactivity, low kinetic stability and is also termed as soft molecule. Highest occupied molecular orbital (HOMO) is directly related to the ionization potential, while energy of the lowest unoccupied molecular orbital (LUMO) is directly related to the electron affinity. The calculated energy values of LUMO and HOMO are 0.030 and -0.353 au in HF level and -0.119 and -0.271 au in DFT methods respectively. The molecular orbital energy gap value of 2NANO is 0.383 au at HF level and 0.152 au in DFT/B3LYP methods. This lower energy gap can be associated with the high chemical reactivity and the possible NLO activity of the molecule
4. Conclusion
A needle shaped transparent non-centrosymmetric crystals of new semi organic compound, 2-nitroanilinium nitrate, are obtained by slow evaporation method. The crystallographic investigations reveal that the asymmetric part of the unit cell contains half of a 2-nitroanilinium cation and half of a nitrate anion. The crystal packing features intricate three dimensional hydrogen bonding network formed by the classical N–H•••O hydrogen bonds. It forms two infinite chain C22(6) motifs running along the a-axis of the unit cell and bifurcated hydrogen bonded ring R12(4) motifs. These two primary motifs are inter-linked to each other to form a ring R66(18) motif stacked along the a-axis of the unit cell. Hirshfeld analyses reveals that the O???H/H???O interactions are contributed with the high percentage of 29.1 % which are characterized by ‘spikes’ in the fingerprint plot. The interactions developed on oxygen atoms represent the closest contacts in this structure. Further, the optimized molecular structure, vibrational frequencies, molecular properties, NBO analysis and computed spectra of 2-nitroanilinium nitrate are found out by quantum chemical calculations with HF and DFT/B3LYP methods invoking 6-311++G(d,p) basis sets. The computed spectra compared with experimental vibrational spectra. Computed geometrical parameters and harmonic frequencies of fundamental, combination and overtone modes are more reliable with the experimental data. The NLO property is confirmed using Nd:YAG laser of wavelength 1064 nm and the Second harmonic generation efficiency of the material is three times better than the standard organic NLO material urea. The calculated first-order hyperpolarizability values are 2.436×10-30 / 10.121×10-30 esu by HF / DFT methods, respectively , which are nearly 3 and 12 times greater than the urea, respectively. The lowering of HOMO and LUMO energy gap clearly reveals the charge transfer interactions taking place within the molecule. This clearly describes that in acid–base hybrid crystals, hydrogen bond plays an important role not only in the formation of crystal structure and its stability, but also in the enhancement of the hyperpolarizability (?) of the crystal.


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