Nonlinear optical (NLO) Materials play an important role in nonlinear optics, optical communication, optical switching, optical disk data storage, laser fusion reactions, optical rectifications and in particular they have a great impact on information technology and industrial applications [1-6]. The approach of combining the high nonlinear optical coefficient of the organic molecules with the excellent physical properties of the inorganics found to be extremely successful in the recent past. Thiourea, which is centrosymmetric, yields excellent non centrosymmetric materials. These materials are formed by combining organic molecules of high polarizability with thermally stable and mechanically robust inorganic molecules. These materials in addition to retaining high optical nonlinearities of organic molecules also possess favourable physical properties. An added advantage is that large single crystals can be grown from slow evaporation solution growth [7,8]. The literature survey reveals that, to the best of our knowledge, no intensive observation of spectroscopic [FT-IR and FT-Raman] and theoretical [HF/DFT] investigation has been reported so far.

BTMS crystal was synthesized by dissolving AR grade thiourea and AR grade magnesium sulphate in the molar ratio 2:1 in distilled water. The saturated solution of magnesium sulphate is slowly added to the saturated solution of thiourea. This is stirred well to get a clear solution. Pure BTMS crystal was synthesized according to the reaction:

2CS[NH₂]₂ + MgSO₄ → Mg[CS(NH₂ )₂]₂SO₄

The solution was purified by repeated filtration. The saturated solution was kept in a beaker covered with polythene paper. For slow evaporation 6 or 7 holes are made in the polythene paper. Then the solution is left undisturbed in a constant temperature bath (CTB) kept at a temperature of 35°C with an accuracy of ± 0.1°C. As a result of slow evaporation, after 75 days colourless and transparent pure BTMS crystals were obtained (Figure 1).

Figure 1: Photograph of grown BTMS crystals.

The FT-IR spectrum of the compound is recorded in Bruker IFS 66V spectrometer in the range of 4000-100 cm^-1. The spectral resolution is ± 2 cm^-1. The FT-Raman spectrum of AMS is also recorded in the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source operating at 1.064 μm line widths with 200 mW power. The spectra are recorded in the range of 4000-100 cm^-1 with scanning speed of 30 cm^-1 min^-1 of spectral width 2 cm^-1. The frequencies of all sharp bands are accurate to ± 1 cm^-1. The transmission spectrum of BTMS was recorded using Varion Cary 5E UV-Vis-NIR Spectro photometer. The Thermogravimetric analysis and Differential thermal analysis (TGA and DTA) curves for BTMS were obtained using Simultaneous Thermogravimetric Analyser (STA) 409 C (NET- ZSCH) made in Germany at a heating rate of 10 °C/min in Nitrogen. The single crystals of BTMS have been subjected to X-ray diffraction studies using an ENRAF NONIUS CAD4 X-ray diffractometer.

In the present work, some of the hybrid methods such as HF and DFT, were carried out using the basis sets 6-311+G(d,p) and 6-311++G(d,p). All these calculations were performed using GAUSSIAN 09W program package on Pentium core i3 processor in personal computer. In DFT methods; Becke’s three parameter hybrids function combined with the Lee-Yang-Parr correlation function (B3LYP), Becke’s three parameter exact exchange-functions (B3) combined with gradient-corrected correlation functional of Lee, Yang and Parr (LYP). The optimized structural parameters are used in the vibrational frequency calculations at DFT (B3LYP,) levels. The minimum energy of geometrical structure is obtained by using level 6-31++G(d,p) and 6-311++G(d,p) basis sets. At the optimized geometry for the title molecule no imaginary frequency modes are obtained, so there is a true minimum on the potential energy surface is found. The calculated frequencies are scaled by 0.85, 0.88, 0.95, 0.98 and 1.25 for HF/6-31++/6-311++G(d,p), B3LYP/631++/6-311++G(d,p) and B3PW91/6-31++G(d,p) method basis set. The electronic properties, such as HOMO-LUMO energies, absorption wavelengths and oscillator strengths are calculated using B3LYP method of the time-dependent DFT (TD-DFT), based on the optimized structure in solvent (DMSO, chloroform and CCl₄) and gas phase. The thermodynamic properties of the title compound at different temperatures have been calculated in gas phase using B3LYP/6-311++G (d,p) method. The observed (FT-IR and FTRaman) and calculated vibrational frequencies and vibrational assignments are submitted in Table 1. Experimental and simulated spectra of IR and Raman are presented in the Figures 2 and 3, respectively.

Table 1: FT-IR and FT-Raman experimental and calculated (scaled) vibrational frequencies of Bis(thiourea) Magnesium Sulphate (BTMS).

Figure 2: Experimental [A] and calculated [B, C and D] FT-IR spectra of Bisthiourea magnesium sulphate.

Figure 3: Experimental [A] and calculated [B, C and D] FT-Raman spectra of Bisthiourea magnesium sulphate.

Molecular geometry

The molecular structure of BTMS belongs to CS point group symmetry. The optimized molecular structure of the molecule is obtained from Gaussian 09 and Gauss view program and is shown in Figure 4. The molecule contains two thiourea group connected with MgSO₄. The structure optimization and zero point vibrational energy of the compound in HF/DFT-B3LYP/6-31++G(d,p) and 6-311++G(d,p) are 96.88, 89.80 and 89.81 Kcal/Mol respectively. The comparative optimized structural parameters such as bond lengths, bond angles and dihedral angles are presented in Table 1.

Figure 4: Molecular structure of Bisthiourea Magnesium Sulphate (BTMS).

The molecular structure is optimized by Berny’s optimization algorithm using Gaussian 09 and Gauss view program and is shown in Figure 4. The comparative optimized structural parameters such as bond length, bond angle and dihedral angle are presented in Table 2. The present compound contains magnesium as a metal atom, sulphate atoms and four amino groups. The calculated energy of HF is greater than DFT method because the assumption of ground state energy in HF is greater than the true energy. Though, the molecular structure belongs to one plane, with respect to magnesium the thiourea on both sides are somewhat titled due to attraction between Mg and H.

Table 2: Optimized geometrical parameters for Bis(thiourea) Magnesium Sulphate (BTMS).

The experimental bond length of C-S and C-N are 1.720 and 1.340Å whereas the calculated bond lengths are 1.736 and 1.329 Å respectively. Though, both the amino groups are coupled with carbon symmetrically, the bond distance of C-N is differed by 0.016 Å between them due to the attraction of H by Mg. Normally, the double bond is to be there between C and S atoms, but one bond alone is there due to 2 lone pair of electrons are transferred from ligand to metal. The calculated bond length of Mg-S is 2.488 Å which is long and strong van der Waals bond. The internuclear distance of N5-H6 and N8-H9 are same and are 0.015 Å higher than other N-H bonds due to the existence of Mg and H attraction. From the optimized parameters, it is inferred that, the organo-metallic compound is very strong due to the complex bonds.

Vibrational assignments

The BTMS molecule consists of 22 atoms, which undergoes 60 normal modes of vibrations. On the assumption of CS group of symmetries, the numbers of vibration modes of the 60 fundamental vibrations of the molecule can be distributed as

Γ_Vib = 36 A′+24 A″

In accordance with the Cs group symmetry, all the 60 fundamental vibrations are to be active both in Raman and IR. However the intensity of certain peaks is so weak, they are found missing in the spectra, which can be verified from the intensity values calculated theoretically. The calculated (scaled) and experimental frequency values, for different methods and basis sets and the corresponding assignments are presented in the Table 1. The unscaled values of frequencies are presented in Table 3. The equivalent FT-IR and FT-Raman spectra were described in Figures 2 and 3 respectively. All the observed bands are assigned to different possible modes of vibrations based on the earlier works on structurally similar molecules, the characteristic frequencies of the functional groups, GAUSSVIEW program and the calculated IR intensity and Raman activity etc.

Table 3: Calculated unscaled frequencies of Bis (thiourea) Magnesium Sulphate (BTMS).

N-H vibrations: The molecule consists of couple of NH₂ groups on both sides; there is a possibility of eight N-H stretching vibrations. In this present case, the N-H stretching frequencies are observed at 3773, 3755, 3381, 3296, 3277, 3192 and 3179 cm^-1. All the bands are assigned to symmetric vibrations. In addition to that, first two vibrational bands are found to be moved high from the expected region. This is mainly due to the presence of sulphate with chain. The in-plane deformation vibrations for the present compound are observed at 1645, 1618, 1518, 1492, 1473, 1105, 1098 and 1083 cm^-1. The first two bands are moved up to the higher region and it is cleared that, these vibrations are favoured. The out-of-plane bending vibrations are set up at 960, 845, 715, 643, 625, 613, 590, 550, 545 and 520 cm^-1. Normally, whenever the metal atom coupled with the organic molecules, the normal vibrational modes of the same are suppressed much. From the N-H vibrations it is observed that, the entire out of plane vibrational modes are affected by other substitutions in the chain.

N=C=N vibrations: The symmetric N=C=N stretching vibrations occur in the region 2155-2130 cm^-1. The in the observed N=C=N stretching vibration was found at 2715, 2143, 2048 and1836 cm^-1 IR spectrum and is due to greater double bond character of carbon to nitrogen on the formation of Mg [TU]₂ SO₄ complex.

C-N vibrations: In this present work, the C-N Stretching vibrations are observed at 1061, 998, 746, and 730 cm^-1 which is making disagreement with the literature due to the loading of sulfur and metal atoms with the molecule. The C-NH₂ in-plane and out-of-plane bending vibrations are appeared at 510, 485, 230 and 210 & 200, 190, 155, 150 and 120 cm^-1 respectively. These two vibrations are affected much by other vibrations which make disagreement with literature values (33-34) [9].

C-S vibrations: In this metal organic compound, the in-plane bending vibrations are found at 470 and 440 cm^-1 and the out-of-plane bending vibrations at 180, 160, 140, 130, 110, 105 and 100 cm^-1. These vibrational bands are pulled down to the lower region of the expected range due to the metal ion.

S=O vibrations: The symmetric S=O stretching vibrations occur in the region 1450-1350 cm^-1. The -1 in the observed IR and S=O stretching stretching vibration was found at 1412 and 1393 cm Raman spectrum and is due to greater double bond character of sulfur to oxygen on the formation of Mg [TU]₂ SO₄ complex. This is mainly due to the presence of sulphate in the chain.

Mg-O-S vibrations: In BTMS, Mg-O-S stretching vibration was found at 435 and 425 cm^-1 in IR spectrum. The in plane deformation vibrations for the present compound is observed at 422 and 390 cm^{- 1}. The first two bands are moved up to the higher region and it is cleared that, these vibrations are favoured. The out of plane bending vibrations are set up at 340, 250 and 240 cm^-1. As all the IR bands have their counterparts in Raman suggests that the title crystal is non-centro symmetric.

Mulliken charge distribution analysis

The charge distribution on the molecule has an important influence on the vibrational spectra. The Mulliken charge levels of the molecule with different interactions were shown in Figure 5. The charge population was indicated in Table 4. Here, the negative charges are accumulated over the N atoms in thiourea even after the magnesium and sulphate ions are added. When the highly electronegative and positive atoms are coupled to formed dipoles, the remaining C and H of the molecule have positive space. Since the addition of magnesium sulphate have been occurred in thiourea, the sulphur atoms become low order negative that is almost neutral. Simultaneously, the chemical property has also changed for the same. This is the main cause for the compound was being optically active.

Table 4: Mulliken atomic charge distribution of BTMS.

Figure 5: Mulliken charge (HF&DFT) of BTMS.

Single crystal XRD analysis

The lattice dimensions and the crystal system have been determined from the single X-ray diffraction analysis (Model: ENRAF NONIUS CAD 4). The determined unit cell parameters and the observed crystal system are reported in the Table 5.

Table 5: Lattice parameters of TU, BTCC, BTZC and BTMS.

UV-Vis-NIR analysis

The Optical transmission spectra of BTMS was recorded using Varion Cary 5E UV– Vis–NIR spectro photometer in the range 200- 2000 nm with high resolution and is shown in Figure 6. The crystal has a good transmission in the entire visible region. The low absorbance behaviour in the entire visible range also confirms colourless nature of the crystal. In the UV-Vis-NIR spectrum, the lower cut off is found near 288 nm, which is an advantage in semi organic nonlinear optical materials over their inorganic counterparts [10].

Figure 6: Transmittance and absorbance spectrum of BTMS.

Second Harmonic Generation (SHG) analysis

The second harmonic generation test was carried out by classical powder method developed by Kurtz and Perry. It is an important and popular tool to evaluate the conversion efficiency of NLO materials. The fundamental beam of 1064nm from Q switched Nd: YAG laser was used to test the second harmonic generation (SHG) property of pure BTMS crystals. Pulse energy 2.9mJ/pulse and pulse width 8ns with a repetition rate of 10Hz were used. The photo multiplier tube (Hamahatsu R2059) was used as detector and 90 degree geometry was employed. The input laser beam was passed through an IR detector and then directed on the microcrystalline powdered sample packed in a capillary tube. The SHG signal generated in the sample was confirmed from the emission of green light from the sample [11]. Potassium dihydrogen orthophosphate (KDP) crushed into samples of identical size is used as reference material. The output of laser beam having the bright green emission of wavelength 532 nm confirms the second harmonic generation output as shown in Figure 7.

Figure 7: SHG graph of BTMS.

TGA/DTA analysis

The Thermogravimetric analysis and Differenial thermal analysis (TGA and DTA) curves for BTMS were obtained using simultaneous thermogravimetric analyser (STA) at 409 °C (NET- ZSCH) made in Germany at a heating rate of 10 ºC/min in Nitrogen were shown in Figure 8. The TGA curves shows that there was a weight loss of about 82.57% in the temperature range 243-289 °C. The weight loss in this range may be due to decomposition of Thiourea present in BTMS. The increase in decomposition temperature when compared to the decomposition temperature of thiourea which is 182 °C may be due to the formation of metal complex. This revealed the thermal stability of BTMS [12].

Figure 8: TGA/DTA curve for BTMS.

NMR analysis

The NMR spectral data was calculated at B3LYP method with 6-311++G(d,p) level on the basis of GIAO method and the chemical shifts of the compound are reported in ppm relative to TMS for ¹H and ¹³C NMR spectra which were presented in Table 6 and the corresponding spectra were shown in Figure 9. In the case of BTMS, the chemical shift of C3 and C4 are 190.34 and 197.05 ppm respectively. The chemical shift is same for C3 and C4 since both the carbons having similar groups. The chemical shift of both carbons is very high; it is also due to the migration of double bond from C-S to C-N. The chemical shift of H6, H7, H9, H10, H12, H13, H15 and H16 are 27.69, 26.93, 27.25, 26.48, 27.57, 27.28, 26.05 and 26.40 ppm respectively. From the entire chemical shift of the molecules it can be inferred that, the chemical property of the metal is directly mingled with organic molecules and this is the main cause for the present metal complex molecule having new chemical property.

Table 6: Calculated ¹H and ¹³C NMR spectral data of BTMS.

Figure 9: Simulated ¹³C and ¹H NMR spectra of BTMS.

Frontier molecular (electronic properties) analysis

The 3D diagram of the frontier molecular orbitals, in IR and UVVisible region for title compound were displayed in Figures 10 and 11 respectively. According to such Figure, the HOMO is mainly localized over the magnesium, sulphate, N atoms and C-S group in which there are two sigma bond interaction taking place over the C-S of thiourea and one delta bond interaction over magnesium sulphate. From this observation, it is clear that, the in and out of phase interactions are present in HOMO and LUMO respectively. The electron transitions were happened between those HOMO and LUMO and thus the compound has been stabilized. During that time, the analogous physical and chemical properties mutually shared between the thiourea and magnesium sulphate. The charge levels were fluctuated due to the electron cloud sharing and were evidenced in the Mulliken charge analysis. The band gap of the present compound was found to be 5.61 eV.

Figure 10: Frontier molecular orbital in IR region.

Figure 11: Frontier molecule orbital lobe formation.

Optical properties analysis

The electronic energy excitation of the present molecule were calculated at the B3LYP/6-311++G(d,p) level using the TD-DFT approach in gas phase and with the solvent of DMSO, Chloroform and CCl₄. The calculated excitation energies, oscillator strength (f) and wavelength (l) and spectral assignments are given in Table 7. The range of UV-Visible from 200 to 380 nm is the portion of the spectrum normally covered by the term ultraviolet. The compound under study is a semiconductor which was designed for the purpose of optoelectronic. In the solvent phases, there was no considerable disparity found. The molecular orbital lobe interaction in UV-Visible is appeared in Figure 11.

Table 7: Theoretical electronic absorption spectra of BTMS (absorption wavelength λ (nm), excitation energies E (eV) and oscillator strengths (f)) using TD-DFT/B3LYP/6-311++G(d,p) method.

Molecular Electrostatic Potential (MEP) analysis

The colour code of these maps is in the range between -7.86 au (deepest red) to 7.86 au (deepest blue) in compound. The positive (blue) regions of MEP are related to electrophilic reactivity and the negative (green) regions to nucleophilic reactivity shown in Figure 12. As can be seen from the MEP map of the title molecule, the negative regions are mainly localized on nickel chloride and sulphur atoms. A maximum positive region is localized on the H of NH₂ groups indicating a possible site for nucleophilic attack. Though, this molecule contains different electron rich atoms, the negative potential regions located at metal combined atoms. From these results, it is found that, the metal atoms coupled strongly in the inertial position of organic lattice site.

Figure 12: MEP and ESP display of BMTS.

Chemical properties

Usually, the chemical applications can be identified from the Chemical properties of the compounds. It is useful for recognizing the substance for the particular purpose. It was very important to know whether this compound is able to have rich potential for doping and starting material for semiconductor application. Accordingly, the chemical hardness and potential, electronegativity and Electrophilicity index were calculated and presented in Table 8. The dipole moment is another important electronic property of the chemical compound which is the ability of electrical and optical polarization of a substance. The large dipole moment leads the compound; very strong intermolecular interactions. The calculated dipole moment value for the present compound was 4.89 Debye. It was comparatively very high and the present molecule was being highly polar where the strong intermolecular interactions induced generally.

Table 8: Chemical parameters of BTMS.

The FT-IR and FT-Raman spectra were recorded and the detailed vibrational assignments using HF and DFT methods with 6-31++G(d,p) and 6-311++G(d,p) basis sets were made for BTMS. The difference between the corresponding wave numbers (observed and calculated) is very small for most of fundamentals. Therefore, the results presented in this work for BTMS indicate that this level of theory is reliable for the prediction of both infrared and Raman spectra of the title compound. The equilibrium geometries of molecule have been determined and compared with X-ray analytical data. The optimization has been done in order to investigate the energetic behaviour and dipole moment of title compound in the gas phase and solvent. The electronic excitation in UV-VIS spectra was analysed. A reduction in absorption at Nd: YAG fundamental (1064 nm) observed in the optical transmission spectrum of BTMS recommends the crystal for nonlinear optical applications. The property of the second harmonic generation of the experimental crystal confirms the nonlinear nature of the crystal. The TGA/DTA curve recorded for the crystal confirmed its thermal stability.