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Vibrational Spectroscopic Investigation on Bisthiourea Magnesium Sulphate (BTMS) Using Experimental and Computational [HF and DFT] Analysis

Durga R1, Anand S1*, Sundararajan RS2 and Ramachandraraja C2

1Department of Physics, AVC College, Mayiladuthurai, Tamil Nadu, India

2Department of Physics, Government Arts College, Kumbakonam, Tamil Nadu, India

*Corresponding Author:
Anand S
Department of Physics, AVC College
Mayiladuthurai, Tamil Nadu, India
Tel: +919443650530
E-mail: [email protected]

Received date: February 17, 2017; Accepted date: March 29, 2017; Published date: April 05, 2017

Citation: Durga R, Anand S, Sundararajan RS, Ramachandraraja C (2017) Vibrational Spectroscopic Investigation on Bisthiourea Magnesium Sulphate (BTMS) Using Experimental and Computational [HF and DFT] Analysis. J Theor Comput Sci 4:153. doi:10.4172/2376-130X.1000153

Copyright: © 2017 Durga R, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Nonlinear optical bisthiourea mixed magnesium sulphate BTMS crystals were synthesized and grown by slow evaporation method using water as solvent. Vibrational spectra were recorded to determine the symmetries of molecular vibrations. These observations suggest that the metals coordinate with thiourea through sulphur. The observed peaks IR and Raman were assigned according to their distinctiveness region. The hybrid computational calculations were carried out for calculating geometrical and vibrational parameters by HF and DFT methods with basis sets and the corresponding results were tabulated. UV-Vis-NIR spectra were recorded to study the optical transparency of the grown crystals. The observed Raman and infrared bands were also assigned and discussed. The optical transmission spectral study was carried out to test the transmitting ability of the crystal in the visible range. The second harmonic generation test of BTMS revealed the nonlinear nature of the crystal. The TGA/DTA curve was also recorded for the experimental crystal. The lattice parameters of the grown crystal have been determined by X-ray diffraction studies.

Keywords

BTMS; SHG; NLO activity; FMO; Optical properties; Hybrid Gaussian

Introduction

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.

Experimental Methods

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).

theoretical-and-computational-grown-BTMS-crystals

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.

Computational Methods

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.

S.
No
Symmetry
Species CS
Observed Frequency (cm-1) Methods Vibrational Assignments
HF B3LYP B3PW91
6-31++G
(d-p)
6-311++G
(d-p)
6-31++G
(d-p)
6-311++G
(d-p)
6-31++G
(d-p)
FT- IR FT- Raman
1 A 3773 w - 3771 3771 3771 3758 3768 (N-H) υ
2 A 3755 w - 3731 3751 3742 3720 3730 (N-H) υ
3 A - 3381 s 3375 3395 3374 3358 3363 (N-H) υ
4 A - 3296 s 3297 3278 3265 3278 3290 (N-H) υ
5 A 3277 s - 3254 3241 3278 3267 3264 (N-H) υ
6 A - 3192 s 3178 3203 3170 3193 3191 (N-H) υ
7 A 3179 s - 3167 3158 3142 3177 3170 (N-H) υ
8 A 2715 w - 2692 2689 2719 2722 2719 (N=C=N) υ
9 A 2143 w - 2135 2132 2137 2141 2138 (N=C=N) υ
10 A 2048 w - 2040 2037 2047 2036 2050 (N=C=N) υ
11 A 1836 w - 1805 1832 1829 1820 1835 (N=C=N) υ
12 A - 1645 s 1623 1634 1628 1652 1668 (N-H) δ
13 A 1618 s - 1603 1610 1606 1610 1617 (N-H) δ
14 A 1518 s - 1517 1507 1510 1546 1546 (N-H) δ
15 A - 1492 s 1489 1481 1477 1478 1487 (N-H) δ
16 A 1473 s - 1469 1461 1472 1462 1466 (N-H) δ
17 A 1412 vs - 1390 1389 1410 1407 1411 (S=O) υ
18 A - 1393 vs 1383 1379 1389 1386 1390 (S=O) υ
19 A - 1105 s 1105 1101 1102 1113 1107 (N-H) δ
20 A 1098 s - 1093 1089 1089 1087 1096 (N-H) δ
21 A 1083 s - 1081 1079 1081 1078 1080 (N-H) δ
22 A 1061 s - 1057 1055 1080 1051 1058 (C-N) υ
23 A 998 m - 1016 1005 988 990 992 (C-N) υ
24 A 960 m - 997 960 952 954 955 (N-H) γ
25 A 845 m - 836 849 843 843 863 (N-H) γ
26 A - 746 m 744 741 738 740 741 (C-N) υ
27 A 730 m - 725 725 711 714 723 (C-N) υ
28 A 715 m - 715 714 705 706 718 (N-H) γ
29 A 643 w - 640 640 656 652 660 (N-H) γ
30 A 625 w - 626 620 623 644 620 (N-H) γ
31 A 613 w - 617 612 612 610 613 (N-H) γ
32 A 590 w - 587 583 576 580 583 (N-H) γ
33 A 550 w - 549 547 564 566 570 (N-H) γ
34 A 545 w - 540 541 552 555 555 (N-H) γ
35 A 520 w - 516 516 517 545 520 (N-H) γ
36 A 510 w - 505 506 509 536 511 (C-N) δ
37 A 485 w - 481 483 487 488 491 (C-N) δ
38 A 470 w - 467 467 480 468 483 (C-S) δ
39 A 440 w - 438 438 439 440 442 (C-S) δ
40 A 435 w - 430 432 438 433 441 (Mg-O (S)) υ
41 A 425 w - 425 422 433 427 437 (Mg-O (S)) υ
42 A 422 w - 421 417 432 423 434 (Mg-O-S) δ
43 A 390 w - 416 386 403 410 406 (Mg-O-S) δ
44 A 340 w - 341 337 338 336 338 (Mg-O-S) γ
45 A 250 w - 249 248 246 248 246 (Mg-O-S) γ
46 A 240 w - 237 237 238 238 239 (Mg-O-S) γ
47 A 230 w - 229 229 228 237 230 (C-N) δ
48 A 210 w - 214 207 208 226 209 (C-N) δ
49 A 200 w - 212 198 207 211 208 (C-N) γ
50 A 190 w - 192 188 200 195 201 (C-N) γ
51 A 180 w - 171 174 181 175 182 (C-S) γ
52 A 160 w - 169 172 168 155 166 (C-S) γ
53 A 155 w - 123 123 119 118 117 (C-N) γ
54 A 150 w - 108 107 114 109 114 (C-N) γ
55 A 140 w - 105 104 112 99 111 (C-S) γ
56 A 130 w - 86 87 86 82 85 (C-S) γ
57 A 120 w - 59 56 54 63 54 (C-N) γ
58 A 110 w - 53 52 51 54 50 (C-S) γ
59 A 105 w - 30 25 33 33 33 (C-S) γ
60 A 100 w - 23 21 22 21 22 (C-S) γ

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

theoretical-and-computational-Bisthiourea-magnesium

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

theoretical-and-computational-magnesium-sulphate

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

Results and Discussion

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.

theoretical-and-computational-Molecular-structure

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.

 
Geometrical
Parameters
Methods
HF B3LYP B3PW91
6-31++G
(d-p)
6-311++G
(d-p)
6-31++G
(d-p)
6-311++G
(d-p)
6-31++G
(d-p)
Bond length(Å)
(S1-C3) 1.7361 1.7341 1.7401 1.7391 1.7322
(S1-Mg17) 2.4885 2.4839 2.4765 2.4716 2.4702
(S2-C4) 1.7361 1.7341 1.7401 1.7382 1.7322
(S2-Mg17) 2.4885 2.484 2.4765 2.4666 2.4702
(C3-N8) 1.3296 1.3296 1.3461 1.3458 1.3432
(C3-N14) 1.3078 1.3071 1.3251 1.3224 1.3217
(C4-N5) 1.3296 1.3071 1.3461 1.3492 1.3432
(C4-N11) 1.3078 1.3296 1.3251 1.3203 1.3217
(N5-H6) 0.9946 0.9958 1.009 1.008 1.0078
(N5-H7) 0.9931 1.0101 1.007 1.0057 1.0061
(N8-H9) 0.9946 0.994 1.009 1.0078 1.0078
(N8-H10) 0.9931 0.9923 1.007 1.0058 1.0061
(N11-H12) 0.9963 0.994 1.0117 1.0108 1.0104
(N11-H13) 1.009 0.9923 1.0394 1.0416 1.0417
(N14-H15) 0.9963 0.9958 1.0117 1.0103 1.0104
(N14-H16) 1.009 1.0101 1.0394 1.0434 1.0417
(Mg17-O18) 1.9795 1.9849 1.9925 2.0136 1.993
(O18-S19) 1.5355 1.5322 1.5949 1.5887 1.5872
(S19-O20) 1.4372 1.4295 1.4686 1.4547 1.464
(S19-O21) 1.4372 1.4295 1.4686 1.4854 1.464
(S19-O22) 1.5356 1.5323 1.5949 1.558 1.5871
Bond angle(?)
(C3-S1-Mg17) 103.4389 103.9473 101.0394 102.5485 100.9222
(C4-S2-Mg17) 103.4412 103.9441 101.0421 107.1544 100.9121
(S1-C3-N8) 117.1778 117.0547 117.0189 116.6413 116.9458
(S1-C3-N14) 123.8518 123.9383 123.8853 124.2817 123.786
(N8-C3-N14) 118.9564 118.9902 119.0762 119.0571 119.2487
(S2-C4-N5) 117.1763 123.9382 117.0185 115.8502 116.9488
(S2-C4-N11) 123.8531 117.055 123.8863 125.8269 123.7826
(N5-C4-N11) 118.9566 118.99 119.0756 118.2945 119.2491
(C4-N5-H6) 121.9032 120.5628 122.0381 121.9452 122.0782
(C4-N5-H7) 119.3295 121.0057 119.0765 118.8191 118.9961
(H6-N5-H7) 118.7575 116.6299 118.8713 118.627 118.9141
(C3-N8-H9) 121.9031 121.8539 122.039 122.1056 122.0789
(C3-N8-H10) 119.3296 119.2926 119.0746 119.1329 118.9952
(H9-N8-H10) 118.7573 118.8379 118.8715 118.7599 118.9132
(C4-N11-H12) 120.8345 121.8541 120.3358 119.2149 120.4376
(C4-N11-H13) 121.1151 119.2922 120.2831 126.2408 120.2488
(H12-N11-H13) 116.3764 118.8383 115.9381 114.0374 116.0717
(C3-N14-H15) 120.8348 120.5615 120.3366 120.3324 120.4368
(C3-N14-H16) 121.1141 121.0056 120.2829 121.2097 120.2506
(H15-N14-H16) 116.3742 116.6313 115.942 117.2738 116.077
(S1-Mg17-S2) 115.796 115.5599 115.3819 116.08 115.154
(S1-Mg17-O18) 105.12 104.301 104.9739 104.2932 104.7559
(S2-Mg17-O18) 126.8779 128.3368 126.4075 120.106 126.9659
(Mg17-O18-S19) 94.9933 95.5088 93.8543 92.1035 93.8338
(O18-S19-O20) 109.0452 109.0645 108.8066 110.6221 108.7984
(O18-S19-O21) 111.0894 111.157 110.7549 107.8245 110.6476
(O18-S19-O22) 98.2129 97.8249 97.9723 98.8904 98.2658
(O20-S19-O21) 116.7933 116.9201 117.9233 116.5139 117.9072
(O20-S19-O22) 111.0883 111.1565 110.7559 113.3861 110.6505
(O21-S19-O22) 109.0448 109.0659 108.8039 108.0517 108.7999
Dihedral angles(?)          
(Mg17-S1-C3-N8) 155.8392 156.3497 154.0423 160.5037 154.1954
(Mg 17-S1-C3-N14) −25.5397 −25.1581 −27.5849 −21.1351 −27.4254
(C3-S1- Mg17-S2) 175.0829 173.6904 175.8918 160.245 176.4973
(C3-S1- Mg17-O18) 29.895 27.4397 32.0952 25.7374 32.3349
(Mg17-S2-C4-N5) 155.8526 −25.1612 154.0442 −152.2027 154.1519
(Mg17-S2-C4-N11) −25.5238 156.3458 −27.5851 29.7805 −27.4747
(C4-S2- Mg17-S1) 175.0787 173.6918 175.8942 165.2823 176.5489
(C4-S2- Mg17-O18) −48.4705 −49.6488 −49.2577 −67.7378 −48.5736
(S1-C3-N8-H9) 177.82 178.0341 174.8979 176.4153 174.9432
(S1-C3-N8-H10) −3.3441 −3.4223 −3.6784 −4.0363 −3.745
(N14-C3-N8-H9) −0.8713 −0.5357 −3.5564 −2.0357 −3.5129
(N14-C3-N8-H10) 177.9646 178.0078 177.8673 177.5127 177.7989
(S1-C3-N14-H15) 170.4248 170.7616 166.1288 172.682 166.5408
(S1-C3-N14-H16) 5.7163 6.5608 7.8969 5.4556 7.6536
(N8-C3-N14-H15) −10.977 −10.7736 −15.5299 −8.9936 −15.1151
(N8-C3-N14-H16) −175.6855 −174.9744 −173.7619 −176.2201 −174.0023
(S2-C4-N5-H6) 177.8161 170.7558 174.9173 −169.4895 174.9679
(S2-C4-N5-H7) −3.3384 6.5548 −3.7001 1.4393 −3.7755
(N11-C4-N5-H6) −0.8776 −10.7785 −3.5351 8.6844 −3.4827
(N11-C4-N5-H7) 177.968 −174.9796 177.8475 179.6133 177.7739
(S2-C4-N11-H12) 170.4263 178.0285 166.1201 −174.7373 166.534
(S2-C4-N11-H13) 5.7049 −3.4152 7.9029 −3.4179 7.6677
(N5-C4-N11-H12) −10.9731 −0.5422 −15.5407 7.2896 −15.1278
(N5-C4-N11-H13) −175.6945 178.0141 −173.7579 178.609 −173.994
(S1- Mg17-O18-S19) −124.4391 −126.1082 −124.2745 −131.8945 −124.8501
(S2- Mg17-O18-S19) 95.5441 93.6065 97.2588 95.8735 96.7014
(Mg17-O18-S19-O20) 115.7481 115.6533 115.2054 130.3477 115.2052
(Mg17-O18-S19-O21) −114.1429 −114.0016 −113.6402 −101.1879 −113.7522
(Mg17-O18-S19-O22) 0.0003 −0.0012 0.009 11.1305 −0.0025

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

ΓVib = 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.

S. No Observed frequency Calculated frequency
HF B3LYP B3PW91
6-31++G
(d-p)
6-311++G
(d-p)
6-31++G
(d-p)
6-311++G
(d-p)
6-31++G
(d-p)
1 3773 3970 3949 3742 3721 3768
2 3755 3970 3949 3742 3720 3768
3 3381 3880 3858 3628 3611 3656
4 3296 3879 3857 3628 3603 3656
5 3277 3829 3814 3603 3591 3627
6 3192 3829 3814 3603 3588 3627
7 3179 3599 3549 3111 3055 3078
8 2715 3590 3539 3090 2992 3056
9 2143 1857 1854 1710 1713 1711
10 2048 1855 1852 1708 1711 1709
11 1836 1805 1797 1663 1655 1669
12 1645 1804 1796 1662 1652 1668
13 1618 1653 1643 1560 1549 1570
14 1518 1649 1639 1557 1546 1568
15 1492 1552 1543 1421 1408 1430
16 1473 1547 1538 1416 1393 1424
17 1412 1390 1389 1294 1268 1319
18 1393 1258 1254 1139 1127 1159
19 1105 1215 1210 1137 1113 1142
20 1098 1215 1210 1135 1087 1142
21 1083 1175 1173 1081 1078 1080
22 1061 1175 1173 1080 1051 1080
23 998 1016 1005 933 934 945
24 960 997 1000 925 900 937
25 845 899 904 843 870 863
26 746 897 904 821 823 843
27 730 756 756 711 714 723
28 715 753 752 705 706 718
29 643 736 736 656 652 660
30 625 729 722 656 644 660
31 613 727 720 645 643 646
32 590 668 671 576 580 583
33 550 647 652 564 566 570
34 545 600 595 552 555 555
35 520 600 594 551 545 554
36 510 581 582 536 536 538
37 485 512 514 487 488 491
38 470 508 508 480 488 483
39 440 504 498 439 440 442
40 435 501 497 438 433 441
41 425 463 464 433 427 437
42 422 458 459 432 423 434
43 390 453 455 403 410 406
44 340 406 407 376 378 376
45 250 390 388 368 350 368
46 240 360 334 314 314 315
47 230 359 333 243 237 256
48 210 320 315 242 226 255
49 200 212 213 207 211 208
50 190 192 198 200 195 201
51 180 171 174 181 175 182
52 160 169 172 168 155 166
53 155 123 123 119 118 117
54 150 108 107 114 109 114
55 140 105 104 112 99 111
56 130 86 87 86 82 85
57 120 59 56 54 63 54
58 110 53 52 51 54 50
59 105 30 25 33 33 33
60 100 23 21 22 21 22

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.

Atoms HF / 6-311++G(d-p) DFT-B3LYP / 6-311++G(d-p)
1S − 0.149 − 0.176
2S − 0.150 − 0.176
3C 0.073 0.348
4C 0.073 0.348
5N −0.432 − 0.612
6H 0.291 0.307
7H 0.516 0.331
8N − 0.371 − 0.612
9H 0.297 0.307
10H 0.313 0.331
11N − 0.371 − 0.604
12H 0.297 0.308
13H 0.313 0.402
14N − 0.432 − 0.604
15H 0.291 0.308
16H 0.516 0.402
17Mg 0.151 0.570
18O − 0.609 − 0.729
19S 0.773 1.407
20O − 0.390 − 0.564
21O −0.390 − 0.564
22O −0.609 − 0.729

Table 4: Mulliken atomic charge distribution of BTMS.

theoretical-and-computational-Mulliken-charge

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.

S. No. Samples a(A?) b(A?) c (A?) α(?) β(?) γ(?) Volume (A? 3)
1 Thiourea 14.82 8.90 7.86 90 90 90 1037.84
2 BTCC 5.80 13.07 6.48 90 90 90 491.22
3 BTZC 5.90 12.75 12.97 90 90 90 976.51
4 BTMS 5.50 7.66 8.56 90 90 90 361.38

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].

theoretical-and-computational-Transmittance-absorbance

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.

theoretical-and-computational-SHG-graph

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].

theoretical-and-computational-curve-BTMS

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.

 
Atom
  Theoretical  
Gas DMSO Chloroform  
CCl4
H6 27.69 26.76 27.02 27.27
H7 26.93 26.58 26.71 26.80
H9 27.25 26.52 26.67 26.86
H10 26.48 26.08 26.15 26.25
H12 27.57 26.67 26.88 27.12
H13 27.28 26.16 26.12 26.14
H15 26.05 26.59 26.68 26.80
H16 26.40 26.43 26.48 26.47
C3 190.34 188.34 188.64 189.29
C4 197.05 190.47 192.08 197.77

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

theoretical-and-computational-NMR-spectra

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.

theoretical-and-computational-Frontier-molecular

Figure 10: Frontier molecular orbital in IR region.

theoretical-and-computational-orbital-lobe-formation

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.

λ (nm) E (eV) ( f ) Major contribution Assignment Region Bands
Gas                                  
853.02 1.4535 0.0004 H®L (86%) n→π*  
Quartz
UV
B-band
(German,
radikalartig)
826.66 1.4998 0.0001 H®L+1 (85%) n→π*
804.60 1.5409 0.0028 H-1®L (78%) n→σ*
DMSO          
256.44 4.8348 0.0021 H®L (92%) n→σ*  
Quartz
UV
B-band
(German,
radikalartig)
251.81 4.9237 0.0003 H-1®L (89%) n→σ*
244.99 5.0607 0.0580 H®L+1 (86%) n→σ*
Chloroform          
310.24 3.9964 0.0016 H-1®L+1 (83%) n→π*  
Quartz
UV
B-band
(German,
radikalartig)
310.06 3.9987 0.0010 H®L+1 (92%) n→π*
292.87 4.2334 0.0016 H-1®L (89%) n→σ*
CCl4          
406.66 3.0489 0.0004 H®L (86%) n→π*  
Quartz
UV
B-band
(German,
radikalartig)
398.93 3.1079 0.0024 H®L+1 (85%) n→π*
380.87 3.2553 0.0009 H-1®L (78%) n→π*

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.

theoretical-and-computational-ESP-display

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.

Parameters Values
HOMO 6.4000 eV
LUMO 0.7804 eV
Energy gap 5.6196 eV
Ionization potential (IP) 6.4000 eV
Electron affinity 0.7804 eV
Electrophilicity index (ω) 2.2936
Chemical potential (μ) 3.5902
Electronegativity (χ) 3.5902 eV
Chemical hardness (h) 2.8098
Chemical softness(S) 0.1779
Dipole moment 4.8945 Debye

Table 8: Chemical parameters of BTMS.

Conclusion

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.

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