Crystal growth, vibrational, optical, thermal and theoretical studies of a nonlinear optical material: 2-Methyl 3,5-dinitrobenzoic acid

Physica B 501 (2016) 5–17

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Physica B journal homepage: www.elsevier.com/locate/physb

Crystal growth, vibrational, optical, thermal and theoretical studies of a nonlinear optical material: 2-Methyl 3,5-dinitrobenzoic acid K. Sangeetha a, L. Guru Prasad b, R. Mathammal a,n a b

Department of Physics, Sri Sarada College for Women, Salem-16, India Department of Science & Humanities, M. Kumarasamy College of Engineering, Karur, India

art ic l e i nf o

a b s t r a c t

Article history: Received 15 July 2016 Received in revised form 2 August 2016 Accepted 3 August 2016 Available online 6 August 2016

Single crystals of 2-methyl 3,5-dinitro benzoic acid with reasonable size have been grown by slow evaporation solution growth method using ethanol as solvent. Quantum chemical calculation of 2-methyl 3,5-Dinitro benzoic acid was carried out by using DFT/B3LYP/6-31 þ G(d,p) method. The powder X-ray diffraction pattern was recorded and indexed. Both the experimental and theoretical vibrational spectrum validates the presence of functional groups. Polarizability, first order hyperpolarizability and the electric dipole moment values have been computed theoretically. The 1H and 13C NMR chemical shift of the molecule was calculated and compared with experimental results. TG/DSC analysis has been employed to understand the thermal and physio-chemical stability of the title compound. Frequency conversion property of the crystal was tested by Kurtz and Perry method. Optical absorption behavior of the grown crystal was examined by recording the optical spectrum and band gap energy was also estimated. The calculated HOMO and LUMO energy shows the charge transfer nature of the molecule. & 2016 Elsevier B.V. All rights reserved.

Keywords: 2-Methyl 3,5-Dinitro benzoic acid Vibrational analysis NMR NBO analysis NLO HOMO–LUMO

1. Introduction Single crystals play a significant role in the present era of rapid scientific and technical advancement where in the application of crystals has unbounded limits. New potential materials have received much attraction because of its usefulness in the field of device technology [1,2]. Many organic materials show extremely high nonlinear optical responses compared to its counterpart inorganic materials [3]. NLO property arises in the organic materials due to the electronic transitions which is faster than the distortions of crystal lattice which occurs in the inorganic materials. Most of the organic materials possess conjugated system which gives rise to a strong π-electron delocalization [4]. NLO properties of the materials depend on the polarizability nature of π-bond [5]. Delocalized π-electrons in the conjugated system can also be enhanced by adding the donor and acceptor groups. Most of the πconjugated molecules with asymmetric electron donating and accepting group gives high values of hyperpolarizability [6]. Properties of the organic materials can be optimized by using molecular engineering and chemical synthesis. By means of modifying the molecular structure of organic materials, the SHG property of the materials can be tailored [7–9]. NLO properties of the crystals get affected due to the influence of hydrogen bonding on dipole alignment [10]. Among the n

Corresponding author.

http://dx.doi.org/10.1016/j.physb.2016.08.006 0921-4526/& 2016 Elsevier B.V. All rights reserved.

available materials, nitro compounds receive much attraction because of its high NLO coefficients [11,12]. 2-Methyl 3,5-dinitrobenzoic acid (MDNBA) is an aromatic compound has both electron donor group (carboxyl) and electro acceptor groups (methyl & nitro-)hence there may be a possibility for the formation of hydrogen bonds in this system. This push-pull system with weak hydrogen bonds may show high value of NLO coefficients. In this manuscript, we report the experimental and theoretical studies on 2-methyl 3,5-dinitrobenzoic acid crystal.

2. Experimental details 2.1. Crystal growth MDNBA compound was purchased from Sigma-Aldrich (U.S.A) with a stated purity of 99% and was used to grow the crystals. In the temperature range 30–50 °C, the solubility of the compound was tested gravimetrically in acetone, ethanol and methanol solvent. It is observed from the solubility test that ethanol is a suitable solvent to grow MDNBA crystals. Supersaturated solution of MDNBA was prepared in ethanol at the room temperature. The prepared solution was filtered and covered with pricked polythene paper and kept undistributed for slow evaporation. Good quality transparent yellow color plate shaped MDNBA crystals were obtained after 10 days and is shown in Fig. 1.

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Table 1 Crystal properties of MDNBA.

Fig. 1. As grown single crystal of MDNBA.

Crystal property

Present investigation

Reported work [13]

Molecular formula Crystal structure Cell parameters a (Å) b (Å) c (Å) β (deg.) Volume (Å)3

C8H6N2O6 Monoclinic

C8H6N2O6 Monoclinic

26.839 5.113 13.879 104.599 1843.007

26.844 5.104 13.885 104.544 1841.6

planes observed in the XRD pattern were indexed using XRDA program and it confirm that the crystal system of the title crystal is monoclinic. The derived crystallographic parameters of MDNBA are charted in Table 1. The calculated values of cell parameters are in good agreement with the reported literature [18].

3. Computational details 4.2. Geometrical structures Quantum chemical computational calculation is the effective tool for interpreting and predicting the vibrational spectra [13]. In order to study the consequence of charge transfer in MDNBA, its electronic property and the optimized structure were computed by using Gaussian09 package [14] and GaussView [15]. Density functional theory (DFT) methods received much attention since one can simulate the electronic structure of the molecules. Structural optimization were carried in B3LYP/6-31 þ G(d,p) level [16,17]. The optimized molecular structure was used to simulate the IR, Raman and hyperpolarizability calculations. The GIAO method is one of the most common method for calculating the nuclear magnetic shielding tensors. 1H and 13C NMR isotropic shielding were calculated using CDCl3 solvent effect by B3LYP/631þ G(d,p) method.

4. Results and discussion 4.1. Powder X-ray diffraction Unit Cell parameters of the grown MDNBA crystal were calculated by recording the powder XRD pattern using the PANalytical PW3040/60 X′pert PRO diffractometer with CuKα1, radiation of 1.54059 Å. The recorded pattern of XRD is shown in Fig. 2. The

Fig. 2. XRD pattern of MDNBA crystal.

Molecular geometry is one of the sensitive indicators of intra and inter-molecular interactions [19]. A complete geometrical optimization have been performed within the C1 point group symmetry and the most stable optimized molecular structure of MDNBA is shown in Fig. 3. The optimized parameters such as bond lengths and bond angles for the geometry of MDNBA are presented in Table 2. 4.2.1. Hydrogen bonds Hydrogen bond is one of the important types of non covalent interaction that is being present in many chemical and biological systems [20]. The strength of hydrogen bonding is determined by the molecular geometry and the donor, acceptor groups. Hydrogen bonds are formed by good donors (O–H, N–H) and good acceptors (N, O and halide) groups and are labeled ‘strong’. MDNBA molecule has both electron donor (-carboxyl) and acceptor (-nitro & methyl) groups and forms a donor–acceptor bridge. Therefore, there is a possibility for the formation of larger number of hydrogen bonding in this system and these weak bonding helps to get high nonlinearity from the material. MDNBA has a molecular environment of adjacent carboxyl group and methyl group and also it

Fig. 3. Molecular structure of MDNBA.

K. Sangeetha et al. / Physica B 501 (2016) 5–17

7

Table 2 Structural parameters calculated for MDNBA obtained by B3LYP/6-31 þ G(d,p) method. Atoms

C1–C2 C1–C6 C1–C7 C2–C3 C3–C4 C4–C5 C5–C6 C2–C8 O8–C7 O9–C7 O16–N15 O17–N15 O20–N19 O21–N19 C4–H18 C6–H22 C11–H12 C11–H13 C11–H14

Bond length Å

Atoms

Experimental

Theoretical

1.4098 1.3854 1.5015 1.3999 1.372 1.377 1.3779 1.4999 1.2686 1.2473 1.216 1.198 1.340 1.274 0.9300 0.9300 0.9600 0.9600 0.9600

1.4188 1.4004 1.5028 1.4103 1.3900 1.3875 1.3873 1.5096 1.2135 1.3517 1.2300 1.2296 1.2293 1.2301 1.0824 1.0828 1.0889 1.0892 1.0894

C1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C2–C1–C7 C2–C1–C6 C1–C6–C5 C6–C1–C7 C3–C2–C11 C1–C2–C11 N15–C3–C2 N15–C3–C4 N19–C5–C4 N19–C5–C6 O9–C7–C1 O8–C7–O9 O8–C7–C1 C3–C4–H18 C5–C4–H18 C1–C6–H22 C5–C6–H22 C2–C11–H12 C2–C11–H13 C2–C11–H14 H12–C11–H13 H12–C11–H14

Bond angle deg. Experimental

Theoretical

114.81 125.49 116.64 121.86 122.82 121.37 119.75 115.80 120.82 124.31 118.80 115.71 118.82 119.31 119.55 124.54 115.86 122.00 122.00 120.00 120.00 109.00 109.00 109.00 109.00 109.00

115.46 124.24 117.67 121.38 123.21 121.39 119.80 113.69 121.60 122.82 120.16 115.00 119.06 119.55 115.31 122.31 123.31 121.11 121.11 119.78 120.41 110.69 110.69 111.39 108.59 108.99

Fig. 4. Dimer structure of MDNBA.

possesses nitrogen group, hence there is a chance for the formation of either type of hydrogen bonding (intra or inter). The oxygen atom of the carbonyl group can either interact with a hydrogen atom of the adjacent methyl group by an intramolecular hydrogen bridge as a monomer. Similarly, in the dimer structure, hydrogen bonding formation may occur between the hydroxyl group and the carboxyl group. Between the dimer structures of the MDNBA, there has a possibility of the formation of the inter-molecular hydrogen bonding. The optimized dimer structure of the molecule MDNBA is depicted in Fig. 4. 4.3. Vibrational analysis MDNBP consists of 22 atoms and it has 60 normal vibrational modes. The point group for the title molecule is C1 point group with 60 degrees of freedom. All the vibrations are active in both IR and Raman. Fig. 5. represents the experimental and theoretical FTIR spectra. The experimental and theoretical Raman spectra are shown in Fig. 6. The vibrational band assignments are done on the

basis of normal coordinate vibrational analysis. The resulting vibrational frequencies for the optimized geometries and the vibrational assignments as well as IR intensities and Raman scattering activities are given in Table 3. Generally, due to the exclusion of scaled frequencies and incomplete incorporation of correlation of electron and the use of finite basis set, theoretically calculated wavenumbers values are larger than the experimental. The overestimations are mostly systematic and can be corrected by following the empirical scaling procedure over the obtained theoretical frequencies fit to the corresponding experimental frequencies. In this study, selective scaling procedures have been applied. For the B3LYP/6-31þ G(d,p) level a scaling factor of 0.956 has been uniformly applied to the computed wavenumbers. We compared the calculated values with the experimental results and it is found that there is an excellent agreement between the experimental and calculated frequencies at the DFT level. 4.3.1. O–H vibrations O–H stretching band and C–O stretching band generally show

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interaction between C–H in-plane bending vibrations and C–C stretching vibrations. The C–H out-of-plane bending vibrations are coupled vibration which may show strong peaks in the region of 900–667 cm  1 [26]. The peaks observed at 1165 and 1144 cm  1 in FTIR and 1147 cm  1 in FT-Raman is assigned to C–H in-plane bending vibrations. The peaks observed at 927 cm  1 in FTIR and 922 and 907 cm  1 in FT-Raman spectrum arises due to C–H outof-plane bending vibrations.

Fig. 5. Comparison of (a) experimental and (b) calculated FTIR spectrum of MDNBA.

Fig. 6. Comparison of (a) experimental and (b) calculated FT-Raman spectrum of MDNBA.

its characteristic bands in the region of 3200–3650 cm  1 and 1000–1260 cm  1 respectively in the vibrational spectrum. The position of O–H stretching band depends on the factors such as concentration, nature of the solvent and the temperature [21]. These factors affect the hydrogen bonding, which in turn also affects the absorption frequency [22]. The stretching vibration of the hydroxyl groups in the title compound is observed at 3574 cm  1 in FTIR and in Raman spectrum, it is observed at 3580 cm  1. The wavenumbers computed here are in good agreement with the experimental results. 4.3.2. C–H vibrations Generally, the carbon hydrogen stretching vibrations traces its vibrational peak in the region of 3100–3000 cm  1 for all aromatic compounds [23–25]. In the present investigation, the FTIR band at 3103 & 3110 cm  1 and Raman band at 3105 and 3107 cm  1 are assigned to C–H stretching vibrations. The position of the CH vibrational bands is shifted to the higher wavenumber region due to steric effect of the nitro group. Medium–weak intensity vibrational bands occur in the region 1300–1000 cm  1 as a result of

4.3.3. Methyl vibrations In general, methyl groups are referred as electron donating substituent in the aromatic ring system. The CH asymmetric stretching and symmetric stretching vibrations traces peaks at 2980 cm  1 and at 2870 cm  1 wavenumber respectively [27,28]. The asymmetric stretching mode of CH is observed in the spectral range around 3007 and 3012 cm  1 in FT-IR and the band at 3002 and 3015 cm  1 in FT Raman are assigned to asymmetric stretching mode. Vibrational bands of methyl group get shifted to higher wavenumber due to intramolecular hydrogen bonding and steric effect of the adjacent carbonyl group. The band observed at 2934 cm  1 in FT Raman is assigned to symmetric stretching mode. The methyl hydrogen atoms may be subjected to hyperconjucation and induction. The twisting, scissoring and rocking vibrations are well found in the characteristic regions. In most of the compounds, the asymmetric scissoring appears at a constant range of 1400–1460 cm  1 [29]. The vibrations observed at 1435 and 1417 cm  1 in FTIR and at 1436 and 1412 cm  1 in FT Raman are consigned to asymmetrical scissoring vibration and a band observed at 1357 cm  1 FT Raman is assigned to symmetrical scissoring vibration. The CH3 rocking vibrations of the compound are identified at 997 cm  1 in the FT-IR spectra and at 993 cm  1 in FT Raman. CH3 twisting modes are observed at 267 cm  1 in FTIR and the same is absent in FT Raman. The CH3 torsion mode of vibration is found at 80 cm  1. The vibrations due to C–H in-plane and out-of-plane bending are in good agreement with theoretically computed values. 4.3.4. Ring vibrations The aromatic ring vibrational modes of MDNBA were also investigated with the help of previous reports [30,31]. The location of carbon–carbon stretching modes of the phenyl group is expected in the range from 1650 to 1200 cm  1. In Figs. 4 and 5, the wave numbers observed in the FTIR spectrum at, 1175, 1281, 1302, 1375, 1405, 1567 and 1585 cm  1 and in FT-Raman spectrum at 1172, 1303, 1373, 1403, and 1582 cm  1 are assigned to C–C stretching vibrations. Vibrational peaks observed at 1056, 900 and 852 cm  1 in the FTIR spectrum and in the FT-Raman spectrum at 893 and 853 cm  1 are assigned to CCC in-plane bending vibrations. The CCC out-plane bending vibrations are noticed at 623,512 and 484 cm  1 in FTIR and at 621 cm  1 in FT-Raman spectrum. 4.3.5. Carboxylic acid vibrations Carboxylic acids have a strong tendency towards the formation of hydrogen bonding and exist almost as dimers in solid as well as in liquid states and in concentrated solutions. The polarity of the C¼ O bond is reduced in the carboxyl group due to the inductive removal of electrons by hydrogen from carbonyl oxygen and leads to shortening of the bond. Owing to the dissimilarity in electro negativity of carbon and oxygen, the bonding electrons are not uniformly shared out between the atoms. Nature of the carbonyl group also depend the lone pair electrons in oxygen. Formation of hydrogen bonds between hydroxyl group and the carbonyl group leads decrease in the CO stretching vibrations wavenumber and normally this vibration shows very strong peaks in the region of 1655–1740 cm  1. This is well established in the present study of MDNBA. The peak identified at 1712 in FT-IR and

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Table 3 Detailed assignment of fundamental vibrations of DBNP by normal mode analysis based on SQM force field calculations. SI. No

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Symmetry species

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A

Observed frequencies (cm  1)

Calculated frequencies (cm  1) with B3LYP/6-31G(d,p) force field

Infrared

Raman

Unscaled (cm  1)

Scaled (cm  1)

IR

Raman

3576 3110 3103 3012 3007 – 1715 1585 1567 1537 1525 1435 1417 1405 1375 1357 1330 1317 1302 1281 1175 1165 1144 1097 1056 997 – 927 – 900 852 770 – – 728 – 671 – 623 – – 512 484 – – – – – – 291 267 – – – – – – 45 – –

3580 3107 3105 3015 3002 2934 1712 1582 – 1540 1521 1436 1412 1403 1373 1355 1333 – 1303 – 1172 – 1147 1093 – – 993 922 907 893 853 – 757 746 – 702 677 642 621 – – – – – – – – 332 303 294 – – – – – – 80 – – –

3747 3255 3249 3156 3147 3076 1797 1660 1640 1612 1598 1504 1484 1474 1444 1424 1398 1383 1366 1344 1232 1223 1201 1151 1107 1054 1050 974 951 942 895 811 796 783 772 742 712 674 655 643 609 541 523 476 443 375 360 350 321 310 292 183 181 172 145 117 85 51 49 37

3582 3112 3106 3017 3008 2940 1718 1587 1568 1541 1527 1438 1418 1409 1380 1361 1336 1322 1306 1285 1177 1169 1148 1100 1058 1007 1004 931 909 900 855 775 761 748 738 709 680 644 626 614 582 517 500 455 423 358 344 335 307 296 279 175 173 164 138 112 81 48 47 35

114.90 20.564 16.76 4.74 6.86 1.14 354.20 175.08 69.12 183.35 114.25 6.56 2.19 10.82 63.80 13.98 132.62 364.81 61.64 22.967 63.890 3.83 31.36 246.08 42.20 4.331 1.244 5.070 8.83 25.83 13.044 15.75 20.53 1.022 9.145 36.70 19.09 42.88 8.503 84.50 51.64 5.530 2.325 1.276 7.204 2.272 1.560 1.021 1.015 1.247 5.949 3.803 0.886 0.492 1.278 2.998 1.999 0.526 0.004 1.959

163.89 42.29 39.43 48.91 70.76 237.9 61.57 20.52 52.14 53.69 41.15 3.98 4.95 2.74 10.07 20.71 137.29 168.33 6.819 1.089 16.308 51.74 2.597 6.72 13.85 1.313 2.97 0.161 0.788 10.04 4.258 0.642 3.511 6.520 9.837 1.469 3.746 3.78 0.302 1.295 2.865 1.345 0.258 1.962 1.635 0.719 4.318 2.057 5.391 0.3438 1.102 0.695 1.665 0.965 0.763 0.628 1.026 2.043 1.454 0.236

1715 in FT Raman cm  1 is attributed to carbonyl (C ¼O) stretching mode [32,33]. The CO stretching vibration is assigned to the peak observed in FTIR at 1093 cm  1 and at 1097 cm  1 in FT-Raman spectrum. Vibrational bands of carbonyl groups get shifted to the lower wavenumber side is due to the presence of acid group and nitro groups in the title molecule and which leads to the electronic and steric effects. Generally, free O–H stretching frequency is observed around at 3700 cm  1. The presence of broad peak around 3576 cm  1 in the experimental FTIR and at 3580 cm  1 in FT-

% PED

νOH(100) νCH(100) νCH(97) νCH3(97) νCH3(97) νCH3(97) νCQO(90), βOH (15), νCC(94), νNO(24), βCH(10) νCC(72), νNO(22), βCH(10) νNO(80), νCC(32), βCH(10) νNO(80), νCC(52), βCH(12), βCH3(88) βCH(80), βRing(18) νCC(70), βCH3(18), βRing(12) βRing(86), βCCH3(72), βOH (15) βCH3(88) νCN(84), βONO(15) νCN(74), βONO(25) νCC(70), βOH (25), βRing(16) νCC(85), βCOH(25),βCH3(14), νCC(85), βCOH(18) βCCH(78), βCOH(16) βCCH(76), βOH(26), βRing(14) νCO(70), βCOH(16), βRing(14) βRing(14), βCH(16) βCCH3(92) βCCH3(82), βRing(12) δCH (92) δCH (96) βRing(92), νCN(15) βRing(82), βCCH3(22), βNO2(16) βNO2(86), βRing(24) τCOOH(92), δRing(12) βNO2(84), δRing(14) δNO2(80), δRing(24) δNO2(86), τRing(15) τCH3(78), τRing(10) τOH(76), τCH3(38), τRing(10) δRing(74), βOH(26), βCH3(18) βOH(76), βRing(10) τOH(76), τRing(10) δRing(84), βCCN(18) δRing(84), βCH3(18) τCH3(18), τOH(76) δRing(74), βCCH3(18) δCCH3(78), βRing(24) δRing(74) Ring Breathing Ring Breathing βCCO(72), βCCH3(34), βCNO(10) δCCH3(78), τRing(10) τRing(70) τRing(74), τCOOH(15) τCH3(88) τCH3(78), βCCN(10) τRing(70) τCH3(88), τCOOH(15) τNO2 τNO2 τAcid, τNO2

Raman confirms the formation of intermolecular hydrogen bonding in MDNBA. All the wave numbers for the above modes coincides with observed spectra. 4.3.6. C–N vibrations Silverstein assigned C–N stretching absorption in this region 1382–1266 cm  1 [34]. In the present work, the bands are observed at 1330 and 1317 cm  1 in the FTIR and at 1333 cm  1 in FT-Raman spectrum. This shift in wavelength arises due to the presence of

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intermolecular hydrogen bonding and inductive effect. 4.3.7. Nitro group vibrations Aromatic nitro compounds have strong absorptions in the region of 1570–1485 cm  1 and 1370–1320 cm  1 [35–37] which arises due to the asymmetric and symmetric stretching vibrations of the nitro group. The strong band observed at 1537 cm  1 in the FTIR and 1540 cm  1 in FT-Raman are assigned for NO2 asymmetric stretching mode. The symmetric stretching vibration of NO2 group is traced at 1525 cm  1 in the FTIR and at 1521 cm  1 in the FTRaman spectrum. Observation of these symmetric vibrational bands at higher wavenumber side is due to the presence of hydrogen bonding and inductive effect. The bands with medium intensity observed in FTIR at 770 and 746 cm  1 are assigned to the NO2 scissoring and wagging mode. The bands traced at 728 and 702 cm  1 are assigned to NO2 rocking and twisting vibration. Owing to the NO2 torsion vibration a band observed at 45 cm  1 in the FT-Raman spectrum. 4.4. UV–vis–NIR spectrum analysis UV–vis–NIR spectrum of MDNBA has been recorded in the wavelength range of 200–1200 nm by using Perkin Elmer Lamda 35 UV–vis–NIR spectrophotometer at room temperature and is shown in Fig. 7a. The crystal has excellent transparency in the visible and NIR region and it is visible that the lower cut-off wavelength lies nearly at 400 nm for MDNBA crystal. These lower cut-off wavelength value and high transparency nature in the visible and NIR region show the suitability of the grown MDNBA crystal for frequency conversion process. An absorption peak observed at 400 nm arises due to electron transition from n to π* states. Energy gap of MDNBA is calculated by using the formula [38].

E=

1. 243X103 eV λ max

and the energy gap of the grown crystal is found as 3.10 eV. Simulated UV–vis–NIR spectrum for the title molecule is computed by using TD-DFT method and is shown in Fig. 7b. Theoretical maximum absorption wavelength was computed at 305 nm. Generally, the absorption for phenyl compounds will occur near 305 nm and which take place due to the charge transfer from π to π* bond. 4.5. NMR spectral analysis The NMR serves as a great resource to determine the structure of an organic compound by revealing the carbon and hydrogen outline. Analysis of 13C NMR and 1H NMR theoretical calculations on chemical shift of MDNBA have been done by Gauge Independent Atomic Orbital (GIAO) method at B3LYP/6-31 þ G(d,p) level with CDCl3 solvent. The theoretical and experimental 13C NMR and 1H NMR spectrum of the title compound are given in Figs. 8 and 9 respectively. Assignments of 13C NMR and 1H NMR, chemical shifts and isotropic shielding constants are tabulated in Table 4. Aromatic carbon trace its signals with chemical shift values from 100 to 150 ppm in the overlapped areas of the spectrum and carbon having double bond with oxygen traces signal in the range 150– 220 ppm [39,40]. In the present investigation, the chemical shift values of aromatic carbons are in the range 29.54–168.29 ppm. MDNBA has two nitrogen atoms in third and fifth positions and these two atoms are having electronegative property. The oxygen atom bonded with the nitrogen atom act as electron withdrawing group, hence deshielding occurs and high value of chemical shift

Fig. 7. (a) Optical transmittance of MDNBA single crystal (b) theoretically calculated UV–vis spectrum of MDNBA.

of C7 and C3 are observed. Chemical shift at 152.31, 150.08, 138.96, 136.65 129.34 and 29.54 ppm arises due to presence of other carbon atoms in the benzene ring. The signal at 168.29 ppm is due to C ¼O group of the carboxylic acid. As the carbon atom is being surrounded by electronegative oxygen atoms, there is more deshielding around it and hence the chemical shift falls at downfield with high ppm value. It is observed from the Table 4, the calculated values of 13C NMR and 1H NMR show good agreement with measured values. The signals at 10.54 ppm and 10.12 ppm occur due to the presence of protons in the aromatic ring. Here, due to electronegative oxygen atoms, more deshielding effect occur which gives raise to the increased chemical shift. The signal at 7.87 ppm is observed due to the proton of carboxyl group. In the title molecule, the hydrogen atom is covered by oxygen atoms, hence there will be more shielding around it and which gives the low ppm chemical shift. The signal which is observed at H12, H13 and H14 are due to the proton of methyl group. 4.6. Molecular electrostatic potential In order to analyze the charge distribution of a molecule in accurate manner the electrostatic potential energy values of the molecule must be calculated [41,42]. The best way to express this

K. Sangeetha et al. / Physica B 501 (2016) 5–17

Fig. 8. (a) Calculated and (b) experimental

data is to visually portrait it, as in an electrostatic potential map. Using the Schrödinger equation, the electron density model of a molecule can be derived with the help of a computer program. The molecular electrostatic potential map visualizes the charge distribution of a molecule by means of color grading. Area of lower potential is represented as red (negative MEP) and indicates that region contains by plenty of electrons or highest electron density. A portion of a molecule which has a negative electrostatic potential is disposed to electrophilic attack and more negative the better. Areas of higher potential are symbolized as blue (positive MEP) and are represented by a relative absence of electrons. This area is the region of nucleophilic attack. MEP analysis were performed in B3LYP/6-31 þG(d,p) basis set in order to predict reactive sites for electrophilic and nucleophilic attack of the investigated molecule. In Fig. 10a, the negative (red and yellow) regions of the MEP are assigned to electrophilic

11

13

C NMR spectrum of MDNBA.

reactivity and the positive (blue) regions are connected to nucleophilic reactivity. For monomer which is seen from the Fig. 10a, the negative region is mainly localized over the oxygen atoms (O12 and O13) of the nitro group. The maximum positive region is being localized on the hydrogen atom (H8) of the hydroxyl group which indicates a possible site for nucleophilic attack which confirms that the oxygen atom (O12 and O13) are the sites for electrophilic attack and hydrogen atom (H8) is the site for nucleophilic attack [43]. The electrostatic potential surface of the title compound is displayed in Fig. 10b. 4.7. Polarizability and hyperpolarizability studies Analysis of organic molecules which has conjugated π-electron systems and large hyperpolarizability using FT-IR FT-Raman spectroscopy are evolved as a subject of research [44]. The

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Fig. 9. (a) Calculated and (b) experimental

potential applications of the compound demands the investigation of structural and bonding features that contributes to the enhancement of hyperpolarizability. First order hyperpolarizability (β) is a third rank tensor and by using the Kleinman symmetry its 27 components are reduced into 10 components [45]. The hyperpolarizability components can be identified in the expansion of Taylor series for the energy in the external field. When the external electric field tends to be weak and homogeneous, this expansion becomes:

1 1 E = E°−μα Fα − ααβFαFβ − βαβγ FαFβFγ 2 6 where Eº is the energy of the unperturbed molecules, Fα is the field at the origin, mα, ααβ and βαβγ are the dipole moment, polarizability and the first order hyperpolarizabilities respectively. Using x, y and z components the magnitude of the total static

1H

NMR spectrum of MDNBA.

isotropic polarizability (m0), first-order hyperpolarizability (βtotal) tensor and dipole moment (m) can be calculated by the following equations:

(

μ 0 = μ x2 + μ y2 + μ z2

1/2

)

α 0 = 1/3(αxx + αyy + αzz ) βtot = (βx + βy + βz )1/2 Using the below equation and GAUSSIAN 09W output, the firstorder hyperpolarizability values were calculated [46]:

βtot =[(βxxx + βxyy + βxzz)2 + (βyyy + βyzz + βyxx)2 +(βzzz + βzxx + βzyy)2 ]1/2 Computed molecular dipole moment, polarizability and the

K. Sangeetha et al. / Physica B 501 (2016) 5–17

Table 4 Experimental and calculated MDNBA.

13

C NMR and

13

1

H NMR chemical shifts (ppm) of

Atoms

Experimental

B3LYP/6-31G þ (d,p)

C7 C3 C5 C2 C1 C6 C4 C11 H22 H18 H10 H13 H12 H14

165.88 151.33 145.23 138.39 135.52 126.97 121.08 16.04 8.803 8.797 8.655 2.598 2.505 2.500

168.29 160.12 152.31 150.08 138.96 136.65 129.34 29.54 10.54 10.12 7.87 4.03 3.692 3.283

first order hyperpolarizability values of MDNBA are depicted in Table 5. The first order hyperpolarizability of the title molecule is 10 times greater than that of urea (0.1947  10  30 esu). None zero value of dipole moment and larger value of hyperpolarizability confirms the appropriateness of the grown material for frequency doubling process. 4.8. HOMO–LUMO analysis HOMO and LUMO energies are the important parameters in quantum chemistry. By analyzing the molecular orbitals (MO) energies one can get how the molecule interact with other species in the crystal. The frozen orbital approximation and the ground state properties are used to calculate the excitation values. This method is very practical particularly for calculating large systems [47]. The total energy, energy gap and the dipole moment affects the stability of a molecule. The frontier orbital surfaces have been drawn to understand the bonding scheme of present compound and it is shown in Fig. 11. Analysis on the HOMO–LUMO energy gap gives knowledge about the intermolecular charge transfer. The information about the molecular stability can also be derived from the investigation of HOMO–LUMO gap [48]. The energy gap value of HOMO and LUMO is  0.42998 eV for MDNBA molecule. From the energy gap value, hardness of the molecule can be determined. Hard molecules have larger energy gap, whereas soft molecules have only smaller energy gap. Compared to soft molecules hard molecules are less polarizable. The energy values of the frontier orbitals are presented in Table 6. The calculated self-consistent field (SCF) energy of MDNBA is 869.1719866 a.u. Electronegativity, hardness (η), softness (ζ) and electrophilicity index (ψ) values of the title molecule has been computed as 0.3733, 0.3733,  1.3394 and  0.18664 respectively. Moreover, lowering in the HOMO and LUMO energy gap confirms the charge transfer interactions take place within the molecule and softness of the molecule. The total density of states (TDOS) spectrum was obtained using the GaussSum 2.2 program and is shown in Fig. 12. DOS spectrum explains the chemical bonding and molecular orbital compositions. DOS plot confirms that more number of electrons can lodge in the LUMO and the material may serve as electron transport one. 4.9. Powder SHG measurement The second harmonic generation (SHG) property of MDNBA crystal was tested by using Kurtz and Perry powder technique [49]. A Q-switched Nd:YAG laser beam of wavelength 1064 nm

Fig. 10. (a) Total electron density mapped with ESP (b) Electrostatic potential surface of MDNBA. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

with an input power of 2.5 mJ, pulses width of 10 ns and the repetition rate of 10 Hz was made to fall on the powdered sample of MDNBA which is filled in a micro capillary tube. In order to eliminate the fundamental and to collect the second harmonic beam, the output beam from the sample has been monochromated. The SHG signal generated from the sample was detected by the photomultiplier tube with the help of lens. It is observed that, the SHG conversion efficiency of MDNBA is 0.8464 times that of KDP. This confirms the usefulness of the grown crystal for the applications in photonic and also in optoelectronic devices.

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K. Sangeetha et al. / Physica B 501 (2016) 5–17

Table 5 Polarizability, hyperpolarizability and dipole moment values of MDNBA.

Table 6 Calculated energy values of MDNBA. DFT/B3LYP/6-31þ G(d,p)

Parameters αxx αxy αyy αxz αyz αzz αtot βxxx βxxy βxyy βyyy βxxz βxyz βyyz βxzz βyzz βzzz βtot mx my mz lTotal

Gas

B3LYP /6-31G(d,p) 164.8533 0.535466 169.94104 2.4658678 6.3676653 74.682554 2.02281  10  23 e.s.u 194.194315 201.52117  284.10728  595.0364 17.30619 4.603720 13.55042  10.051302 14.19459 5.8439392 3.29738  10  30 e.s.u  0.578432 1.8446373 0.1343834 1.937867

 869.1719 Hartrees Etotal (Hartree) EHOMO (eV)  0.40164 ELUMO (eV) 0.02834 ΔEHOMO  ELUMO gap (eV)  0.42998 EHOMO  1 (eV)  0.40983 ELUMO þ 1 (eV) 0.04027 ΔEHOMO  1  ELUMO þ 1 gap (eV)  0.4501

Fig. 12. The total density of states of MDNBA.

analysis is being carried out by investigating all feasible interactions between donor (occupied) and acceptor (unoccupied) groups. The hyperconjugative interaction energy has been deduced from the second-order perturbation approach [51,52]. NBO analysis have been performed for the monomer structure of the title molecule at the DFT/ B3LYP/6-31 þG(d,p) level [53]. The stabilization energy E(2) associates for each donor (i) and acceptor (j), with the delocalization i/j is estimated as;

E( 2) = ∆Eij = qi[

Fig. 11. Frontier molecular orbitals of MDNBA.

5. Natural bond orbital analysis The bonding interactions in MDNBA have also been studied with the aid of natural bond orbital (NBO) analysis [50]. NBO

2 ⎤ F ( i, j ) ⎥ ( εj − εi ) ⎥⎦

In NBO analysis large E(2) value represents the intensive interaction between the electron-donors, the electron-acceptors. The potential interactions are given in Table 7. The donor–acceptor interaction occurs due to the delocalization of electron density occurs between occupied Lewis-type orbitals and formally unoccupied (anti-bond or Rydberg) non-Lewis NBO orbitals. The NBO occupancy of C3–N15 bond in MDNBA is larger compared to other bonds which indicates that the strength of the bond is larger than the other bonds. For each NBO the occupancy lies between 0 and 2. From this analysis, it is observed that the C1–C2, C1–C6, C2–C3, C1– C7, C3–C4, C2–C11 bonds are having three occupancy values. Intramolecular bonds are formed due to the overlapping of orbitals like s(c–c), s*(c–c), π(c–c), π*(c–c) bonds. The interactions that occur in the title molecule having lone pair O9 with that of antibonding C7–O8 and the lone pair O8 with that of antibonding ,C7–O9 results in the stabilization of 46.06 kj/

K. Sangeetha et al. / Physica B 501 (2016) 5–17

15

Table 7 Second order perturbation theory analysis of Fock Matrix in NBO basis corresponding to the intramolecular bonds of MDNBA. Donor (I)

Types of Bond

Occupancy

Acceptor (J)

Type of Bond

Occupancy

E (2) kcal/Mol

E (i)  E (j) a.u.

F (i,j)

C1–C2

s

1.97068

C1–C6

s

1.96826

C1–C6

π

1.62090

C1–C7

s

1.97164

C2–C3

s

1.97270

C2–C3

π

1.61046

C3–C4

s

1.96939

C2–C11

s

1.98141

C3–N15

s

1.98757

C4–C5

s

1.97302

C4–C5

π

1.62632

C4–H18

s

1.97271

C5–C6

s

1.97651

C5–N19

s

1.98769

C7–O8

s

1.99633

C7–O8 C7–O9 O9–H10

π s s

1.98538 1.99544 1.98619

C11–H12 C11–H13 C11–H13 N15–O16 N15–O17 N15–O17 N19–O20 N19–O20 N19–O21 LP(1)O8 LP(2)O8 LP(1)O9 LP(2)O9 LP(1)O16 LP(2)O16 LP(1)O17 LP(2)O17 LP(1)O20 LP(2)O20 LP(1)O21 LP(2)O21

s s π s s s s π s

1.98106 1.96661

C3–N15 C1–C6 C2–C3 C5–N19 C1–C2 C2–C11 C2–C3 C7–O8 C4–C5 O9–H10 C2–C3 C5–C6 C3–C4 C1–C7 C1–C2 C4–C5 N15–O17 C1–C6 C2–C3 C2–C11 C4–C5 C3–C4 C1–C6 C1–C2 C4–C5 C1–C2 C5–C6 C3–N15 C3–C4 N19–O20 C1–C6 C2–C3 C5–C6 C4–C5 C1–C6 C1–C6 C3–C4 C1–C2 C1–C7 C1–C6 C1–C6 C1–C7 C7–O8 C2–C3 C2–C3 C2–C3 C3–C4 C2–C3 N15–O17 C4–C5 N19–O20 C5–C6 C1–C7 C7–O9 C7–O8 C11–H12 N15–O17 N15–O17 C3–N15 N15–O16 C5–N19 N19–O21 C5–N19 N19–O20

s* s* s* s* s* s* π* π* π* s* s* s* s* s* s* s* s* s* s* s* s* s* s* s* s* s* s* s* s* π* π* s* s* s* s* s* s* s* s* π* s* s* s* s* s* π* s* s* π* s* π* s* s* s* s* s* s* s* s* s* s* s* s* s*

0.10103 0.02772 0.01792 0.10629 0.02738 0.01344 0.35566 0.23818 0.35248 0.01437 0.02772 0.02080 0.01928 0.07134 0.02738 0.35248 0.60524 0.31653 0.02772 0.01344 0.02137 0.01928 0.31653 0.02738 0.02137 0.02738 0.02080 0.10103 0.01928 0.61792 0.31653 0.02772 0.02080 0.02137 0.01792 0.01792 0.01928 0.02738 0.07134 0.31653 0.01792 0.07134 0.23818 0.02772 0.02772 0.35566 0.01928 0.02772 0.60524 0.02137 0.61792 0.02080 0.07134 0.09679 0.23818 0.23818 0.06138 0.06138 0.10103 0.06344 0.10629 0.05614 0.10629 0.05565

4.43 3.40 2.17 4.38 3.59 3.22 24.94 18.19 17.24 3.09 2.11 2.68 3.99 3.17 2.07 26.04 18.16 14.21 4.23 2.75 2.30 3.06 2.68 2.02 1.67 1.66 4.03 3.71 2.24 26.72 24.43 4.18 4.31 4.08 2.19 1.50 1.42 1.18 1.25 2.47 1.20 5.04 3.69 3.66 0.69 4.87 0.61 0.80 6.94 0.80 7.34 0.83 2.67 33.56 46.06 3.93 2.78 19.20 4.04 19.30 4.18 19.42 4.19 19.33

0.99 1.26 1.25 0.99 1.25 1.11 0.27 0.27 0.27 1.10 1.20 1.23 1.28 1.13 1.26 0.28 0.15 0.29 1.27 1.14 1.29 1.18 1.18 1.17 1.36 1.33 1.30 1.01 1.29 0.15 0.29 1.07 1.10 1.29 1.28 1.35 1.35 1.61 1.48 0.40 1.49 1.15 1.35 1.03 1.02 0.49 1.63 1.61 0.33 1.63 0.32 1.64 1.10 0.61 0.34 1.21 1.22 0.71 1.07 0.70 1.07 0.71 1.07 0.71

0.060 0.059 0.046 0.060 0.060 0.031 0.074 0.065 0.062 0.052 0.045 0.051 0.064 0.054 0.046 0.077 0.049 0.058 0.065 0.050 0.049 0.054 0.050 0.043 0.043 0.042 0.065 0.056 0.048 0.061 0.077 0.065 0.062 0.065 0.047 0.040 0.039 0.039 0.039 0.03 0.038 0.069 0.027 0.055 0.024 0.048 0.028 0.032 0.051 0.032 0.052 0.033 0.049 0.130 0.114 0.087 0.053 0.105 0.060 0.105 0.061 0.106 0.061 0.106

1.99418 1.99572 1.98656 1.99576 1.98520 1.99580

mol and 33.56 kj/mol respectively, which donates larger delocalization [54]. The maximum energy transfer is from O9 and O8 to C7–O8 and C7–O9 as shown in Table 7. Conjugation to the antibonding orbital of π*(N19–O20) contributes energy of 26.72 kcal mol  1. It is visible from Table 7, there are strong intramolecular hyper conjugative interactions of electrons in MDNBA. These interactions increase the electron density (ED) in

C–C antibonding orbital that weakens the respective bonds. The NBO analysis of MDNBA explains the evidences of the formation of H-bonded interaction which takes place between LP (2) O9 and (C11–H12) antibonding orbitals [55]. The stabilization energy E(2) which is associated with hyperconjugative interactions (LP(2) O9)-s*(C11–H12) is obtained as 3.93 kcal mol  1 which clearly confirms the formation of intramolecular hydrogen

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K. Sangeetha et al. / Physica B 501 (2016) 5–17

Acknowledgments The authors are sincerely thankful to the SHG measurement facility extended by Department of Inorganic and physical chemistry, Indian Institute of Science, Bangalore. The authors are also thankful to Sophisticated Analytical Instrumentation Facility (SAIF), IIT , Chennai, Cochin, VIT Vellore and St. Joseph’s College, Trichirappalli, India for providing spectral measurements.

References

Fig. 13. TG/DTA curves of MDNBA.

bonding. The stabilization energy E(2) associated with hyperconjugative interactions (O9–H10)-s*(C7–O8) is obtained as 3.69 kcal mol  1 respectively. These interactions resulted in intramolecular charge transfer causing stabilization of MDNBA. 5.1. Thermal analysis TGA pattern of MDNBA was recorded in the temperature range of 40–800 °C in nitrogen atmosphere with the heating rate of 10 °C min  1 and the recorded curve is shown in Fig. 13. It is observed from the Fig. 13 that the grown material has only one state of weight loss and it starts at 204 °C. No appreciable weight loss is noticed below to this temperature, which confirms the non-inclusion of the solvent in the grown material. In DTA, at first stage, a sharp exotherm peak observed at 204 °C which arises due to the melting of the sample. The sharpness of this peak confirms the purity and crystallinity of the grown crystal. In the second stage, the DTA spectrum with endothermic peak is observed at 322 °C which represents the decomposition of MDNBA. Thus from the thermal analysis, it is confirmed that the grown crystal has thermal stability up to 204 °C.

6. Conclusions Optically transparent single crystal of 2-methyl 3,5-Dinitro benzoic acid have been grown by slow evaporation solution growth method. Powder XRD analysis discloses that the grown crystal crystallizes in monoclinic system and the crystal is stabilized through hydrogen bonds. The vibrational nature of the functional group present in the molecule is investigated by analyzing the vibrational spectrum. The UV–vis–NIR absorption spectrum confirms the transparency nature of the crystal in the range of 400–1100 nm and the energy gap value of the grown crystal is 3.10 eV. The frequency conversion efficiency of the grown crystal is 0.8464 times greater than that of KDP. Both the conversion efficiency and transparency character of the crystal confirms the suitability of the materials for optical applications. Experimentally obtained 1H and 13C NMR chemical shifts are compared with theoretically computed values. The calculated HOMO and LUMO energies authenticate the charge transfer occurs in the molecule when it is excited.

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