Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 165–175

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Conformational stability, vibrational (FT-IR and FT-Raman) spectra and computational analysis of m-trifluoromethyl benzoic acid V. Balachandran a,⇑, V. Karpagam b, G. Santhi c, B. Revathi a, G. Ilango d, M. Kavimani a a

Research Department of Physics, AA Government Arts College, Musiri 621211, India Department of Physics, Sri Saradha College for Women, Perambalur 621212, India c PG Department of Physics, Government Arts College, Karur 639005, India d Department of Physics, M.I.E.T. Engineering College, Tiruchirappalli 620007, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Optimized geometrical parameters of

mTFBA obtained by B3LYP, LSDA and MP2 methods.  Global minimum energy for four conformers of mTFBA have been studied.  HOMO and LUMO analysis have been calculated.  Electrostatic potential surface have been calculated.  Thermodynamic parameters have been calculated.

a r t i c l e

i n f o

Article history: Received 21 May 2014 Received in revised form 15 August 2014 Accepted 24 August 2014 Available online 3 September 2014 Keywords: m-Trifluoromethyl benzoic acid DFT calculations PED Thermodynamic functions IR and Raman NCA

C1

C3

C4

a b s t r a c t In this work, the vibrational characteristics of m-trifluoromethyl benzoic acid have been investigated and both the experimental and theoretical vibrational data indicate the presence of functional groups in the title molecule. The density functional theoretical (DFT) computations were performed at the B3LYP/631G (d, p), LSDA/6-31G (d, p), MP2/6-31G (d, p) levels to derive the optimized geometry, vibrational wavenumbers. Furthermore, the molecular orbital calculations such as natural bond orbitals (NBO), HOMO–LUMO energy gap and Mapped molecular electrostatic potential (MEP) surfaces, The Mulliken charges, the first-order hyperpolarizability were also performed with the same level of DFT. The thermal flexibility of molecule in associated with vibrational temperature was also illustrated on the basis of correlation graphs. The detailed interpretation of the vibrational spectra has been carried out with the aid of potential energy distribution (PED) results obtained from MOLVIB program. The delocalization of electron density of various constituents of the molecule has been discussed with the aid of NBO and HOMO–LUMO energy gap analysis. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Recent spectroscopic studies of benzoic acid and its derivatives have been motivated by their biological and pharmaceutical importance. The aromatic acids are crystalline substances, generally slightly soluble in water and well soluble in polar organic ⇑ Corresponding author. Tel.: +91 0431 2591338; fax: +91 4326 262630. E-mail address: [email protected] (V. Balachandran). http://dx.doi.org/10.1016/j.saa.2014.08.086 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

C2

solvents (alcohol, chloroform, benzene). Aromatic acids have all the properties characteristic of the carboxylic acids of the aromatic series. In medicine, aromatic acids are employed as weak antiseptics, and their salts as carriers of specifications [1]. Benzoic acid acts as a better inhibitor compared to salicylic acid with equal concentration [2]. The industrial applications are as a corrosion inhibitor, as an additive to nucleating agents for polyolefin, as a dye intermediate, as a stabilizer in photographic processing, and as a catalyst.

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C1

C2

C3

C4

Fig. 1. Various possible conformers of m-trifluoromethyl benzoic acid.

Organic substances produced naturally in the higher plants, are used for controlling growth or other physiological functions. Benzoic acid is one of the most commonly used preservatives in cosmetics, foodstuffs, and drug preparations [3,4]. To the best of our knowledge, very few fields have been developed specifically for carboxylic acids. However, the lower acids like formic, and acetic acids have been extensively studied, and a few studies exist for the higher acids. Quantum chemical calculations involving carboxylic acids have to account for the electron-rich carboxyl group. Consideration of these factors motivated us to undertake the vibrational spectroscopic studies of the title compound for the

electron-rich carboxyl group. The goal of present study is to give a complete description of the molecular geometry and molecular vibrations of the title compounds. The assignments of bands in the vibrational spectra of molecules are an essential step in the application of vibrational spectroscopy for solving various structural chemical problems. The change in electron density (ED) in the r⁄ antibonding orbitals and E(2) energies have been calculated by natural bond orbital (NBO) analysis to give clear evidence of stabilization originating in the hyperconjugation of hydrogen bonded interactions. The Mulliken population analysis and the HOMO–LUMO energy are

Table 1 Total energies (Hartrees) and kJ/mol of different conformations of m-trifluoromethyl benzoic acid. Conformer

C1 C2 C3 C4 a

Energy

Potential energy barrier

B3LYP/6-31G (d, p)

kJ/mol

LSDA/6-31G (d, p)

kJ/mol

MP2/6-31G (d, p)

kJ/mol

B3LYP/6-31G (d, p)

LSDA/6-31G (d, p)

MP2/6-31G (d, p)

757.66401a 757.63097 757.61420 757.60491

475441.44a 475420.71 475410.19 475404.36

753.82914a 753.79341 753.77553 753.76823

473035.03a 473012.60 473001.38 472996.81

0.7549693a 0.7549347 0.75492181 0.7549117

473.7505510a 473.72879149 473.7206890 473.71438261

0 0.033045 0.049810 0.059101

0 0.03573 0.04993 0.055432

0 0.000034 0.000048 0.000054

Global minimum energy.

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also calculated. From the spectroscopic point of view, spectra of derivatives of benzoic acid have been studied extensively and in some cases reported Rauhut and Pulay [5]. However to the best of our knowledge a complete vibrational analysis of m-trifluoromethyl benzoic acid (mTFBA) has not yet been made. Therefore the present investigation has been undertaken to study the vibrational spectra of this molecule completely and to identify the various modes with greater wavenumber accuracy. Experimental details The compound under investigation namely mTFBA was obtained by Lancaster Chemical Company, UK, which is of spectroscopic grade, and hence used for recording the spectra as such without any further purification. The room temperature Fourier transform infrared spectrum of the title compound was measured in the region 4000–400 cm1 region at a resolution of ±1 cm1 using a BRUKER IFS-66V FT-IR spectrometer equipped with KBr pellet were used in the spectral measurements. The FT-Raman spectrum of mTFBA was recorded on a BRUKER IFS66V model interferometer equipped with an FRA-106 FT-Raman accessory in the region 3500–100 cm1 stokes region using the 1064 nm line of a Nd:YAG laser for excitation operating at 200 mW power.

Fig. 2. Optimized molecular structure of m-trifluoromethyl benzoic acid.

Ii ¼ Computational details The entire calculation was performed at B3LYP/6-31G (d, p), LSDA/6-31G (d, p), MP2/6-31G (d, p) methods on personal computer using Gaussian 09W [6] program package, invoking gradient geometry optimization [7]. The optimized structural parameters were used in the vibrational frequency calculations at the B3LYP/ 6-31G (d, p), LSDA/6-31G (d, p), MP2/6-31G (d, p) level. The natural bonding orbital (NBO) calculations were performed using NBO program as implemented in Gaussian 09W [6] package at DFT level in order to understand various second-order interactions between the filled and unfilled orbitals of the system, which is a measure of the intramolecular delocalization or hyperconjugation. By combining the results of the GAUSSVIEW program [8] along with symmetry considerations, vibrational frequency assignments were made with a high degree of accuracy. The Raman activities (Si) calculated by the Gaussian-09 program were converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering.

f ðt0  ti Þ4 Si ti ½1  expðhcti Þ=kT

where t0 is the exciting frequency (in cm1 units), ti is the vibrational wavenumber of the ith normal mode, h, c, and k are universal constants, and f is the suitably chosen common normalization factor for all the peak intensities. The simulated FT-Raman and FT-IR spectra were plotted from the calculated intensity values using pure normal Lorentzian band shape with a band width of ±10 cm1. The analysis for the vibrational modes of mTFBA was presented in some detail in order to better describe the basis for the assignments. The Cartesian representation of the theoretical force constant has been computed at the fully optimized geometry by assuming the molecule belongs to Cs point group symmetry. The calculated frequencies are scaled by 0.887 for stretching with DFT method. The transformation force field from Cartesian to internal local symmetry coordinates, scaling the subsequent normal coordinate analysis (NCA), calculation of potential energy distribution (PED) were done on a PC with the version V7.0–G77 of the MOLVIB program written by Sundius [9,10].

Table 2 Optimized geometrical parameters m-trifluoromethyl benzoic acid obtained by B3LYP/6-31G (d, p), LSDA/6-31G (d, p), MP2/6-31G (d, p). Parameters

C1AC2 C1AC6 C1AC7 C2AC3 C2AH11 C3AC4 C3AC12 C4AC5 C4AH16 C5AC6 C5AH17 C6AH18 C7AO8 C7AO9 O9AH10 C12AF13 C12AF14 C12AF15

Bond lengths B3LYP/6-31G (d, p)

LSDA/6-31G (d, p)

MP2/6-31G (d, p)

1.4 1.4 1.48 1.4 1.08 1.4 1.49 1.4 1.08 1.4 1.08 1.08 1.24 1.38 0.98 1.39 1.4 1.39

1.39 1.4 1.46 1.39 1.09 1.4 1.47 1.39 1.09 1.39 1.09 1.1 1.24 1.37 0.99 1.38 1.38 1.38

1.41 1.41 1.49 1.41 1.09 1.41 1.5 1.41 1.09 1.41 1.09 1.09 1.25 1.4 0.98 1.41 1.41 1.41

X-ray data from Ref. [11].

Exp. value

Parameters

1.38 1.40 – 1.38 – 1.39 – 1.37 1.03 1.38 0.93 – 1.25 1.27 1.11

C2AC1AC6 C2AC1AC7 C6AC1AC7 C1AC2AC3 C1AC2AH11 C3AC2AH11 C2AC3AC4 C2AC3AC12 C4AC3AC12 C3AC4AC5 C3AC4AH16 C5AC4AH16 C4AC5AC6 C4AC5AH17 C6AC5AH17 C1AC6AC5 C1AC6AH18 C5AC6AH18

Bond angles B3LYP/6-31G (d, p)

LSDA/6-31G (d, p)

MP2/6-31G (d, p)

120 122 118 119 120 121 121 120 120 120 120 120 120 120 120 120 118 122

121 122 118 119 120 121 121 120 119 120 119 121 120 120 120 120 120 119

121 121 118 119 120 121 121 119 119 119 120 120 120 120 120 119 121 125

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Results and discussion Molecular geometry In order to find the most optimized geometry, the energy calculations were carried out for mTFBA, using DFT method and basis set for various possible conformers. The computationally predicted various possible conformers obtained for the title compound are shown in Fig. 1. The total energies obtained for these conformers are listed in Table 1, the structure optimizations have shown that the conformer of Fig. 1 (C1) have produced the global minimum energy. The optimized molecular structure with the numbering of atoms of the title compound is shown in Fig. 2. The predicted geometrical parameters such as bond lengths, bond angles of mTFBA calculated at B3LYP/6-31G (d, p), LSDA/631G (d, p), MP2/6-31G (d, p) levels of theory are presented in Table 2. The observed FT-IR and FT-Raman spectra of mTFBA are shown in Figs. 3 and 4, respectively. The optimized structure of the title molecule was compared with other similar system for which the crystal structures have been solved [11]. Inclusion of electron correlation in the DFT to a certain extent makes the wavenumber values smaller in comparison with the MP2 wavenumber data [11].

Vibrational assignments A detailed description of vibrational modes can be given by means of normal coordinate analysis. For this purpose, the full set of 70 standard internal coordinates containing 22 redundancies

were defined as given in supplementary Table 1. From these, a nonredundant set of local symmetry coordinates were constructed by suitable linear combinations of internal coordinates following the recommendation by Rauhut and Pulay [5] and they are presented in supplementary Table 2. The theoretically calculated DFT force fields were transformed to this later set of vibrational coordinates and used in all subsequent calculation. The observed and calculated wavenumbers and normal mode descriptions for the title compound are reported in Table 3. When using computational methods to predict normal vibrations for relatively complex polyatomic molecules, scaling strategies are used to bring computed wavenumbers. The vibrational frequencies obtained from DFT method were suitably scaled using the various scale factors for stretching, in-plane bending, out-of-plane bending and ring vibrations.

Carbon–hydrogen vibrations The aromatic structure shows the presence of CH stretching vibrations. The vibrations assigned at 3004, 2985, 2976 and 2965 cm1 by B3LYP/6-31+G (d, p), 3002 2986, 2972 and 2968 cm1 by LSDA/6-31+G (d, p) and 3006, 2989, 2980 and 2971 by MP2/6-31+G (d, p) level. The value computed by DFT method (B3LYP, LSDA) shows good agreement with FT-IR at 3000 cm1. In general, in-plane and out-plane CAH deformation vibrations occur in the region 13,001,0001 cm1 and 6,001,000 cm1, respectively. Hence in the presence study, the CH in-plane bending vibrations computed at 1175, 1096, 1075 and 906 cm1 by B3LYP/6-31+G (d, p) method. The values at 1170, 1092, 1073 and 905 cm1 by LSDA/6-31+G (d, p) methods

Observed Observed

B3LYP/6-31+G(d,p)

Raman intensity

Transmittance (%)

B3LYP/6-31G(d,p)

LSDA/6-31+G(d,p)

LSDA/6-31G(d,p)

MP2/6-31G(d,p)

MP2/6-31+G(d,p)

3500 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1) Fig. 3. Observed and stimulated FT-IR spectra of m-trifluoromethyl benzoic acid.

3000

2500

2000

1500

1000

500

Wavenumber (cm-1) Fig. 4. Observed and stimulated FT-Raman spectra of m-trifluoromethyl benzoic acid.

Table 3 Vibrational wavenumbers observed using FTIR and FT Raman band positions (cm1) and vibrational assignments for m-trifluoromethyl benzoic acid. Species

Calculated frequency

Raman

B3LYP/6-31G (d, p)

LSDA/6-31G (d, p)

MP2/6-31G (d, p)

B3LYP/6-31G (d, p)

LSDA/6-31G (d, p)

MP2/6-31G (d, p)

3091 – – – – – 1625 1602 1442 – 1377 –

3632 3264 3245 3237 3220 1731 1670 1649 1546 1489 1401 1388 1366 1333 1236 1196 1182 1139 1134 1092 1081 1045 1036 994 980 858 855 765 728 722 650 639 638 584 568 505 467 438 406 369 324 307 241 174 129 128 63 21

3522 3176 3172 3159 3149 1733 1665 1646 1516 1478 1451 1403 1341 1290 1192 1182 1158 1148 1112 1079 1077 1019 1009 958 944 861 835 758 725 708 647 641 626 581 566 502 460 427 402 364 317 304 231 170 126 123 70 25

3605 3238 3224 3213 3198 1710 1648 1633 1534 1479 1400 1396 1367 1343 1241 1204 1182 1148 1129 1084 1058 1025 886 869 844 843 770 718 709 643 643 633 619 563 510 498 449 405 401 368 315 302 244 165 130 121 39 21

3095 3004 2985 2976 2965 1697 1620 1606 1440 1436 1378 1346 1309 1286 1175 1142 1136 1125 1096 1075 1008 906 867 851 829 821 765 706 659 620 606 568 541 520 506 460 415 386 369 346 306 284 180 141 110 72 39 16

3090 3002 2986 2972 2968 1696 1623 1602 1440 1431 1375 1341 1316 1285 1170 1140 1133 1125 1092 1073 1002 905 863 850 828 820 761 705 658 621 604 566 546 525 501 463 415 387 369 345 303 285 182 140 111 70 36 15

3096 3006 2989 2980 2971 1699 1626 1609 1443 1435 1381 1350 1325 1289 1176 1146 1135 1127 1096 1078 1003 905 868 853 825 821 763 705 659 623 610 570 549 526 512 466 416 389 372 348 308 289 186 145 112 74 35 18

3000 (vw)

1693 1625 1602 – 1430 – 1340 1318 1284 1170 1142 – 1125 1090 1074

(vs) (ms) (ms) (s) (vs) (vs) (vs) (vs) (vs) (vs) (vs) (vs)

908 (s) 863 (w) – 829 (s) 761 (vs) 704 (vs) 659 (s)

(ms)

(vs) (ms) (w) (s)

– – – 1132 (vw) – 1090 (vw) 1070 (vw) 1000 (vs) 863 (vw) – 819 (vw) 761 (vs) 655 (s) –

600 (vw) 545 (ms) – – – – – – – – – – – 111 (w) – – –

– – 500 (vw) – 414 (vw) – – 342 (w) – 285 (vw) – 143 (s) – 71 (vw) 35 (vs) 14 (s)

Scaled frequency

TED

t OH(100) t CH(99) t CH(98) t CH(96) t CH(96) t C@O(85) t CC(73), t CO(12) t CC(70), t CO(12) t CC(75), t CH(18), t CO(10) t CC(65), t CO(18), t CH(10) t CAO(65), d OH(18), t CC(14) t CC(75), t CO(16), t CH(10) t CC(69), t CO(18), t CH(10) t CC(70), d CO(15), d CH(10) d CH(60), d OH(22), t CC(11) d OH(70), t CO(18), t CC(10) CF3 ass(54), t CO(15), t CH(10) CF3 sym(64), t CO(15), t CH(10) d CH(45), d OH(20), t CC(11) d CH(73), t CC(18) d C@O(75), t CC(12), t CH(08) d CH(40), t CC(17), t CO(13) t CC(68), t CO(12), t CH(08) c CH(32), c CC(18), CF3opr(10) c CH(68), c CC(15), CF3ipr(12) c CH(34), c CC(20), c CO(15) c CH(65), c CF3(15), c CC(13) d CC(65), d CH(12), t CO(10) d CC(58), t CC(15), t CO(10) CF3 ss(40), t CO(15), t CH(10) CF3 ipd(54), t CC(15), t CO(10) CF3 opd(30), d CH(20), d CC(8) c OH(67), c CC(15), c COOH(10) d CC(45), d CH(12), t CO(10) CF3 ipr(48), d CH(18), d CC(12) CF3 opr(30), c CH(20), c CC(10) c CF3(53), c CC(15), d CC(40), d CH(18), t CO(10) d CC(42), d CH(15), t CO(10) c CC(64), c CO(12), c CH(8) c CC(45), s CF3(12) c CC(54), c CH(18), c CO(12) c CC(46), c OH(22) c CC(65), c CH(12) d CAO(75), t CC(12), t CH(08) c CO(52), c OH(18), s CF310) c C@O(38), c CC(18), c CH(10) s CF3(75)

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A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A00 A00 A00 A00 A0 A0 A0 A0 A0 A00 A0 A00 A00 A00 A0 A0 A00 A00 A00 A00 A00 A0 A00 A00 A00

Observed frequency FT-IR

t – stretching, d – in-plane, c – out-plane, w – weak, m – medium, vw – very weak, s – strong, vs – very strong, ass – asymmetric stretching, ss – symmetric stretching, ipb – in-plane bending, opb – out-of-plane bending, ipr – in-plane rocking, opr – out-of-plane rocking, s – torsion.

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show good agreement with 1170, 1090, 1074 and 908 cm1 in FTIR spectrum. In general, the CAH vibrations (stretching, in-plane and out-of-plane bending) computed by DFT method show good agreement with experimental observations. The CAH out-of-plane vibration in Raman spectrum at 761 cm1 and 829, 761 cm1 in IR spectrum shows good agreement with computed value at 829, 761 cm1 in DFT (LSDA) method [12]. Carbon–carbon vibrations The ring carbon–carbon stretching vibrations occur in the region 1625–1430 cm1. In general, the bands are of variable intensity and are observed at 16,251,590 cm1, 15,901,575 cm1, 1540–1470, 1465–1430 and 13,801,280 cm1 as given by Varasanyi [13] for the fine bands in this region. In the present work, the wavenumbers very strong band observed in the FT-IR spectrum at 1625, 1602, 1430, 1340, 1318, 1284 and 863 cm1 have been assigned to CAC stretching vibrations in mTFBA. The same vibrations appear in the FT-Raman spectrum at 1625, 1602, 1442, 863 cm1 for mTFBA. The theoretically computed values by LSDA/ 6-31+G (d, p) method at 1623, 1602, 1440, 1431, 1341, 1316, 1285 and 863 cm1 for mTFBA showed excellent agreement with experimental data. The in-plane deformation vibration is at higher wave numbers than the out-of-plane vibrations, Shimanouchi et al. [14] gave the wave number data for these vibrations for different benzene derivatives from normal coordinate analysis. The theoretically computed values by DFT method show excellent agreement with the experimental data. Small changes in wavenumbers observed for these modes are due to the changes in force constants/reduced mass ratio resulting mainly from the extent of mixing between the ring and the substitution group [15]. COOH vibrations Carboxylic acid is formed by strong hydrogen bonding in the solid state. Vibrational analysis of carboxylic acid group is made on the basis of carbonyl group and hydroxyl group. The C@O stretch of carboxylic acid is identical to the C@O stretch in ketones, which is expected in the region 1740–1660 cm1 [16]. The C@O bond formed by Pp–Pp between C and O, intermolecular hydrogen bonding reduces the frequencies of the C@O stretching absorption to a greater degree than does intermolecular H bonding because of the different electro negatives of C and O, the bonding is not equally distributed between the two atoms. The lone pair of electrons on oxygen also determines the nature of the carbonyl group. In our present study, very strong intense band observed in FT-IR spectrum at 1693 cm1 is assigned to C@O stretching vibration while the B3LYP, LSDA and MP2 scaled value at 1697, 1696 and 1699 cm1, respectively are assigned to C@O stretching vibration for our title molecule. The computed anharmonic frequency nearly coincides with the experimental spectrum. The free hydroxyl group absorbs strongly in the region 3700– 3584 cm1, where as the existence of intermolecular hydrogen bond formation can lower the OAH stretching frequency in the range 3500–3200 cm1 with increase in intensity and breadth [17,18]. The calculated wavenumber at 3095, 3090 and 3096 cm1 in B3LYP, LSDA and MP2 methods, respectively, shows deviation when we compared with FT-Raman spectral value at 3091 cm1 may be due to the presence of strong intermolecular hydrogen bonding to the neighboring carbonyl group. The OAH frequency of present IR and Raman studies provides more accurate value of the frequency. The in-plane OAH deformation vibration usually appears as strong band in the region 1445–1100 cm1 in the FT-IR spectrum [19]. The theoretically predicted band at 1142 cm1 in B3LYP, and 1140 cm1 in LSDA is assigned to in-

plane bending vibration of OAH group for title molecule is in good agreement with recorded FT-IR band at 1142 cm1. The OAH outof-plane deformation vibration lies in the region 280–312 cm1 for free OAH and in the region 600–720 cm1 for associated OAH [13]. The recorded spectrum at 545 cm1 in FT-IR spectrum is assigned to OAH out-of-plane bending vibration with PED contribution of 67%. Methyl vibrations The title molecule under consideration possesses one CF3 group in the side substituted chain. For the assignments of CF3 group frequencies one can expect that nine fundamentals can be associated to each CF3 group. These vibrations are CF3 ss (symmetric stretching), CF3 ips (in-plane stretching), CF3 ipb (in-plane bending), CF3 sb (symmetric bending), CF3 ipr (in-plane rocking), CF3 opr (outof-plane rocking), t CF3 (twist), CF3 ops (out-of-plane stretching), CF3 opb (out-of-plane bending) vibrations, respectively. Methyl groups are generally referred as electron donating substituent’s in the aromatic ring system. The CAF methyl group stretching vibrations are highly localized and generally observed in the range 3000–2800 cm1 [20,21]. The CF3 asymmetric stretching very weak band in FT-Raman at 1132 cm1 is coincides with computed value at1136, 1133, 1135 cm1 and symmetric stretching very strong band in FT-IR at 1125 cm1 is coincides with computed value at1125, 1125, 1127 cm1 in B3LYP, LSDA and MP2 methods. The computed values found at 506, 501, 512 cm1 and 460, 463, 466 in B3LYP, LSDA, MP2 are assigned to CF3 in-plane rock and out-of-plane rock of mTFBA molecule. The recorded to weak band observed at 500 cm1 in FT-Raman spectrum are assigned to CH3 in-plane rock of mTFBA. The DFT method also shows good agreement with observed values of mTFBA. The asymmetrical CH3 deformation vibrations are computed at 606, 604, 610 cm1 in B3LYP, LSDA and MP2 for mTFBA. Similarly, the symmetric CH3 deformation vibrations are computed at 568, 566, and 570 in B3LYP, LSDA and MP2 for mTFBA. These fundamental values show good agreement with the observed band at 600 cm1 in FT-IR spectrum. The computed values at 415, 415, 416 and 16, 15, 18 cm1 are assigned to CF3 wagging and twisting vibrations by B3LYP, LSDA and MP2 methods, respectively. The observed band 14 in FT-Raman spectrum. This assignment is also supported by the literature.

Table 4 Mulliken atomic charge of m-trifluoromethyl benzoic acid performed at B3LYP, LSDA, and MP2 methods with 6-31+G (d, p) basis sets. Mulliken atomic charge S. No.

Atom

B3LYP

LSDA

MP2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

C1 C2 C3 C4 C5 C6 C7 O8 O9 H10 H11 C12 F13 F14 F15 H16 H17 H18

0.032429 0.0897 0.03916 0.08669 0.14529 0.10503 0.427019 0.40383 0.56352 0.395789 0.202856 0.698518 0.2779 0.27018 0.28092 0.172333 0.151669 0.181602

0.047101 0.08522 0.03475 0.09859 0.18601 0.12176 0.291168 0.343 0.51744 0.408406 0.217333 0.509774 0.22045 0.21304 0.2238 0.192702 0.177087 0.200476

0.04036 0.10534 0.11251 0.12018 0.15173 0.1378 0.511443 0.39372 0.61093 0.411224 0.242722 0.862125 0.31603 0.30779 0.32011 0.203142 0.178149 0.207672

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NBO analysis Natural bonds orbital analysis picture of mTFBA because all orbital are mathematically chosen to include the highest possible percentage of the electron density. Interaction between both filled and virtual orbital spaces information correctly explained by the NBO analysis, it could enhance the analysis of intra- and intermolecular interactions. The second-order Fock matrix was carried out to evaluate donor (i)–acceptor (j) i.e. donor level bonds to acceptor level bonds interaction in the NBO analysis [22]. The result of interaction is a loss of occupancy from the concentration of electron NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associates with the delocalization i–j is estimated as:

Eð2Þ ¼ DEij ¼ qi

F 2ði;jÞ

where qi is the donor orbital occupancy, are ei and ej diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. Natural bond orbital analysis provides a convenient basis for investigating charge transfer or conjugative interaction in molecular systems. Some electron donor orbital, acceptor orbital and the interacting stabilization energy resulted from the second-order micro-disturbance theory are reported [23,24]. The larger E(2) value the more intensive is the interaction between electron donors and acceptor i.e., the more donation tendency from electron donors to electron acceptor and the greater the extent of conjugation of the whole system [25]. Delocalization of electron density between occupied Lewis type (bond or lone pair) NBO orbital and formally unoccupied (anti bond or Rydberg) non-Lewis NBO orbital correspond to a stabilizing donor–acceptor interaction. NBO analysis has been performed on the mTFBA molecule at the B3LYP level in order to elucidate, the intra-molecular rehybridization and delocalization of electron density within the molecule supplementary Table 3.

ej  ei

HOMO

171

LUMO

HOMO-1

LUMO-1

HOMO-2

LUMO-2

Fig. 5. The atomic orbital compositions of the frontier molecular orbital of m-trifluoromethyl benzoic acid.

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The intra molecular interaction is formed by the orbital overlap between r(C6AH18) and r⁄(C1AC2) bond orbital, which results intra-molecular charge transfer causing stabilization of the system. The most important interactions in the mTFBA molecule having lone pair O8 (1) with that of anti bonding C7 results the stabilization of 1.67 kJ/mol. The interaction between lone pair O9 with anti-bonding C7 resulting stabilization energy 14.45 kJ/mol, which denotes larger delocalization, is shown in supplementary Table 4. Natural charge and electron population analysis Natural charges and electron population analysis [26] performed on the electronic structures of mTFBA clearly describes the distribution of electrons in core, valance and Rydberg atomic orbital’s. Usually the electron cloud between bonded atoms is symmetrical, when the two atoms are similar but if one of atoms has a greater tendency to attract the electron cloud then it shifts slightly towards that atom. According to an electrostatic point of new of the molecule, these electronegative atoms have a tendency to donate an electron. The natural population analysis showed mTFBA molecule is distributed on the sub-shells as follows:

Core : 25:99104 ð99:955 % of 26Þ

VðrÞ ¼

X

ZA  jRA  rj

Z

qðr0 Þ jr0  rj

dr

where the summation runs over all the nuclei A in the compound and polarization and reorganization effects are neglected. ZA is the charge of the nucleus A, located at RA and q(r0 ) is the electron density function of the molecule. To predict reactive sites for electrophilic and nucleophilic attack for the investigated compound, molecular electrostatic potential (MEP) was calculated at B3LYP/6-31+G (d, p) optimized geometries. The different values of the electrostatic potential at the surface are represented by different colors; red represents regions of most electro negative, electrostatic potential, blue represents regions of the most positive electrostatic potential and green represents region of zero potential. Potential decreases in the order red < orange < yellow < green < blue. The MEP surface provides necessary information about the reactive sites. The electron total density onto which the electrostatic potential surface has been mapped is shown in Fig. 5. The negative regions V(r) were related to electrophilic reactivity and the positive ones to nucleophilic reactivity. As easily can be seen in Fig. 5, this figure provides a visual representation of the chemically active sites and comparative reactivity of atoms [30]. Frontier molecular orbital’s (FMOs)

Valance : 69:82544 ð99:7506 % of 70Þ Rydberg : 0:18352 ð0:1912 % of 96Þ

Mulliken population analysis The natural population analysis of mTFBA obtained by Mulliken [27] population analysis with B3LYP, LSDA and MP2 level using different methods with same basis set. Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system because of atomic charge effect, dipole moment, molecular polarizability, electronic structure and a lot of properties of molecular systems. Mulliken atomic charges calculated at the B3LYP, LSDA and MP2 methods with 6-31+G (d, p) basis set are collected in Table 4. It is worthy to mention that C1, C7, C12, H10, H11, H16, H17, H18 atoms of mTFBA exhibit positive charge, while C2, C3, C4, C6, O8, O9, F13, F14 and F15 atoms exhibit negative charges. The maximum negative charge values of about 0.03916, 0.03475, 0.11251 C3 atom and C12 have a maximum positive charge values of about 0.698518, 0.59774, 0.862125 in the molecule at B3LYP/6-31+G (d, p) and LSDA/6-31+G (d, p), MP2/6-31+G (d, p) level of theory.

Molecular electrostatic potentials (MEP) Molecular electrostatic used extensively for interpreting potentials have been and predicting the reactive behavior of a wide variety of chemical system in both electrophilic and nucleophilic reactions, the study of biological recognition processes and hydrogen bonding interactions [28]. V(r), at a given point r(x, y, z) in the vicinity of a compound, is defined in terms of the interaction energy between the electrical charge generated from the compound electrons and nuclei and positive test charge (a proton) located at r. Unlike, many of the other quantities used at present, and earlier as indices of reactivity V(r) is a real physical property that can be determined experimentally by diffraction or by computational methods. For the systems studied the molecular electrostatic potential values were calculated as described previously, using the equation [29].

The most important orbital’s in molecule is the frontier molecular orbital’s, called highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). These orbitals determine the way of molecule interacts with other species. The frontier molecular energy gap helps to characterize the chemical reactivity and kinetic stability of the molecule. A molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is also termed as soft molecule [31]. The low values of frontier orbital gap in mTFBA make it more chemical reactive and less kinetic stable. The frontier molecular orbital’s plays an important role in the electric and optical properties [32]. The conjugated molecules are characterized by a small highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) separation, which is the result of a significant degree of intramolecular charge transfer from the end-capping electron acceptor groups through p-conjugated path [33]. The 3D plot of the frontier orbital’s HOMO and LUMO of mTFBA molecule is shown in Fig. 6. The positive phase is red and negative phase one is green.1 Many organic molecules, conjugated p electrons are characterized by large values of molecular first hyperpolarizabilities, were analyzed by means of vibrational spectroscopy [34,35]. In most cases, even in the absence of inversion symmetry, the strongest band in the FT-Raman spectrum is weak in the FT-IR spectrum vice versa. But the intramolecular charge transfer from the donor–acceptor group in a single–double bond conjugated path can induce large variations of both the dipole moment and the polarizability, making FT-IR and FT-Raman activity strong at the same time. The analysis of wavefunction indicates that the electron absorption corresponds to the transition from the ground state to the excited state and is mainly described by one. An electron excitation from the high occupied molecular orbital to the lowest unoccupied molecular orbital (HOMO–LUMO). Generally, the energy gap between the HOMO and LUMO decreases, it is easier for the electrons of the HOMO to be excited. The higher energy of HOMO, the easier it is for HOMO to donate electrons whereas it is easier for LUMO to accept electrons when the energy of LUMO is low. 1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.

V. Balachandran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 165–175

173

where E is the total energy, N is the number of electrons of the chemical species, l is the chemical potential and V(r) is the external potential, which is identified as the negative of the electronegativity (v) as defined by Iczkowski and Margrave [40]. According to Koopman’s theorem [41], the entries of the HOMO and the LUMO orbital’s of the molecule are related to the ionization potential (I) and the electron affinity (A), respectively, by the following reactions:

I ¼ EHOMO A ¼ ELUMO Absolute electronegativity (v) and absolute hardness (g) of the molecule are given by [42], respectively. Softness (r) is a property of compared the measures the extent of chemical reactivity. It is the reciprocal of hardness.

v ¼ ð1 þ AÞ=2 g ¼ ð1  AÞ=2 r ¼ g1 Recently Parr et al. [43] have defined a new descriptor to quantity of global electrophilic power of the compound as electrophilicity index (x) in terms of chemical potential and hardness as follows:





l2 2g



All the calculate values of quantum chemical parameters of the molecule in both basis sets of DFT are presented in Table 5. Thermodynamic function analysis

Fig. 6. Electrostatic potential contour map of m-trifluoromethyl benzoic acid.

Global and local reactivity descriptors Based on the density functional descriptors, global chemical reactivity descriptors of title molecule such as global hardness (g), chemical potential (l), global softness (r), electronegativity (v), ionization potential (I), electron affinity (A) and global electrophilicity (x) as well as local reactivity have been defined [36–39] as follows:



1 2





@2 E @N2 VðrÞ

 @E 

¼

 

1 @l 2 @N VðrÞ

l ¼ @N VðrÞ  @E  v ¼ l ¼  @N VðrÞ

On the basis of vibrational calculation at B3LYP/6-31+G (d, p) the value of thermodynamic parameters such as zero-point vibrational energy (ZPVE), dipole moment and the total energy of a molecule is sum of translational, rotational, vibrational and electronic energies. i.e., E = Et + Er + Ev + Ee. The statistical thermo chemical analysis of mTFBA is carried out considering the molecule to be at room temperature of 298.15 K and 1 atm pressure. The title molecule having rotational symmetry number 1 and the total thermal energy has been arrived as the sum of electronic, translational, rotational and vibrational energies. The variations in the zero point vibrational energy seem to be insignificant. The thermodynamic quantities such as entropy Stotal, heat capacity at constant (Cp)total, enthalpy (H  E)/T and entropy (S) for various ranges (100–1000 K) of temperatures are determined and these results are presented in supplementary Table 5. The correlation equations between heat capacity, Gibb’s free energy, entropy, enthalpy changes and temperatures were filled by parabolic formula and the corresponding filling factors (R2) for these thermodynamic properties are 0.999, 0.999, 0.999 and 1.000, respectively. The corresponding filling equations are as follows and the correlation graphics of those shown in Fig. 7 for B3LYP/6-31+G (d, p), respectively. For mTFBA by B3LYP/6-31+G (d, p)

ðC p Þ ¼ 4:767 þ 0:144T  0:00007T 2 ðR2 ¼ 0:999Þ ðH  E=TÞ ¼ 2:124 þ 0:105T þ 0:00004T 2 ðR2 ¼ 0:999Þ G  E=T ¼ 56:6  0:100T þ 0:00003T 2 ðR2 ¼ 0:999Þ ðS Þ ¼ 58:18 þ 0:206T þ 0:000001T 2 ðR2 ¼ 1Þ One of important parameters of thermodynamics is the partition function. The partition function links thermodynamics, spectroscopy and quantum theory. The different types of partition functions are translational, rotational, vibrational and electronic partition functions. The partition functions can be used to calculate heat capacities, entropies, equilibrium constants and rate con-

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Table 5 Comparison of HOMO, LUMO energy gaps and related molecular properties of m-trifluoromethyl benzoic acid. Molecular properties

Energy (a.u.)

B3LYP/6-31+G (d) HOMO 0.2904 LUMO 0.0779 HOMO-1 0.2923 LUMO+1 0.0470 HOMO-2 0.2965 LUMO+2 0.0196 LSDA/6-31+G HOMO LUMO HOMO-1 LUMO+1 HOMO-2 LUMO+2

(d) 0.2673 0.1253 0.2839 0.0964 0.2847 0.0357

MP2/6-31+G HOMO LUMO HOMO-1 LUMO+1 HOMO-2 LUMO+2

(d) 0.3806 0.0468 0.3878 0.0834 0.4743 0.1810

Energy gap (a.u.)

Ionization:ionization potential (I)

Electron affinity (A)

Global hardness (g)

Electro negativity (v)

Global softness (r)

Chemical potential (l)

Global Electrophilicity (x)

0.3683

0.29041

0.07798

0.46101

0.53899

2.16915

0.53899

0.31508

0.3393

0.29230

0.04703

0.47648

0.523515

2.09870

0.52351

0.28759

0.2768

0.29651

0.01964

0.49018

0.50982

2.04006

0.50982

0.26514

0.3926

0.26739

0.12530

0.43735

0.56265

2.28864

0.56265

0.361924

0.3803

0.28391

0.0964

0.4518

0.5482

2.21336

0.5482

0.33258

0.3205

0.28475

0.03575

0.482125

0.51787

2.07415

0.51787

0.278137

0.4275

0.38064

0.04686

0.47657

0.52343

2.09832

0.52343

0.28744

0.4712

0.38785

0.08340

0.4583

0.5417

2.18197

0.5417

0.32013

0.6553

0.47434

0.18100

0.4095

0.5905

2.44200

0.5905

0.42575

Conclusion

300 250

Thermodynamic parameters

200 150 100 50 Cp (G-E/ T) (H-E/ T) S

0 -50 -100 -150

0

200

400

600

800

1000

Temperature (K) Fig. 7. Thermodynamic parameters m-trifluoromethyl benzoic acid.

stants. All these thermodynamic parameters provide helpful information for further study on mTFBA. They can be used to compute the other thermodynamic parameters according to relationships of thermodynamic functions and to determine the direction of chemical reactions according to the second law of thermodynamics [44]. Molecular polarizability One of the objectives of the present investigation is to study the effect of the methods B3LYP/6-31+G (d, p) and LSDA/6-31+G (d, p), MP2/6-31+G (d, p) on molecular polarizability of mTFBA using the Gaussian 09W program. In this study, the computation of the molecular polarizability of mTFBA was reported. Here, is a second-rank tensor property called the dipole polarizability and mean polarizability hai is evaluated using the equation [45].

hai ¼

1 ðaxx þ ayy þ azz Þ 3

The calculated polarizabilities using DFT methods for the mTFBA molecule are summarized in supplementary Table 6.

The present investigation thoroughly analyzed the conformational stability, HOMO–LUMO, NBO and the vibrational spectra, both infrared and Raman of mTFBA molecule with B3LYP/6-31+G (d, p) and LSDA/6-31+G (d, p), MP2/6-31+G (d, p) methods. All the vibrational bands observed in the FT-IR and FI-Raman spectra of these compounds are assigned to the various modes of vibration and most of the modes have wavenumbers in the expected range. The complete vibrational assignments of wave numbers are made on the basis of potential energy distribution (PED). The scaled DFT (B3LYP, LSDA) results are the best over the MP2/6-31+G (d, p) method. The electrostatic potential surfaces (ESP) together with complete analysis of the vibrational spectra, both IR and Raman and electronic spectra help to identify the structural and symmetry properties of the title molecule. The excellent agreement of the calculated and observed vibrational spectra reveals the advantages of higher basis set for quantum chemical calculations. NBO analysis provides an efficient method for studying inter and intra molecular interaction in molecular system. The stabilization energy has been calculated from second order perturbation theory. The NBO analysis confirms the hyper conjugation interaction. The strengthening and increase in wave number is due to the hyper conjugation interaction. Natural Bond Orbital analysis shows the differences in interaction of energies are due to the substitution of OH and CF3 groups, respectively. Finally the calculated HOMO and LUMO energies shows that charge transfer occur in the molecules, which are responsible for the bioactive property of the biomedical compound mTFBA. Thermodynamic analysis reveals that all the thermodynamic parameters calculated are directly proportional to temperature. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.08.086. References [1] G. Melentyeva, L. Antonova, Pharmaceutical Chemistry, Mir Publishers, Moscow, 1988. pp. 375–393. [2] S. Bilgic, Mater. Chem. Phys. 76 (2002) 5258.

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Conformational stability, vibrational (FT-IR and FT-Raman) spectra and computational analysis of m-trifluoromethyl benzoic acid.

In this work, the vibrational characteristics of m-trifluoromethyl benzoic acid have been investigated and both the experimental and theoretical vibra...
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