Molecular geometry, conformational, vibrational spectroscopic, molecular orbital and Mulliken charge analysis of 2-acetoxybenzoic acid

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 329–336

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Molecular geometry, conformational, vibrational spectroscopic, molecular orbital and Mulliken charge analysis of 2-acetoxybenzoic acid P. Govindasamy a,⇑, S. Gunasekaran b, S. Srinivasan c a

Department of Physics, Karpagam University, Eachanari, Coimbatore 641021, TN, India Research and Development St. Peter’s Institute of Higher Education and Research, St. Peter’s University, Avadi, Chennai 600054, TN, India c Department of Physics, Presidency College, Chennai 600005, 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

 The spectroscopic investigation of

2-acetoxybenzoic acid has been performed using HF and DFT methods.  The complete assignments are performed on the basis of the potential energy distribution (PED).  The HOMO and LUMO energy gap shows that charge transfer within the molecule.  The electrophilic and nucleophilic attack region in the molecule were identified.

a r t i c l e

i n f o

Article history: Received 26 January 2014 Received in revised form 1 March 2014 Accepted 20 March 2014 Available online 13 April 2014 Keywords: 2-Acetoxybenzoic acid FTIR FT-Raman ESP HF DFT

a b s t r a c t The Fourier transform infrared (FT-IR) and FT-Raman spectra of 2-acetoxybenzoic acid (2ABA), a painkiller agent were recorded in the region 4000–450 cm 1 and 5000–50 cm 1 respectively. Hartree Fock (HF) and Density functional theory (DFT) methods have been used to determine its optimized geometrical parameter, atomic charges, and vibrational wavenumbers and intensity of the vibrational bands of the title molecule. The computed vibrational wave numbers were compared with the FT-IR and FT-Raman experimental data. The computational calculations were done at HF and DFT/B3LYP level with 6-311++G(d,p) basis set. The complete vibrational assignments were performed on the basis of the potential energy distribution (PED) analysis. The Mulliken charges, UV–Visible spectral analysis and HOMO–LUMO energy gap have been calculated and reported. The B3LYP method of calculated parameters is a good complement with the experimental findings. The thermodynamic properties like entropy, heat capacity and zero vibrational energy have been calculated and discussed. The electrostatic potential (ESP) contour surface and molecular electrostatic potential (MESP) of the molecule were constructed. Ó 2014 Elsevier B.V. All rights reserved.

Introduction The 2-acetoxybenzoic acid (2ABA) is also referred as aspirin, belongs to non-steroidal anti-inflammatory drug (NSAIDS). The

⇑ Corresponding author. Tel.: +91 9884766203. E-mail address: [email protected] (P. Govindasamy). http://dx.doi.org/10.1016/j.saa.2014.03.056 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

hydroxyl and carboxyl group which react with acetic acid to form 2-acetoxybenzoic acid also known as acetylsalicylic acid. It is the most common widely used antiseptic and anti pyretic agent. It is useful in the relief of headache, muscle and joint aches. 2ABA is also effectively reducing fever inflammation, rheumatoid arthritis, rheumatoid fever and mild infection. Its derivatives are important for the preparation of other pharmaceutical products, dyes, flavors and preservatives.

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Michal Pignone et al. examined the cancer mortality reducing effect of aspirin for primary prevention [1]. Wang et al. studied the effect of aspirin on ERK and PI3K/AKT signaling in a rat model of APE and evaluate the prognostic values of brain natriuretic peptide (BNP), Troponin (TnT) and D-Dimer. Aspirin reduces lung damage and improves prognosis [2]. Rolando et al. synthesized and evaluated a series of water-soluble (benzoyloxy) methyl esters of acetylsalicylic acid (ASA) as pro drugs [3]. Ulla Derthasching et al. investigated the effects of aspirin on platelet function and coagulation in human endotoxemia [4]. Vibrational spectra of benzoic acid and substituted benzoic acid have been studied by various workers. Amalanathan et al. studied the infrared absorption spectra of 3,5 dinitrobenzoic acid have been recorded in the solid phase [5]. The stokes and anti stokes laser Raman spectra of 2,3,5-tri-iodobenzoic acid have been recorded in the region 150–4000 cm 1 [6]. The theoretical ab initio, DFT, and normal coordinate analysis give information regarding the nature of the electronic structure, the functional groups, and orbital interactions and mixing of vibrational frequencies. The structural characteristics of vibrational spectroscopic analysis of 2ABA by the quantum mechanical ab initio and DFT methods have not been studied. Thus, considering the pharmaceutical and industrial importance of benzoic acid and its nitro derivatives, an extensive experimental and theoretical ab initio and DFT studies were carried out on 2ABA to obtain a complete reliable and accurate vibrational assignments and structural characteristics of the compound. Experimental The powder form of 2ABA was purchased from leading pharmaceutical company in Chennai and used as such without further purification. FT-IR spectrum of powder 2ABA recorded in the range 4000–450 cm 1 on Bruker IFS 66 V spectrophotometer using KBr pellet technique with 4.0 cm 1 resolution. The FT-Raman spectrum recorded using 1064 nm line of Nd:YAG laser as excitation wavelength in the 4000–50 cm 1 region on Bruker IFS 66 V spectrophotometer with FRA 106 Raman module which was used as an accessory. The UV–Visible spectral measurements were carried out using a Varian Cary 5E UV–NIR spectrophotometer. The spectral measurements were carried out at Sophisticated Analysis Instrumentation Facility, IIT Madras, India. Computational details

optimized structural parameters, bond length, bond angles and dihedral angles of 2ABA are presented in Table 1. The experimental data on the geometric structure of the related molecule was compared with theoretical values [13,14]. From the theoretical values, It is found that most of the optimized geometrical parameters are slightly larger than the experimental values, due to that the theoretical calculations belonging to isolated molecules gases phase while the experimental results belongs to molecules in solid state. Comparing bond length bond angles at B3LYP level with those of HF level the former level is larger than later while B3LYP calculated values correlates well compared with the experimental data. The calculated geometrical parameters represents a good approximation and they are the basis for the calculating other parameters, such as vibrational frequencies and thermodynamic properties. The C@O bond distances in the acetic acid group such as C1AO8 and C11AO12 are 1.2081 Å and 1.1939 Å predict the double bond character, were as other CAO bonds (C1AO9, C3AO10, C11AO12) with higher bond length values in the range 1.3568– 1.3794 Å depicts the single bond character. The magnitudes of the CAC bond length on benzene ring are ranged from 1.3890 to 1.4065 Å at B3LYP level, which is longer than double bond length of C@C but shorter than that of the single CAC bond length 1.3899 Å. The introduction the substituted group (hydroxyl and methyl group) causes slight differences between them it may be due to steric repulsion between them. The CCC bond angles in the benzene ring slightly distorted from the hexagonal geometry was revealed by the bond angle C3AC2AC7 (118.6°). The hydroxyl and methyl groups are slightly deviates from the planar geometry as seen from the dihedral angles values of C1AC2AC3AC4 = 178.8° and C3AO10AC11AO12 = 178.6°. The conformational structural features of COH, the selected degree of torsional freedom, T(C2AC1AO9AH18), is rotated from 180° to 180° in steps of 15° and the molecular energy profile is obtained with the DFT method. The PES scan for the selected above the torsional angle mentioned is shown in Fig. 2. The conformational energy profile shows two global minima observed at ±180°. It is clear from Fig. 2, there is one local minima observed at 15° for the above mentioned torsional angle. Therefore the most stable conformer is for ±180° torsional angle. Also, the crystal structure for the title compound is not available, we carried out potential energy scan has been done for the torsional angle C1AC2AC3AO10 and its profile is presented in Fig. 3. So that the relative orientation of acetoxy group with

All the theoretical computations were performed at Restricted Hartree Fock and DFT levels on a Pentium IV/1.6 GHz personal computer using the Gaussian 09W Program package [7]. The geometries were first optimized at the HF level of theory employing the 6-311++G(d,p) basis set. DFT employed the B3LYP keyword which invokes Becke’s three parameter hybrid method [8] using correlation function of Becke et al. [8]. The optimized structural parameters were used in the vibrational frequency calculations at both RHF and DFT levels to characterize all stationary points as minima. The calculated frequencies are scaled by 0.921 for HF and 0.972 B3LYP method [9,10]. The assignments of calculated wave numbers are aided by the animation option of Gauss View 5.0 graphical interface [11]. Furthermore, theoretical vibrational spectrum of the title compound is interpreted by means of PEDs using the VEDA 4 program [12]. Result and discussion Molecular geometry and potential energy surface scan The molecular structure and numbering of the atoms of 2ABA adopted in the study are shown in Fig. 1. The comparative

Fig. 1. Structure of 2-acetoxybenzoic acid (2ABA).

P. Govindasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 329–336 Table 1 Optimized geometrical parameters for 2-acetoxybenzoic acid calculated at HF/6311++G(d,p) and B3LYP/6-311++G(d,p) methods. Geometrical parameter Calculated with

Exp

HF/6-311++G(d,p) B3LYP/6-311++G(d,p) Bond length (Å) C1AC2 C1AO8 C1AO9 C2AC3 C2AC7 C3AC4 C3AO10 C4AC5 C5AC6 C6AC7 O10AC11 C11AO12 C11AC13

1.4900 1.1840 1.3269 1.3932 1.3923 1.3833 1.3608 1.3826 1.3854 1.3810 1.3567 1.1730 1.5055

1.4875 1.2081 1.3568 1.4065 1.4028 1.3933 1.3794 1.3909 1.3944 1.3890 1.3898 1.1939 1.5065

Bond angle (°) C2AC1AO8 C2AC1AO9 O8AC1AO9 C1AC2AC3 C1AC2AC7 C3AC2AC7 C2AC3AC4 C2AC3AO10 C4AC3AO10 C4AC3AC5 C4AC5AC6 C5AC6AC7 C2AC7AC6 C3AO10AC11 O10AC11AO12 O10AC11AC13 O12AC11AC13

125.1044 112.7378 122.1376 121.1844 119.9419 118.8597 120.2184 121.5957 117.8530 120.1547 120.2947 119.4363 121.0103 124.8405 117.9361 117.5860 124.4397

125.7168 112.2784 121.9871 121.2974 120.0791 118.6061 120.2338 121.6294 117.7413 120.2808 120.1250 119.6615 121.0680 123.8721 117.5149 117.0006 125.4775

Dihedral angles (°) O8AC1AC2AC3 O8AC1AC2AC7 O9AC1AC2AC3 O9AC1AC2AC7 C1AC2AC3AC4 C1AC2AC3AO10 C7AC2AC3AC4 C7AC2AC3AO10 C1AC2AC7AC6 C3AC2AC7AC6 C2AC3AC4AC5 O10AC3AC4AC5 C2AC3AO10AC11 C4AC3AO10AC11 C3AC4AC5AC6 C4AC5AC6AC7 C5AC6AC7AC2 C3AO10AC11AO12 C3AO10AC11AC13

25.7476 152.8768 155.8666 25.5100 179.0974 7.6699 0.4594 173.6921 177.6620 0.9934 1.5436 175.0253 90.5734 96.0399 1.1882 0.2523 1.3501 179.9992 2.1460

25.5675 152.8907 155.9449 25.5970 178.8611 8.5219 0.3808 172.9978 177.4789 1.0204 1.4965 174.3952 91.3207 95.8856 1.2209 0.1723 1.3019 178.6657 0.4220

1.3970

1.3780 1.3760 1.4320 1.3760 1.3800 1.3590 1.1910 1.4810

109.0000 121.2000

119.8000

120.2000 119.7000

126.3000

158.4000 19.9000

carboxylic group is revealed from this conformational analysis. It is observed that the stable geometry was obtained at a dihedral angle value of 15°. From the conformational analysis we found that there is no local minimum was obtained. Comparing the potential energy scan profile for these two dihedral angles, it is observed that the relative orientation of acetoxy group with carboxylic group favors the hydrogen bonding between O8


H19 (2.365 Å) than the rotation of OH group in carboxylic acid (O8


H19 bond distance is 2.576 Å). Vibrational assignments The optimized molecular structure of 2ABA belongs to C1 point group symmetry. The title molecule contains methyl and acetyl

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group with benzene ring. The observed and simulated FT-IR and Raman spectra are shown in Figs. 4 and 5. The vibrational band assignment for 2ABA molecule sis of C1 point group symmetry. The molecule has 21 atoms, hence, which possess 57 normal modes of vibrations. The HF and B3LYP calculated harmonic frequencies along with experimentally obtained FTIR and FT-Raman spectral measurements are tabulated in Table 2. The comparison of frequencies calculated at HF and B3LYP method with the experimental values reveals the over estimation of the calculated vibrational modes due to the neglect of anharmonicity in real systems. Inclusion of electron correlation in the density functional theory to certain extends males the frequency values smaller in comparison with calculated frequency data. Reduction in the computed harmonic vibrations, although the basis set sensitive are only marginal as observed in the DFT values using 6-311++G(d,p). Any way notwithstanding the level of calculations, it is customary to scale down the calculated harmonic frequencies in order to develop the agreement with the experiment. CAH vibrations The hetero aromatic structure shows presence of CAH stretching vibration in the band region 3000–3100 cm 1 [15,16]. In the present investigation, a very strong band observed in FT-IR spectrum at 3000 cm 1 and a weak band at 3091 cm 1 is occurred at FT-Raman spectrum is assigned to CAH stretching vibration. The HF and B3LYP calculated wave numbers agree closely with the experimental spectral values as seen in Table 2. These assignments are well supported by calculated PED values. OAH vibrations The hydroxyl stretching and bending can be identified by their broadness and strength of the band, which is dependent on the extent of hydrogen bonding. The hydroxyl stretching vibrations are generally observed in the region around 3500 cm 1 [17]. The peak is broader and its intensity is higher than that of free OAH vibration which indicates involvement in an intra-molecular hydrogen bond. In bonded from, a broad and intense band appears in the region 3500–3200 cm 1 [18]. The observed broad intense very weak FT-IR band at 3489 cm 1 corresponds to OAH stretching mode. The HF calculated scaled wavenumber at 3791 cm 1 and B3LYP calculated scaled wavenumber at 3659 cm 1 value shows fairly agreement with the experimental values. CAC vibrations The CAC stretching vibrating gives rise to characteristic bonds in both IR and Raman spectra covering the spectral region ranging from 1625 to 1400 cm 1 [19–21]. A very strong CAC stretching vibration observed at 1605 cm 1 in FTIR spectra and a medium intense peak occurs at 1606 cm 1in FT-Raman spectrum is assigned to CAC stretching vibration mode. A better agreement between the experimental and B3LYP calculated CAC vibrational mode is observed in Table 2. The PED contribution of the aromatic stretching modes indicates that these are highly pure fundamental modes. CAO vibrations Generally the CAO occur in the region 1260–1000 cm 1 [22,23]. In the present study, the very strong bands observed at 1218 and 1186 cm 1 FTIR spectra and a weak band obtained at 1192 cm 1 in FT-Raman spectrum is assigned as CAO stretching vibration. These vibrational assignments are in line with the B3LYP method. C@O vibrations The C@O stretching is a characteristic frequency of carboxylic acid [24]. The carbonyl C@O stretching vibrations is expected to occur in the region 1744–1787 cm 1 [25]. In the present study this

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Fig. 2. PES scan for the selected torsional angle T(C1AC2AO9AH18) of freedom.

Fig. 3. PES scan for the selected torsional angle T(C1AC2AC3AO10) of freedom.

Fig. 5. Experimental and simulated Raman spectra of 2-acetoxybenzoic acid.

Fig. 4. Experimental and simulated FT-IR spectra of 2-acetoxybenzoic acid.

mode appears as a very strong band at 1605 cm 1 in FT-IR spectrum and very weak and medium bands in FT-Raman spectrum at 1627 and 1606 cm 1 respectively. The calculated wave numbers are closely agreeing with the experimental data. HOMO–LUMO analysis The most important orbital in the molecules are the frontier molecular, called highest occupied molecular orbital HOMO and lowest unoccupied molecule orbital LUMO. The HOMO energy characterizes the ability of electron giving, LUMO characterizes the ability electron accepting and the gap between HOMO and LUMO characterizes the molecule, the molecular chemistry

stability [26]. These orbital determines the way the molecule interacts with other species. The frontier gap helps to identify 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. The lower value for frontier orbital gap in case of 2ABA makes it more reactive and less stable. The HOMO is the orbital that primarily acts as an electron donor and the LUMO is the orbital that largely acts as the electron acceptor. The energies of highest occupied molecular orbitals HOMO and lowest unoccupied LUMO were calculated using HF/6-311++G(d,p), B3LYP/6-311++G(d,p) are presented in Table 3. The plot of the HOMO and LUMO orbital computed at both level for 2ABA molecule are illustrated in Fig. 6. The positive phase is red1 and negative one is green. It can be seen from figure that, the HOMO of 2ABA presents a charge density localized on the all atoms. The modes in each HOMO and LUMO are placed symmetrically. The energies gap HOMO–LUMO explains the eventual charge transfer interaction with in the molecule, which influences the biological activity of the molecule. As the energy gap between the HOMO and LUMO decrease, it is easier for the electron of HOMO to be excited.

1 For interpretation of color in ‘Figs. 6 and 10’, the reader is referred to the web version of this article.

P. Govindasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 329–336

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Table 2 Vibrational band assignment 2-acetoxybenzoic acid at HF/6-311++G(d,p) and B3LYP/6-311++G(d,p) method. Modes no.

1

Experimental frequency (cm 1)

Calculated frequency (cm

)

t(IR)

Un scaled

Scaled

IR intensity

Un scaled

Scaled

IR intensity

4116 3376 3358 3349 3332 3308 3275 3201 2028 1984 1790 1760 1649 1606 1598 1586 1531 1492 1407 1375 1338 1329 1310 1244 1197 1179 1163 1131 1119 1112 1091 1011 995 902 877 850 813 779 700 691 626 624 611 589 499 497 466 397 361 290 226 161 129 110 100 60 55

3791 3109 3093 3084 3069 3047 3016 2948 1868 1827 1649 1621 1519 1479 1472 1461 1410 1374 1296 1266 1232 1224 1207 1146 1102 1086 1071 1042 1031 1024 1005 931 916 831 808 783 749 717 645 636 577 575 563 542 460 458 429 366 332 267 208 148 119 101 92 55 51

174 3 6 10 2 12 5 5 608 571 55 41 105 15 37 3 66 154 93 147 189 386 93 34 79 51 20 6 1 67 2 35 4 12 49 14 55 43 48 8 86 23 17 20 6 7 6 2 2 4 0 1 2 1 3 1 2

3764 3211 3197 3189 3175 3157 3122 3055 1849 1789 1640 1611 1512 1485 1476 1469 1392 1364 1329 1292 1241 1204 1188 1182 1150 1080 1063 1058 1007 1006 980 924 897 812 800 780 743 714 645 642 595 563 556 546 463 459 427 368 337 265 207 146 125 100 84 56 49

3659 3121 3107 3100 3086 3069 3035 2969 1797 1739 1594 1566 1470 1443 1435 1428 1353 1326 1292 1256 1206 1170 1155 1149 1118 1050 1033 1028 979 978 953 898 872 789 778 758 722 694 627 624 578 547 540 531 450 446 415 358 328 258 201 142 122 97 82 54 48

106 3 4 7 2 7 2 3 467 404 45 20 65 15 39 2 44 103 1 15 85 167 239 242 46 185 3 24 50 40 2 85 6 6 40 16 39 43 16 30 72 22 6 16 8 1 5 1 1 4 0 1 2 1 3 1 1

HF/6-311++G(d,p)

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

t(Raman)

3489vw

3091w 3000vs 1605vs

1627vw 1606m

1456vs

1306vs 1293w 1218vs 1192w 1186vs 1134vs

1045vw 1012vs 969s 916vs 885s 803vs

785vw

704s 644s

562m 552vw 542m

425vw

131m

69vs

Vibrational assignment with PED% B3LYP/6-311++G(d,p)

tsymOH(100) tsymCH2(97) tsymCH2(85) tasymCH2(86) tasymCH(98) tasymCH2(92) tasymCH2(99) tsymCH3(90) tsymOC(89) tsymOC(83) tsym(CC)4(65) tsym(CC)2(43) + dCCC(11) dHCC(45)

+ sHCCO(14) tsymCC(12) + d(HCC)2(46) d(HCH)2(70)

d(HCH)2(67)

+ dHCCO(12)

d(HCH)3(89)

tsymOC(16) + tsymCC(16) + dHOC(30) + dOCO(13) tsym(CC)4(72) tsymCC(20) + d(HCC)2(51) tsymOC(45) tsymCC(16) + dHOC(38) + dHCC(13) tsymOC(17) + tsymCC(11) + dOCC(10) + dHCC(12) d(HCC)2(32)

tsymCC(20) + tsymOC(10) + d(HCC)2(28) tsymOC(38) + dCCC(20) tsym(CC)3(51) + dHCC(10) s(HCCO)3(43) + cOCOC(18) + dHCH(11) T(HCCC)2(30) + s(HCCO)2(20) s(HCCC)2(33) + sHCCO(10) s(HCCC)3(81) tsymOC(20) + d(CCC)3(37) s(HCCC)3(70) sHCCC(13) + cOCOC(46) + cCCCC(14) tsymCC(15) + s(HCCC)2(34) + sOCCC(11) tsymCC(11) + dCCC(16) tsymCC(10) + sHCCC(11) + cOCOC(16) tsym(CCCC)2(28) + cOCOC(12) dOCC(11)

+ dOCO(11) + dCCC(13) + dCCO(11) + dOCO(34) sHOCC(80) dOCC(15) + sCCCC(11) + dOCOC(12) + cOCCC(12) cHCCO(22) + sOCOC(47) dOCC(30) + dCCC(14) dOCC(26) + dCCO(16) dOCC(37) dCOC(13) + sCCCC(32) dCCC(13) + dOCC(23) + dCCO(21) tsymCC(15) + dCCC(12) + dOCC(23) sCOC(22) + s(CCCC)3(37) dOCC(11) + dCCC(59) dCOC(14) + s(CCCC)3(64) + cCCCC(46) s(HCCO)2(40) + sCOCC(20) + sCCOC(11) dCOC(16) + s(CCCC)2(28) + s(OCCC)2(30) + cOCCC(11) dCOC(14) + sCOCC(21) + cOCCC(21) sCOCC(14) + sCCOC(64) + sOCOC(11) sOCCC(62) + sCOCC(21) d(CCC)2(30)

m – medium; w – weak; s – strong; vw – very weak; vs – very strong; t – stretching; d – bending; c – out of plane bending; s – torsion; asym: asymmetric; sym: symmetric. Potential energy distribution (PED).

a

The higher the energy of HOMO, the easier it is for HOMO to donate electrons where as it is easier for LUMO to accept electrons when the energy of the LUMO is low [27]. The energy values of HOMO and LUMO levels for 2ABA molecule are 9.7447 eV and 0.8326 eV by HF method with 6-311++G(d,p) basis set and 7.3678 eV and 2.0710 eV by B3LYP method with 6-311++G(d,p) basis set respectively. The energy differences are 10.5774 eV and 5.2967 eV. The decrease in the energy gap between

HOMO and LUMO facilitates intra molecule charge transfer which makes the material to be NLO active. UV–Visible spectral analysis The UV–Vis spectrum of the 2ABA was taken in DMSO solution and given in Fig. 7. The calculated HF and B3LYP approach as this method produced good coherent with the experimental data. An

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Table 3 HOMO–LUMO energy values of 2-acetoxybenzoic acid calculated by HF/6311++G(d,p) and B3LYP/6-311++G(d,p) methods. Parameters

HF/6-311++G(d,p)

HOMO energy (eV) LUMO energy (eV) HOMO–LUMO energy gap (eV)

9.7447 0.8326 10.5774

B3LYP/6-311++G(d,p) 7.3678 2.0710 5.2967

absorption maximum (kmax; nm) for lower-lying singlet states of molecule was calculated by the time dependent (TD) DFT method [28]. The theoretical and experimental maximum absorption wavelength, excitation energies, absorbance and oscillator strength are collected in Table 4. The absorption values were observed at 276 nm in experimentally obtained UV–Visible spectral values which is coherent with the B3LYP values 269 nm, whereas the HF method produces much underestimated values (219 nm).

Mulliken charges distribution The Mulliken charge is directly related to the vibrational properties of the molecule and quantifies how the electronic structure charges under atomic displacement. It is therefore related directly to the chemical bonds present in the molecule. It affects dipole moment, polarizability, electronic structure and more properties of molecular system [29]. The net atomic charges of 2ABA molecule, obtained by means of Mulliken population analysis, are tabulated in Table 5 and plotted in Fig. 8. The total charge of the molecule is equal to zero the calculated results reveal that the negative charge is delocalized in oxygen atoms. In 2ABA molecule, the atoms constituting the hydrogen bonds are the ones possess the smallest negative charges. While all the hydrogen atoms in the molecule have positive charges very similar negative charges are noticed for the oxygen atoms O8, O9, O10 and O12 the carbon atoms C1 and C11 attached with as oxygen atoms have more positive charges due to electronegative character of oxygen atoms [30,31]. Very similar values of positive charges are observed for hydrogen atoms connected with carbon atoms of the phenyl ring. It worth mentioning that the biggest values of charge are noticed for H18 may be due to hydrogen bonding.

Fig. 7. Observed UV–Visible spectrum of 2-acetoxybenzoic acid.

Thermodynamic properties On the basis of vibrational analysis at HF and B3LYP method with the basis set 6-311++G(d,p) level, the standard statistical thermodynamic functions heat capacity, entropy and enthalpy for the 2ABA compound were obtained from the theoretical harmonic frequencies and listed in Table 6. The variation in Zero Point Vibrational energies (ZPVEs) seems to be important [29,32]. The values of some thermodynamic parameters such as zero point vibrational energy, thermal energy, specific heat capacity, rotational constant, entropy and dipole moment of 2ABA by HF and B3LYP methods at room temperature and unit atmospheric pressure are listed. The ZPVE energy is much lower in the B3LYP method than HF method. The biggest value of ZPVE value of 2ABA is 105.44 kJ mol 1 obtained at HF level. As a result of HF and B3LYP calculation the highest dipole moment was calculated 6.1227 Debye at HF whereas the little smallest one was observed 5.6659 at B3LYP method [33,34]. However, specific heat and

Fig. 6. HOMO and LUMO plots of 2-acetoxybenzoic acid.

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P. Govindasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 329–336 Table 4 Wavelength (nm), oscillatory strength (f) and energy (eV) of 2-acetoxybenzoic acid calculated by HF/6-311++G(d,p) and B3LYP/6-311++G(d,p) methods. Calculated with

Experimental wavelength

HF/6-311++G(d,p)

B3LYP/6-311++G(d,p)

kmax (nm)

E (eV)

f

kmax (nm)

E (eV)

f

kmax (nm)

219.16 214.24 189.44

5.6573 5.7872 6.5440

0.0511 0.0227 0.0070

269.18 257.22 237.59

4.6060 4.8202 5.2185

0.0464 0.0068 0.0122

276.00

Table 5 The Mulliken charge distribution calculated by HF/6-311++G(d,p) and B3LYP/6311++G(d,p) methods of 2-acetoxybenzoic acid molecule. Atoms

Atomic charges (Mulliken)

Parameter

HF/6-311++G(d,p) C1 C2 C3 C4 C5 C6 C7 O8 O9 O10 C11 O12 C13 H14 H15 H16 H17 H18 H19 H20 H21

Table 6 Calculated thermodynamic parameters of 2-acetoxybenzoic acid employing HF/6311++G(d,p) and B3LYP/6-311++G(d,p) methods.

B3LYP/6-311++G(d,p)

1.5527 0.2884 0.6216 0.2036 0.1202 0.1535 0.0494 0.9360 0.8628 1.1033 1.5541 0.9595 0.0500 0.0637 0.0459 0.0468 0.0737 0.3527 0.0349 0.0176 0.0238

1.3388 0.2815 0.5614 0.1643 0.0699 0.1118 0.0257 0.7797 0.7612 1.0191 1.4183 0.8259 0.1126 0.0616 0.0430 0.0429 0.0696 0.3073 0.0537 0.0300 0.0340

Thermal energy (kJ/mol) Total Translational Rotational Vibrational

Entropy (cal mol Total Translational Rotational Vibrational

105.3580 0.8890 0.8890 103.5800 1

k 1) 43.3480 2.9810 2.9810 37.3860

k 1) 105.3040 41.4710 31.0100 32.8230

108.5270 41.4710 31.0650 35.9910

5.3270 1.2019 2.7689 6.1227

4.8896 1.1521 2.6204 5.6659

Dipole moment (Debye)

Total B3LYP/6-311++G(d,p) HF/6-311++G(d,p)

1.5

1.0

Charges (ev)

1

B3LYP/6-311++G(d,p) 97.9843 1.0862 0.7456 0.4934

112.3610 0.8890 0.8890 110.5830

Molar capacity at constant volume (cal mol Total 40.2950 Translational 2.9810 Rotational 2.9810 Vibrational 34.5560

lx ly lz

0.5

0.0 C1 C2 C3 C4 C5 C6 C7 O8 O9 O10 C11 O12 C13 H14 H15 H16 H17 H18 H19 H20 H21

-0.5

HF/6-311++G(d,p)

Zero-point vibrational energy (kJ/mol) Rotational constant (GHz) 105.4405 1.1019 0.7624 0.5027

Atoms

-1.0

Fig. 8. Mulliken charge distribution in 2-acetoxybenzoic acid.

entropy were calculated the smallest value for HF, but the highest values were obtained B3LYP. Analysis of molecular electrostatic potential Molecular electrostatic potential (MESP) at a point in the space around a molecule gives an indication of the net electrostatic effect produced at that point by the total charge distribution (electron + nuclei) of the molecule and correlates with dipole moments, electro negativity, partial charges and chemical reactivity of the molecule. It provides a visual method to understand the relative polarity of the molecule. An electron density isosurface mapped with electrostatic potential surface depicts the size, shape, charge density and site of chemical reactivity of the molecule. Such

mapped electrostatic potential surface has been plotted for the title molecule. The electrostatic potential contour surface of 2ABA is presented in Fig. 9. A projection of mapped molecular electrostatic potential surface of the title molecule is given in Fig. 10. The color scheme for the MESP surface is red, electron rich, partially negative charge; blue, electron deficient, partially positive charge; light blue, slightly electron deficient region; yellow, slightly electron rich region; green, neutral; respectively. The calculated ESP contour surface and 3D MESP shows the negative regions are electrophilic regions, these are mainly over the oxygen atoms of acetoxy group (O8, O9, O10, O12), the positive regions are the nucleophilic regions and these are over the carbon atoms connected with oxygen atom and over the hydrogen atoms of the title molecule. The electrostatic potential has been used primarily for predicting sites and relative reactivity’s towards electrophilic attack, and in studies of biological recognition and hydrogen bonding interactions [35,36]. In the case of 2ABA molecule the negative regions are mainly localized on the oxygen atoms. The O10 atoms has a higher electro negativity value, it would consequently have a higher electron density around them. The ESP and MESP shows that the negative potential site are on electronegative oxygen atoms as well as the positive potential site are around the hydrogen and carbon atom. From these results it can be inferred that the H atoms indicates the strongest attraction and O atom indicated the strongest repulsion of the electron density plot for molecule shows a uniform distributions.

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P. Govindasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 329–336

atomic charges obtained are tabulated that gives a proper understanding of the atomic theory. Thermodynamic properties zero point vibrational energies rotational constant, thermal energy, molar capacity, entropy, dipole moment are also calculated Furthermore, the charge density distribution and sites of chemical reactivity of the presently studied molecule has been obtained by mapping molecular electrostatic potential surface (MESP) and electrostatic potential surface (ESP) contour surface. References

Fig. 9. Electrostatic potential contour surface of 2-acetoxybenzoic acid.

Fig. 10. 3D molecular electrostatic potential of 2-acetoxybenzoic acid.

Conclusions The DFT calculated geometrical parameters were found to be in good agreement with experimental data. A complete vibrational analysis of 2-acetoxybenzoic acid is performed by HF and DFT/ B3LYP level with 6-311++G(d,p) basis set. The observed and calculated scaled wavenumbers are found to be good in agreement with experimental FTIR and FT-Raman spectral values. On the basis of calculated potential energy distribution results assignments of the fundamental vibrational frequencies have been made satisfactorily. The lowering of HOMO–LUMO energy gap, a quantum chemical descriptor, explains the charge transfer interactions taking place within the molecule. The energies of important MOs and the kmax of the compounds were also evaluated from TD-DFT method. As a result B3LYP/6-311++G(d,p) basis set, gas phase method values are closer to the experimental values. The Mulliken

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