Vibrational (FT-IR and FT-Raman), electronic (UV–vis) and quantum chemical investigations on pyrogallol: A study on benzenetriol dimers

Accepted Manuscript Title: Vibrational (FT-IR and FT-Raman), electronic (UV-Vis) and quantum chemical investigations on pyrogallol: A study on benzenetriol dimers Authors: S. Selvaraja, P. Rajkumar, K. Thirunavukkarasu, S. Gunasekaran, S. Kumaresan PII: DOI: Reference:

S0924-2031(17)30235-7 https://doi.org/10.1016/j.vibspec.2018.01.003 VIBSPE 2764

To appear in:

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Received date: Revised date: Accepted date:

22-8-2017 3-12-2017 8-1-2018

Please cite this article as: Selvaraja S., Rajkumar P., Thirunavukkarasu K., Gunasekaran S., Kumaresan S., Vibrational (FT-IR and FT-Raman), electronic (UV-Vis) and quantum chemical investigations on pyrogallol: A study on benzenetriol dimers, Vibrational Spectroscopy https://doi.org/10.1016/j.vibspec.2018.01.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Vibrational (FT-IR and FT-Raman), electronic (UV-Vis) and quantum chemical investigations on pyrogallol: A study on benzenetriol dimers S.Selvaraj a,*, P.Rajkumar a, K.Thirunavukkarasu a, S.Gunasekaran b, S.Kumaresan a a

PG and Research Department of Physics, Arignar Anna Government Arts College, Cheyyar

- 604407, Tamil Nadu, India. b

St.Peters University, Avadi, Chennai–600054, Tamil Nadu, India.

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*

Corresponding authors E-mail: [email protected]

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Graphical Abstract

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Spectroscopic properties of pyrogallol were analyzed experimentally and theoretically. Vibrational and electronic spectra of pyrogallol were compared with its isomers (phloroglucinol and hydroxyquinol). HOMO-LUMO energy gaps were predicted for isomeric benzenetriols. Optimized monomer and dimer structure of benzenetriols were investigated. Experimental and theoretical FT-IR, FT-Raman and UV-Vis spectra show good agreement.

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HIGHLIGHTS

Abstract

Pyrogallol was identified for the first time from the natural extract of Abrus precatorius

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Linn seeds using GC-MS technique. The vibrational analysis of pyrogallol was investigated in solid phase by FT-IR and FT-Raman spectroscopic techniques in the region 4000-400 cm-1 and 4000-40 cm-1, respectively. The optimized molecular geometry, wave number and intensity of the vibrational bands of pyrogallol and its isomers were obtained by DFT method with different basis sets. The electronic transition energy and intensities, HOMO-LUMO energy gap have been computed with the ZINDO, CIS, TDDFT theory for isomers and the differences were compared with UV-Vis absorption spectra. The observed wave numbers in FT-IR, FT-Raman

and UV-Vis spectra were analyzed and assigned to various fundamental modes of vibration in the molecules. Experimentally observed vibrational frequencies and electronic transitions were assigned with the help of theoretical vibrational and electronic spectra, respectively. The differences between observed and calculated wave number values of most of the fundamental modes are very small. The calculated parameters were found to be in best agreement with experimental findings, thereby confirming the monomer and dimer structure of the identified

Keywords: Vibrational spectra; Electronic spectra; GC-MS; DFT; Dimer

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1. Introduction

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molecule as well as its isomers.

Pyrogallol also known as 1, 2, 3 benzenetriol, is an organic compound that belongs to the phenol family and is one of the isomers of benzenetriols. Pyrogallol has remarkable pharmaceutical activity, which acts as antioxidant, antibacterial, antiseptic, antidermatitis,

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fungicide, pesticide, anti mutagenic dye, candidicide and insecticide [1-3]. Several kinds of

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experimental and theoretical analysis on benzenetriols have been reported by few researchers

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[4-10]. A theoretical interaction between phloroglucinol and water has been computationally investigated by L.Mammino et al [11]. Spectroscopic studies and quantum chemical

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calculations on some similar benzenediols such as hydroquinone, catechol and resorcinol have been reported by many research workers [12-19]. The photo catalytic degradation on ZnO films

V.M.Guerin et.al [20].

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and Density Functional Theory (DFT) study on 4-Chlorophenol has been carried out by

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To the best of our knowledge the literature survey reveals that, neither theoretical calculation nor detailed spectroscopic analyses on monomer and dimer structure of

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benzenetriols (Pyrogallol, Phloroglucinol and Hydroxyquinol) are yet to be reported. These insufficiencies motivated us to perform detailed / extensive experimental and computational spectroscopic studies for better understanding of the vibrational modes, HOMO-LUMO, molecular geometry, dimer formation and electronic excitation of pyrogallol and its isomeric

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compounds. The quantum chemical calculations are performed not only to confirm the structure, but also to gain detailed structural and spectroscopic insight of monomer and dimer of these isomers. In addition, an attempt has been made to identify pyrogallol from the methanolic extract (extracted by soxhlet extraction method) of Abrus precatorius Linn seeds using Gas Chromatography – Mass Spectrometry (GC-MS) technique, the results are

visualized in Fig. S1 (Supplementary material) and represented in Table S1 (Supplementary material). 2. Materials and methods 2.1. Spectroscopic analysis Pyrogallol was purchased from the Avra Synthesis Private Limited, India and used as

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such for the spectral measurements. The Fourier Transform Infra Red (FT-IR) spectrum of pyrogallol was recorded in the region 4000 - 400 cm-1 using Perkin Elmer-Spectrum Two FTIR/ATR Spectrometer system with 0.5 cm-1 resolution. The FT-Raman spectrum of pyrogallol

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was recorded in the region 4000 - 40 cm-1 on a Bruker RFS 27 Stand alone FT-Raman Spectrometer system with 2 cm-1 resolution using the 1064 nm line of Nd:YAG laser for excitation operating at 100 mW power. The Ultraviolet Visible (UV-Vis) spectrum of

pyrogallol was measured in the wavelength ranging 1100 -190 nm, Bandwidth 0.5 - 4 nm

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(variable) using Perkin Elmer-Lambda 35 UV Winlab V6.0 Spectrometer at room temperature.

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2.2. Computational methods

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The theoretical calculations were performed at DFT/B3LYP levels [21-24] included in

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the Gaussian 09W package program [25] together with the 6-31G(d,p), 6-31+G(d,p), 6311G(d,p) and 6-311+G(d,p) basis set functions of the DFT utilizing gradient geometry

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optimization. The calculations of electronic excitations were carried out using the Zerner’s Intermediate Neglect of Differential Overlap (ZINDO), Configuration Interaction Singles

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(CIS) and Time Dependent Density Functional Theory (TD-DFT) methods. The calculated theoretical values are usually higher than the experimental values due to electron correlation effects and partly basis set deficiencies [26]. Therefore, attempts have been made with different

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basis sets to find high degree of accuracy. Also, by combining the results of Chemcraft program [27] with symmetry considerations, vibrational and electronic spectral assignments were made with a high degree of accuracy. There is always some ambiguity in defining internal

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coordination; however, the defined coordinate forms complete set and matches quite well with the motions observed with help of the Chemcraft program. 3. Results and discussion 3.1. Geometrical molecular structure

The optimized geometrical parameter depicts a good approximation, as it is the basis for calculating vibrational frequencies, thermo dynamical properties and other parameters. In view of position of phenolic groups, benzenetriol has 3 isomers. The optimized geometrical structure parameters of benzenetriol monomers were calculated by DFT/B3LYP levels with the 6-31G (d,p), 6-31+G (d,p), 6-311G (d,p) and 6-311+G (d,p) basis sets and also the same parameters for benzenetriol dimeric structure were calculated by DFT/B3LYP levels with the 6-31G (d,p) basis set, in accordance with the labeling of atoms scheme is shown in Fig. 1.

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However, all the bond lengths and bond angles for monomer structures of benzenetriol were

computed with the DFT-B3LYP level shows excellent agreement as it is represented in the

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Tables S2, S3 and S4 (Supplementary material). Also the optimized bond lengths, bond angles

and dihedral angles for dimer structure of benzenetriol are presented in Tables S5 and S6 (Supplementary material).

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Four types of prominent bonds such as C-O, O-H, C-C and C-H are identified and bond lengths are calculated in both monomer and dimer structure of benzenetriol compounds. The

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C-C and C-H bond lengths of aromatic phenyl ring are calculated and are in the range of 1.383

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to 1.410 Å and 1.081 to 1.088 Å, respectively. The C-O and O-H bond lengths fall in the range

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of 1.360 to 1.384 Å and 0.9613 to 0.9694 Å, respectively. These calculated values are found to be closely related to the standard values. The bond angles calculated in the range 106.90 to 113.52◦ for COH bonds indicate that the oxygen lone pairs have a resonance interaction with

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the carbon pushing the hydrogen to the COH plane. The CCO bond angles calculated to be in the range 114.33 to 124.97◦, show good agreement with standard values. The CCC and CCH

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bond angles of phenyl ring are in the range of 118.48 to 121.81◦ and 119.20 to 122.01◦, respectively. However, the CCC bond angles showed acceptable differences which may be due

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to the OH group attached to the six membered phenyl rings, which can distort symmetry and also normal values. Similarly, the monomer structural parameters of benzenetriols are nearly the same as for its dimer structure.

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The isomers of benzenetriol form dimer readily as they can form H bond through

intermolecular interaction. From the geometrical structure of pyrogallol, it is evident that the proximity of –O-H groups at adjacent positions makes it easy to form dimer. In the case of hydroxyquinol, the two –O-H groups at adjacent positions in one molecule can form two O-HO bonds with the same –O-H group in another molecule. This is evident from the shortest bond length in the range 1.841 – 1.873 Å. But in the case of symmetric benzenetriols as there are OH groups at alternate carbon atoms, the dimer is formed through H bonding at a single carbon

site. This is evident from the longer bond length in the range of 1.950 Å. The same kind effects are also observed in the bond angles (O-H...O, H-O…H and C-O…H) and dihedral angles (CO-H-O) as represented in Table S6 (Supplementary material). No correlation can be made because of non-availability of experimental data in the literature, but this theoretical study reveals formation of dimers in these benzenetriols. 3.2. Vibrational assignments

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According to the theoretical calculations, by default pyrogallol structure has set to C 1 point group symmetry. The molecule has 15 atoms and 39 modes of fundamental vibrations.

All the 39 fundamental vibrations are active in both FT-IR and FT-Raman spectra. The

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experimental FT-IR and FT-Raman frequencies with theoretical calculations for various modes of vibrations are assigned and presented in Table 1. The harmonic vibrational frequencies are calculated for phloroglucinol and hydroxyquinol have been summarized in Tables S7 and S8

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(Supplementary material), respectively. All the calculated modes of vibration are numbered

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from the largest to the smallest within each fundamental wave number. Comparison of the vibrational modes (calculated at DFT/B3LYP level) with the experimental values, they show

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pronounced shifts in the spectra due to hydrogen bond formation. The experimental FT-IR and

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FT-Raman spectra of pyrogallol were shown in Fig. 2. The simulated spectra of isomeric benzenetriols at DFT/B3LYP levels using different basis sets are shown in Figs. S2, S3 and S4

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(Supplementary material). From the results, a good correlation was found between experimental values and theoretical values by DFT/6-31G(d,p) and 6-311+G(d,p) basis sets.

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O-H vibrations

The O-H group vibrations [28] are likely to be most sensitive due to the environment,

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so they show pronounced shifts in the spectra of the hydrogen bonded species. The nonhydrogen bonded or a free hydroxyl group absorbs strongly in the region 3800-3000 cm-1. Intramolecular hydrogen bonding if present in 5 or 6 membered ring system may reduce the O-H stretching band to 3550-3200 cm-1. In primary and secondary alcohols, the O-H in-plane

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bending may couple with C-H bending vibrations to produce two bands near 1420 and 1330 cm-1; though these bands are of little practical value, their presence should been taken for structural confirmation. The spectra of alcohols and phenols determined in the liquid state show a broad absorption band in the region 710-517 cm-1 due to out-of-plane bending vibrations [29] of O-H group, while the OH in-plane bending vibrations [30] are occurring in the region 14201330 cm-1. In the experimental data of pyrogallol, three bands observed at 3416, 3370, 3231

cm-1 have been assigned to O-H stretching vibrations, three medium bands observed at 1360, 1311, 1287 cm-1 is assigned to O-H in-plane bending vibration, four bands observed at 762, 702, 666, 576 cm-1 have been assigned to O-H out-of-plane bending vibrations in FT-IR spectra. A medium peak observed in FT-Raman spectrum at 1341 cm-1 is assigned to O-H inplane bending vibrations, also one sharp band observed at 713 cm-1 is assigned to O-H out-ofplane bending vibrations. In the experimental data of phloroglucinol [7, 8], one band observed at 3645 cm-1 is assigned to OH stretching vibration, OH in-plane bending vibrations observed 1

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and assigned at 1500, 1316 cm-1, and OH out-of-plane observed and assigned at 684, 676 cm. The simulated wave numbers at 3854-3191 cm-1 for pyrogallol, 3840-3217 cm-1 for

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phloroglucinol, 3857-3207 cm-1 for hydroxyquinol have been assigned to O-H stretching

vibrations. The O-H in-plane bending vibrations were calculated at 1414-1284 cm-1 for pyrogallol, 1418-1324 cm-1 for phloroglucinol, 1419-1330 cm-1 for hydroxyquinol in DFT/B3LYP method is assigned to O-H in-plane bending vibrations. The O-H out-of-plane

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bending vibrations are calculated at 739-576 cm-1 for pyrogallol, 764-600 cm-1 for

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phloroglucinol, 778-599 cm-1 for hydroxyquinol have been assigned to O-H out-of-plane

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bending vibrations. The wave number of isomers are calculated through DFT/B3LYP levels

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are in good agreement with experimental values. C-H vibrations

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The C-H stretching vibrations [31, 32] appear in the range of 3300-3000 cm-1, C-H inplane bending vibrations [33-35] in the range of 1450-1000 cm-1 and C-H out-of-plane bending vibrations are in the range of 1000-750 cm-1 in aromatic compounds. In the present

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investigation of pyrogallol, two very weak bands observed in FT-IR at 3231, 3050 cm-1 and one sharp band with one shouldering band observed at 3072, 3059 cm-1 in FT-Raman were

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assigned to C-H vibrations. The eight bands of C-H in-plane-bending are observed at 1360, 1311, 1287 1240, 1184, 1138, 1059, 993 cm-1 in FT-IR and five bands at 1551, 1528, 1341, 1155, 1065 cm-1 in FT-Raman was assigned to C-H in-plane-bending vibrations. The four

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bands of C-H out-of-plane bending vibrations observed at 993, 846, 829, 762 cm-1 in FT-IR and one band at 829 cm-1 in FT-Raman was assigned to C-H out-of-plane bending vibrations. In the experimental data of phloroglucinol, one band observed at 3074 cm-1 and it is assigned to C-H vibrations. The C-H in-plane and out-of-plane bending vibrations are observed and are assigned in the range of 1500-1005 cm-1 and 819-801 cm-1, respectively. The simulated wave numbers at DFT/B3LYP levels in the range 3209-3155 cm-1 have been assigned to C-H stretching vibrations, 1578-998 cm-1 have assigned to C-H in-plane bending vibrations, 928-

722 cm-1 have been assigned to C-H out-of plane bending vibrations for pyrogallol. In the range of 3239-3144 cm-1 have been assigned to C-H stretching vibrations, 1566-1005 cm-1 have been assigned to C-H in-plane bending vibrations, 835-756 cm-1 have been assigned to C-H out-ofplane bending vibrations for phloroglucinol. For the hydroxyquinol, in the range 3229-3154 cm-1 have been assigned to C-H stretching vibrations, 1561-1112 cm-1 have been assigned to C-H in-plane bending vibrations, 984-768 cm-1 have been assigned to C-H out-of-plane

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bending vibrations. C-C vibrations

The C-C ring stretching vibrations are very prominent and highly characteristics of the

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aromatic rings. The C-C stretching vibrations of phenyl group were expected in the range of

1650-1200 cm-1 [36, 37]. For pyrogallol, there are six C-C ring vibrations, hence, the C-C ring vibrations of pyrogallol found at 1634, 1618, 1518, 1484, 1360, 1311, 1287, 1240 cm-1 in FT-

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IR and 1625, 1551, 1528, 1341 cm-1 in FT-Raman spectrum were assigned to C-C ring

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vibrations. From the experimental data of phloroglucinol, the C-C ring vibrations observed and assigned at 1640, 1636, 1500, 1316, 1210 cm-1. The theoretical values for pyrogallol were

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calculated in the range 1684-1194 cm-1 and have been assigned to C-C vibrations at

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DFT/B3LYP levels; the results showed that the experimental and theoretical data are in good agreement. The similar respective bands are assigned for phloroglucinol in the range 1689-

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1324 cm-1, and to the hydroxyquinol compound calculated in the range 1689-1279 cm-1 have been assigned to C-C ring vibrations.

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C-O vibrations

The C-O stretching vibrations or linkage in alcohols and phenols occur as a strong band

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in the region 1260-1000 cm-1. The C-O stretching mode may be coupled with the adjacent CC stretching modes. Phenols were absorbed in the region 1260-1180 cm-1 for C-O stretching vibrations [38-40]. In the present study, five bands of C-O stretching vibrations occur in the region at 1240, 1184, 1138, 1059, 993 cm-1 in FT-IR spectrum and two bands at 1155, 1065

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cm-1 in FT-Raman spectrum are assigned to C-O stretching vibrations for the pyrogallol. From the experimental data of phloroglucinol, the C-O stretching vibrations observed and assigned at 1210, 1159, 1153, 1015, 1008, 1005 cm-1. The theoretically predicted frequencies by DFT/B3LYP methods at 1281-998 cm-1 are assigned as C-O stretching to pyrogallol and its proven excellent correlation in experimental data. The corresponding bands assigned for

phloroglucinol in the range 1249-1005 cm-1, and to the hydroxyquinol compound were calculated in the range 1237-1112 cm-1. 3.3. Electronic spectra The results of the ZINDO, CIS and TDDFT calculations of electronic transition energy of isomeric benzenetriols along with oscillator strength (f) are presented in Tables 2, S9 (Supplementary material) and S10 (Supplementary material), they are compared with

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experimental data of pyrogallol in methanol. The wavelength belonging to the HOMO-LUMO transition and thus the maximum wave length appears at 270 nm. The present experiment shows, one medium band at 228 nm, a medium shouldering band at 265 nm and a weak broad

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band at 340 nm, it is good agreement with theoretical values. The experimental UV-Vis spectrum of pyrogallol is shown in Fig. 3 and the comparison of simulated electronic spectra of benzenetriols at ZINDO, CIS and TDDFT methods are shown in Figs. S5, S6 and S7,

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respectively (Supplementary material).

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3.4. HOMO-LUMO analysis

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In general, the energy of highest occupied molecular orbital (HOMO) characterizes the

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ability of electron donation, while the energy of lowest unoccupied molecular orbital (LUMO) characterizes the ability of electron reception. Also the HOMO-LUMO energy gap describes

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the molecule from the point of view of chemical reactivity and kinetic stability [41-43]. Therefore, in the present study, the HOMO-LUMO energies for benzenetriols were calculated. The electronic properties of pyrogallol corresponds to the transition from ground to first excited

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state and is mainly explained by one electron excitation from the -0.21060 eV to 0.01960 eV, as well as the way of molecule interaction with other species. As the energy gap between the

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HOMO-LUMO decreases, it is easier for HOMO to donate the electrons, where as LUMO to accept the electrons [44]. The energy of HOMO and LUMO calculated using ZINDO, CIS and TDDFT methods are presented in Tables 3, S11 (Supplementary material) and S12 (Supplementary material). As can be seen from the theoretical results, the narrowest energy

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gap was found by TDDFT/6-311G(d,p) method. The 3D plot of the HOMO-LUMO orbitals for pyrogallol, phloroglucinol and hydroxyquinol molecules are shown in Fig. 4. The positive phase is symbolized in red color and negative phase symbolized in green color, the HOMOLUMO energy gap explains the eventual charge transfer interactions within the molecule, with perfect matching of λmax with theoretical values was obtained. 4. Conclusions

Pyrogallol, one of the isomers of benzenetriols, was identified from the natural extract of Abrus precatorius Linn seeds using GC-MS technique reported for the first time. The vibrational and structural analyses of the identified compound have been done with the help of FT-IR and FT-Raman spectra. The DFT study reveals that pyrogallol and its isomers exhibit an excellent correlation in confirmation with the structure of monomer and structure of dimeric benzenetriols. Electronic spectrum of pyrogallol was compared with those of its isomers as well as HOMO-LUMO energies along with their molecular stability. The theoretically

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calculated parameters and experimentally observed data were in good agreement to confirm the structure of pyrogallol and its isomers.

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Acknowledgement

The authors thank Prof. E.M. Subramanian, Department of Chemistry, Pachaiyappas' College for Men, Kanchipuram, Tamil Nadu, India, for valuable discussions. The authors are

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thankful to SAIF, St.Peters University, Chennai and SAIF, IIT Madras, Chennai for recording

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spectral measurements. Also wish to extend thanks to SIF, School of Advanced Sciences, VIT, Vellore for recording spectral measurements.

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Figure Caption Figure Captions Fig. 1. Optimized monomer (left) and dimer (right) structure of isomeric benzenetriols at B3LYP/631G(d,p) basis set Fig. 2. Experimental vibrational spectra of pyrogallol

IP T

Fig. 3. Experimental UV-Vis spectrum of pyrogallol

A

CC E

PT

ED

M

A

N

U

SC R

Fig. 4. HOMO - LUMO plots of isomeric benzenetriols at TDDFT/6-311G (d,p) basis set.

Figure Captions Fig. 1. Optimized monomer (left) and dimer (right) structure of isomeric benzenetriols at B3LYP/6-31G(d,p) basis set Fig. 2. Experimental vibrational spectra of pyrogallol

IP T

Fig. 3. Experimental UV-Vis spectrum of pyrogallol

A

CC E

PT

ED

M

A

N

U

SC R

Fig. 4. HOMO - LUMO plots of isomeric benzenetriols at TDDFT/6-311G (d,p) basis set.

Dimer

IP T

Monomer

A

N

U

SC R

a)Pyrogallol

CC E

PT

ED

M

b) Phloroglucinol

c) Hydroxyquinol

A

Fig. 1. Optimized monomer (left) and dimer (right) structure of isomeric benzenetriols at B3LYP/6-31G(d,p) basis set

IP T SC R U N A M ED PT CC E A

Fig. 2. Experimental vibrational spectra of pyrogallol

IP T SC R U N A M ED PT CC E A

Fig. 3. Experimental UV-Vis spectrum of pyrogallol

Ground state

Eg= 0.2302 eV

EHOMO= -0.21060 eV

IP T

Excited state

ELUMO= 0.01960 eV

A

N

U

a) Pyrogallol

SC R

VVeeV

ELUMO= 0.02417 eV eee

b) Phloroglucinol

A

CC E

PT

ED

EHOMO= -0.22296 eV

M

Eg= 0.2471 eV

Eg= 0.2106 eV

EHOMO= -0.20115 eV

ELUMO= 0.00947 eV

c) Hydroxyquinol Fig. 4. HOMO - LUMO plots of isomeric benzenetriols at TDDFT/6-311G (d,p) basis set.

Table 1 Experimental and theoretical vibrational frequencies of pyrogallol

7 8

3847 3827 3785

3072, 3059 1625

3071

3208 3177

3209 3180

3191 3160

3193 3164

υCH + υOH υCH

1616

3171 1684 1662

3175 1672 1653

3155 1670 1647

3158 1664 1645

υCH (1138+846) (1184+702) υCC υCC

1551, 1528 -

-

1578

1566

1563

1558

υCC+βCH

1488

1523

1512

1510

1507

υCC+βCH

1341 -

1364 -

1414 1380 1353 1309

1404 1361 1337 1291

1401 1372 1339 1294

1394 1363 1332 1284

βCH+υCC+βOH βCH+υCC+βOH βCH+υCC+βOH βCH+υCC+βOH

1155

1288 1158, 1140 1062, 1054 991 847, 831 764 713, 707 580 -

1266 1194 1182 1164

1268 1204 1182 1173

1261 1195 1181 1166

βCH +υCC+υCO βCH+υCC+υCO βCH +υCO βCH+υCO

1094

1092

1092

βCH+υCO

1009 914 840 818 730 710 665 582 578

1001 928 839 831 739 706 674 584 576

1002 910 839 814 722 705 677 588 577

998 928 839 831 737 703 686 587 576

βCH+υCO γCH γCH γCH γCH+γOH γOH γOH γOH γOH

1281 1210 1186 1176

19

1059

1065

1100

20 21 22 23 24 25 26 27 28

993 846 829 762 702 666 576

829 713 -

A

CC E

11 12 13 14

M

15 16 17 18

1518, 1484 1360 1311, 1287 1240 1184 1138

6311G (d,p)

6311+G (d,p)

3854 3836 3791

3854 3838 3793

υOH υOH υOH

IP T

3431

ED

10

-

631+ G (d,p) 3849 3832 3790

PT

9

1981 1882 1634, 1618 -

6-31G (d,p)

SC R

6

Vibrational assignments

B3LYP

U

4 5

3416, 3370 3231 3050

Literatur e a,b,c (cm-1)

N

1 2 3

Theoretical wave number (cm-1)

A

S.No.

Experimental wave number (cm-1) FT-IR FTRaman

a

Taken from Ref [5]

b

Taken from Ref [7]

c

Taken from Ref [9]

A

CC E

PT

ED

M

A

N

33 34 35 36 37 38 39

IP T

32

SC R

30 31

560, 547 550 548 545 δring 540 515 521 521 517 515 516 515 δring 502, 506 501 501 502 502 δring 481 438, 419 427 420 393 δring 424, 410, 403 338 331 327 322 γOH 318 319 322 322 322 314 γOH 304 306 304 306 γOH 298 293 289 245 γOH 268 268 250 247 248 241 γOH+ δring 187 197 173 150 γOH 156 153 151 149 119 γOH+ δring υ-stretching; δ-bending / deformation; β-in plane bending; γ-out of plane bending

U

29

Table 2 Experimental and theoretical UV-Vis spectral characteristics of pyrogallol Stat Experime e ntal values λ (nm)

Calculated values

ZINDO

CIS

TDDFT/B3LYP

6-311G(d,p)

6-31G(d,p)

6-311G(d,p) λ(n m)

f

231

0.00 03 0.00 79 0.00 02

λ(n m)

f

λ(n m)

f

λ(n m)

f

λ(n m)

f

0.00 76 0.00 96 0.77 58

184

0.00 62 0.00 61 0.00 02

187

0.00 53 0.00 58 0.00 03

221

0.00 85 0

270

S2

228

244

S3

-

212

178 152

181 169

206

224

SC R

265

194

0

217

N

U

S1

IP T

6-31G(d,p)

ZINDO

Pyrogallol CIS TDDFT/B3LYP 6-31G(d,p) 6-311G(d,p) 6-31G(d,p) 6-311G(d,p)

ED

Parameters (eV)

M

A

Table 3 HOMO-LUMO energies and energy gap of pyrogallol

-0.30998

LUMO energy

0.03200

0.17885

0.13895

0.03898

0.01960

HOMO- LUMO energy gap

0.34541

0.48419

0.44893

0.24029

0.2302

HOMO-1 energy

-0.32315

-0.31943

-0.32334

-0.21663

-0.22537

LUMO+1 energy

0.03400

0.18092

0.15428

0.04020

0.02720

(HOMO-1)(LUMO+1) energy gap

0.35715

0.50035

0.47762

0.25683

0.25257

-0.31341

A

CC E

PT

HOMO energy

-0.30534

-0.20131

-0.21060

15