Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

DFT study of conformational and vibrational characteristics of 2-(2-hydroxyphenyl)benzothiazole molecule Urmila Pandey a, Mayuri Srivastava b, R.P. Singh a, R.A. Yadav b,⇑ a b

Department of Chemistry, UP (PG) Autonomous College, Varanasi 221002, India Lasers and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India

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

 Vibrational and conformational

The atomic orbital compositions of the frontier MO of the lowest energy conformer (C-I) of HBT are reproduced here.

characteristics of 2-(2-hydroxyphenyl)benzothiazole have been investigated.  Out of four possible conformers, C-I, C-II, C-III, and C-IV, the C-I is the most stable conformer.  C-I and C-II are planar while C-III and C-IV are non- planar structures.  HOMO–LUMO and NBO analysis have also been performed.

a r t i c l e

i n f o

Article history: Received 28 December 2013 Received in revised form 1 February 2014 Accepted 19 February 2014 Available online 21 March 2014 Keywords: Optimized molecular geometries Conformational study Vibrational study HOMO–LUMO NBO analysis

a b s t r a c t The conformational and IR and Raman spectral studies of 2-(2-hydroxyphenyl)benzothiazole have been carried out by using the DFT method at the B3LYP/6-311++G level. The detailed vibrational assignments have been done on the basis of calculated potential energy distributions. Comparative studies of molecular geometries, atomic charges and vibrational fundamentals of all the conformers have been made. There are four possible conformers for this molecule. The optimized geometrical parameters obtained by B3LYP/6-311++G method showed good agreement with the experimental X-ray data. The atomic polar tensor (APT) charges, Mulliken atomic charges, natural bond orbital (NBO) analysis and HOMO– LUMO energy gap of HBT and its conformers were also computed. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Heterocyclic compounds are widely distributed in nature and are essential to life in various ways. For e.g., most of the sugars and their derivatives, including vitamin C, exist in the form of five ⇑ Corresponding author. Tel.: +91 542 2368593; fax: +91 542 2368390. E-mail addresses: [email protected], [email protected] (R.A. Yadav). http://dx.doi.org/10.1016/j.saa.2014.02.091 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

membered (furan) or six membered (pyran) rings containing one Oxygen atom. Most member of vitamin B group possess heterocyclic ring containing N [1]. Benzothiazole, a heterocyclic compound, weak base, with varied biological properties is widely found in medicinal and bioorganic chemistry with application in drug discovery. Benzothiazole moieties are part of compounds showing various biological properties e.g., as antimicrobial [2–6], anticancer [7–10], anthelmintic [11],

62

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

antidiabetic [12], antifungal and antivirial activities. In the last few years it has been reported that benzothiazole, its bioisosters and derivatives had antimicrobial activities against Gram negative, Gram positive bacterias (e.g. Enterobacter, Pseudomonaz aeruginosa, Escherichia coli and Staphylococcus epidermides, etc.) and yeast (e.g. Candida albicans) [13]. Benzothiazole is bicyclic ring system with multiple applications, which is present in various marine or terrestrial natural compounds having useful biological properties. 2-Substituted benzothiazoles are also heterocyclic systems found to exhibit diversal biological activities such as central nervous system depressant and antitumour, antileishmanial, antibacterial activities. The survey of literature related to benzothiazole derivatives having conjugated system with donor and acceptor end groups are well known pharmaceutical substances. Pla-Dalmau [14] have prepared several 2-(2-hydroxyphenyl)benzothiazole (abbreviated as HBT) derivatives and studied the transmittance fluoroscence and light yield charactristics in a polystyrene matrix for its application in platistic scintillation. The crystal structure of HBT has been determined by Perstenson by X-ray method [15]. Purkayasha and Chattopadhyay [16] perfomed semiempirical AMI-SCI calculations to rationalize the photophysical behavior of HBT and theoretically generated the potential energy curves (PEC) for the excited state intramolecular proton transfer (ESIPT) process in different electronic states of HBT. Vivie-Riedle et al. [17] reported the involvement of skeletal deformation in the ultrafast excited state proton transfer in HBT and the identification of the vibrational modes active in the process. Liu et al. [18] synthesized five complexes of Metal(HBT)2 and made chemical and elemental analysis, IR spectroscopy, thermo gravimetric/differential thermo gravimetric analyses (TG–DTG). Experimental Raman and IR spectra and vibrational assignment of HBT have been reported earlier [19,20]. Resonance Raman spectra of HBT were measured with excitation in the range of the S0–S1 absorption band by Pfeiffer et al. [19]. They also extended their work to include deuterium shift effect of the transfer hydrogen in the resonance Raman and IR spectra [20]. HBT, its derivatives and family of molecules in azole category have been dealt with by several research workers mainly in explaining the various aspects of excited state intramolecular proton transfer [21–30]. In present work the detailed conformational study, APT and Mulliken atomic charges, optimized molecular geometries and fundamental vibrational wavenumbers along with their intensities in the IR spectrum, Raman activities and depolarization ratios of the Raman lines have been calculated using DFT method B3LYP method by employing the basis set 6-311++g basis set for the HBT molecule. The detailed vibrational assignment has been made on the basis of the calculated potential energy distributions (PEDs). The HOMO–LUMO and NBO analyses have also been performed for the four conformers of the HBT molecule.

Computational details There are two C2 axis in the HBT molecule hence four conformers are possible. The optimized molecular geometries, APT charges, Mulliken atomic charges and fundamental vibrational wavenumbers along with their corresponding intensities in IR spectrum, Raman activities and depolarization ratios of the Raman bands for the four conformers of HBT have been computed at the B3LYP/6-311++G level using the Gaussian 09 software[31]. The molecular structures of HBT were optimized by taking different orientations of OH group with respect to benzothiazole ring at B3LYP/6-311++G level for the four possible conformers. The geometries were optimized by minimizing the energies with respect to all the geometrical parameters without imposing any molecular symmetry constraints. GAR2PED software has been used

to obtain PEDs which helped in the vibrational assignments of the fundamental frequencies. Results and discussion Molecular geometries Fig. 1 shows atomic numbering scheme of the most stable conformer of HBT. The atomic numbering and optimized structures of the four conformers of HBT are shown in Fig. 2(a–d). The symmetries of the four possible conformers, their calculated total energies and energy differences from the lowest energy conformer(C-I) are given in Table 1. The conformer C-I being the lowest energy conformer, is the most stable structure. The optimized geometrical structures for all the four conformers of the HBT molecule along with the experimental parameters obtained from X-ray [15] for C-I are given in Table 2. Most of the calculated geometrical parameters for the four conformers of HBT have nearly the same magnitudes. However, some of the bond lengths, bond angles and dihedral angles show significant variations in their magnitudes. It could be seen from the Table 2 that the bonds C1AC2 of the ring R1 and the C14AC16 bond of the ring R3 are the longest in comparison to the rest five CAC bonds of the respective rings. It is probably because of the presence of the N atom at position 12 for the ring R1 and the O atom for the ring R3 at the position 14. The CAH bond lengths are nearly equal for all the conformers. The bond length C7AN12 for the C-I conformer gets elongated as compared to those in the C-II, C-III and C-IV conformers by 0.007, 0.014 and 0.016 Å respectively. This could be explained as follows: In C-I, H atom directly attached to O has ability to form hydrogen bonding with the N atom present at the position 12. Thus, due to delocalization of electrons double bond character between C7 and N12 reduces to single bond and hence, the bond length increases. The C-III and C-IV conformers are non-planar and hence, delocalization is not favorable. In C-II, there is no condition of hydrogen bonding. The bond lengths C7AC14 and C16AO24 in the C-I conformer are calculated to be shortened as compared to those in the C-II, C-III and CI-V conformers by 0.015, 0.016 and 0.014 Å respectively and the O24AH25 bond length in C-I conformer lengthened by 0.025, 0.022 and 0.025 Å as compared to C-II, C-III and C-IV conformers respectively. This is because of strong tendency of formation of hydrogen bond between the N and H atoms in C-I conformer. The bond angle a(N12AC7AC12) in C-IV conformer is found to be the largest in comparison to the other conformers. This could be due to the presence of two electronegative atoms O and N in the vicinity of each other which repel each other resulting in increment of the angle. The bond angle a(N12AC7AC14) is found to be the largest and the angle (S13AC7AC14) to be the lowest in the C-IV conformer in comparison to C-I, C-II and C-III. This is because of presence of two electronegative atoms O and N in close vicinity causing repulsion of one another. The bond angle a(C2AN12AC7) which is 112.0° in C-IV conformer is found to be the smallest among C-I, C-II, C-III and C-IV.

Fig. 1. Atomic numbering scheme of HBT.

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

63

Fig. 2. Front and Lateral view of (a) conformer C-I, (b) conformer C-II, (c) conformer C-III, and (d) conformer C-IV, of HBT. Table 1 Conformers, their total energy and point group for HBT. S.No.

Conformers

Point groups

Total energy (Hartree)

Relative energy Kcal/mol

1 2 3 4

C-I C-II C-III C-IV

Cs Cs C1 C1

1029.18488483 1029.17129414 1029.17011280 1029.16592044

0 8.534 9.287 11.923

This could be also as a consequence of presence of the N and O atoms close to one another. Because of absence of favorable condition for H-bonding in C-III conformer, the bond angle a(C14ANC16AO24) is found to be the least among C-I, C-II, C-III and C-IV. Similarly because of the absence of favorable condition of hydrogen bonding in C-III conformer the bond angle a(C14AC16AO24) is found to be the largest (124.4°) compared to those of C-I, C-II and C-IV respectively (123.0, 118.1 and 118.7°). If one compares the bond angle a(C16AO24AH25) which is 108.3, 110.3, 111.5 and 109.4° in C-I, C-II, C-III and C-IV respectively, it is found to be the lowest in C-I due to the presence of favorable condition for the hydrogen bonding between N and H. From Table 2, it could be seen that all the dihedral angles for the four conformers are nearly equal, except the angles N12AC7AC14AC15, N12AC7AC14AC16, S13AC7AC14AC15, S13AC7A C14AC16, C14AC16AO24AH25 and C19AC16AO24AH25. The dihedral angles N12AC7AC14AC15, S13AC7AC14AC16, C14AC16AO24AH25 increase in going from C-III to C-IV while N12AC7AC14AC16, S13A C7AC14AC15, C19AC16AO24AH25 decrease in going from C-III to C-IV. These angles are either 0° or 180° in the C-I and C-II conformer. APT charges APT (atomic polarizability tensor) charges are sum of charge tensor and charge flux tensor, leading to a charge–charge flux model [32]. The APT charges for the four conformers of HBT(C-I, C-II, C-III

and C-IV) calculated at the B3LYP/6-311++G level are collected in Table 3. In the ring R1, all the C atoms possess positive charge, except C4 and C5, in all the conformers, in addition to C1 in C-II conformer and C6 in C-IV conformer. The magnitudes of positive charge on C7 carbon atom is the highest followed by that on the C2 carbon, because these are attached to electronegative atoms. In the ring R3, all the C atoms are positive except the atoms C14, C17 and C18 in all the conformers. The C16 atom has the highest positive charge due to direct attachment of O atom to it. Its magnitude is the highest in the C-III conformer. From Table 3, it could be seen that the magnitude of charges on the N, O and S atoms are negative because these are electronegative elements and since O is the strongest electronegative amongst these, its negative charge is the highest. The O atom has the highest negative charge in the C-I conformer in comparison to the other conformers because of hydrogen bonding between the N and H atoms. Similarly N has higher negative charge in the C-I than C-II, C-III and C-IV conformers. The H25 atom bears the highest positive charge in comparison to the other hydrogen in all the conformers because it is directly attached to an electronegative atom, O. The magnitude of the charge at the H25 atom is the highest in the C-I conformer. Mulliken atomic charges Mulliken atomic charges [33] depend on the basis set and have an important role in the application of quantum chemical calculation for the molecular systems since atomic charges effect dipole moment, polarizability, electronic structure and much more properties of molecular system. Mulliken atomic charges (in unit of e) at various atomic sites of the four conformers of HBT have been calculated at the B3LYP/6-311++G level and are also collected in the Table 3. From Table 3, it could be seen that for the ring R1 all the carbon atoms bear positive charges, except C1 in all the conformer, C2 in C-I and C-IV, C6 in C-II and C7 in C-I, C-III and C-IV. The

64

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

Table 2 Experimental and geometrical parametersa of the four conformers of HBT. S.No

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

Parametersa

r(C1AC2) r(C1AC6) r(C1AS13) r(C2AC3) r(C2AN12) r(C3AC4) r(C3AH8) r(C4AC5) r(C4AH9) r(C5AC6) r(C5AH10) r(C6AH11) r(C7AN12) r(C7AS13) r(C7AC14) r(C14AC15) r(C14AC16) r(C15AC17) r(C15AH18) r(C16AC19) r(C16AO24) r(C17AC20) r(C17AH21) r(C19AC20) r(C19AH22) r(C20AH23) r(O24AH25) a(C2AC1AC6) a(C2AC1AS13) a(C6AC1AS13) a(C1AC2AC3) a(C1AC2AN12) a(C3AC2AN12) a(C2AC3AC4) a(C2AC3AH8) a(C4AC3AH8) a(C3AC4AC5) a(C3AC4AH9) a(C5AC4AH9) a(C4AC5AC6) a(C4AC5AH10) a(C6AC5AH10) a(C1AC6AC5) a(C1AC6AH11) a(C5AC6AH11) a(N12AC7AS13) a(N12AC7AC14) a(S13AC7AC14) a(C2AN12AC7) a(C1AS13AC7) a(C7AC14AC15) a(C7AC14AC16) a(C15AC14AC16) a(C14AC15AC17) a(C14AC15AH18) a(C17AC15AH18) a(C14AC16AC19) a(C14AC16AO24) a(C19AC16AO24) a(C15AC17AC20) a(C15AC17AH21) a(C20AC17AH21) a(C16AC19AC20) a(C16AC19AH22) a(C20AC19AH22) a(C17AC20AC19) a(C17AC20AH23) a(C19AC20AH23) a(C16AO24AH25) d(C6AC1AC2AC3) d(C6AC1AC2AN12) d(S13AC1AC2AC3) d(S13AC1AC2AN12)

C-I

C-II

C-III

C-IV

1.413 1.397 1.747 1.403 1.377 1.386 1.083 1.406 1.084 1.390 1.084 1.084 1.300 1.787 1.471 1.407 1.407 1.386 1.082 1.396 1.366 1.396 1.083 1.389 1.086 1.084 0.963 121.4 109.4 129.3 119.6 115.1 125.3 119.0 119.4 121.7 120.9 119.7 119.4 121.0 119.6 119.4 118.2 121.1 120.7 114.4 121.8 123.8 112.4 88.8 118.2 124.3 117.6 121.8 117.4 120.8 120.7 118.1 121.1 119.7 120.0 120.4 120.4 119.2 120.3 119.8 120.6 119.6 110.3 0.0 180.0 180.0 0.0

1.413 1.395 1.751 1.401 1.382 1.387 1.083 1.404 1.084 1.391 1.084 1.083 1.293 1.800 1.470 1.408 1.410 1.385 1.083 1.398 1.364 1.398 1.083 1.387 1.083 1.084 0.966 121.7 109.0 129.3 119.4 115.5 125.1 119.0 119.3 121.7 121.0 119.6 119.4 121.0 119.7 119.4 118.0 121.3 120.8 114.1 124.0 121.8 112.5 88.9 117.8 123.9 118.3 121.5 117.8 120.7 120.0 124.4 115.7 119.4 120.1 120.5 120.5 118.0 121.5 120.3 120.2 119.5 111.5 0.1 179.3 179.1 1.5

1.415 1.396 1.748 1.401 1.382 1.387 1.083 1.405 1.084 1.390 1.084 1.083 1.291 1.794 1.472 1.402 1.411 1.390 1.084 1.397 1.360 1.393 1.083 1.390 1.086 1.084 0.963 121.4 109.1 129.5 119.5 115.5 125.0 119.1 119.2 121.7 120.9 119.7 119.4 121.0 119.6 119.4 118.1 121.2 120.7 114.8 126.1 119.1 112.0 88.6 120.1 121.7 118.2 121.9 118.5 119.6 119.9 118.7 121.4 119.3 120.1 120.6 120.8 119.1 120.1 120.1 120.4 119.5 109.4 0.1 179.7 179.8 0.6

b

Calculated

Experimental

1.412 1.394 1.753 1.400 1.382 1.388 1.083 1.404 1.084 1.391 1.084 1.083 1.307 1.781 1.456 1.408 1.421 1.383 1.085 1.402 1.343 1.401 1.083 1.385 1.083 1.084 0.988 121.5 109.5 129.1 119.7 114.7 125.7 118.9 119.6 121.5 121.0 119.6 119.5 121.0 119.7 119.4 118.1 121.2 120.7 114.1 123.5 122.5 112.8 89.0 121.6 119.9 118.5 121.6 119.1 119.3 119.4 123.0 117.6 119.3 120.2 120.5 120.6 117.9 121.5 120.6 119.9 119.5 108.3 0.0 180.0 180.0 0.0

1.372 1.399 1.757 1.369 1.404 1.386 1.02 1.365 0.99 1.377 0.94 0.96 – 1.749 1.481 1.389 1.411 1.356 1.04 1.422 1.305 1.359 1.07 1.349 0.88 0.92 – 122.1 109.2 – 119.5 115.4 – 118.8 – – 121.6 – – 120.6 – – 117.3 – – 116.1 – 120.8 110.8 88.6 121.6 – 120.3 121.5 – – 116.4 – – 118.3 – – 120.0 – – 123.4 – – – – – – –

65

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73 Table 2 (continued) S.No

74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 a b

Parametersa

d(C2AC1AC6AC5) d(C2AC1AC6AH11) d(S13AC1AC6AC5) d(S13AC1AC6AH11) d(C2AC1AS13AC7) d (C6AC1AS13AC7) d(C1AC2AC3AC4) d(C1AC2AC3AH8) d(N12AC2AC3AC4) d(N12AC2AC3AH8) d(C1AC2AN12AC7) d(C3AC2AN12AC7) d(C2AC3AC4AC5) d(C2AC3AC4AH9) d(H8AC3AC4AC5) d(H8AC3AC4AH9) d(C3AC4AC5AC6) d(C3AC4AC5AH10) d(H9AC4AC5AC6) d(H9AC4AC5AH10) d(C4AC5AC6AC1) d(C4AC5AC6AH11) d(H10AC5AC6AC1) d(H10AC5AC6AH11) d(S13AC7AN12AC2) d(C14AC7AN12AC2) d(N12AC7AN13AC1) d(C14AC7AS13AC1) d(N12AC7AC14AC15) d(N12AC7AC14AC16) d(S13AC7AC14AC15) d(S13AC7AC14AC16) d(C7AC14AC15AC17) d(C7AC14AC15AH18) d(C16AC14AC15AC17) d(C16AC14AC15AH18) d(C7AC14AC16AC19) d(C7AC14AC16AO24) d(C15AC14AC16AC19) d(C15AC14AC16AO24) d(C14AC15AC17AC20) d(C14AC15AC17AH21) d(H18AC15AC17AC20) d(H18AC15AC17AH21) d(C14AC16AC19AC20) d(C14AC16AC19AH22) d(O24AC16AC19AC20) d(O24AC16AC19AH22) d(C14AC16AO24AH25) d(C19AC16AO24AH25) d(C15AC17AC20AC19) d(C15AC17AC20AH23) d(H21AC17AC20AC19) d(H21AC17AC20AH23) d(C16AC19AC20AC17) d(C16AC19AC20AH23) d(H22AC19AC20AC17) d(H22AC19AC20AH23)

C-I Calculated

Experimentalb

0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 179.9 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

C-II

C-III

C-IV

0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 0.0 180.0 180.0 0.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0

0.1 180.0 178.9 1.0 1.7 179.0 0.1 180.0 179.2 0.6 0.3 179.7 0.0 179.9 179.9 0.1 0.0 180.0 180.0 0.0 0.0 179.9 180.0 0.1 1.1 177.9 1.7 178.6 31.6 147.1 145.0 36.3 179.8 0.2 1.5 179.0 179.8 1.0 1.5 177.7 0.4 179.3 180.0 0.2 0.5 179.8 178.8 0.6 7.1 172.0 0.6 179.8 179.7 0.1 0.6 179.8 178.7 0.9

0.0 180.0 179.7 0.3 0.7 179.6 0.1 179.9 179.7 0.3 0.0 179.6 0.1 179.9 180.0 0.0 0.0 180.0 -180.0 0.0 0.1 180.0 180.0 0.0 0.6 179.0 0.7 178.9 139.2 41.1 40.4 139.4 179.5 0.9 0.8 179.4 179.5 2.0 0.8 177.7 0.3 179.3 178.9 0.7 0.3 179.9 178.1 1.407 178.0 0.5 0.3 179.8 179.8 0.3 0.221 179.9 179.3 0.6

Bond lengths in Å and bond angles and dihedral angles in degrees. Ref. [15].

N12 atom has positive charge except in the C-IV conformer, where it bears charge 0.014 a.u. The magnitude of charge on S13 is negative and it is the least in the C-II conformer and highest in the C-IV conformer. For the ring R3, similar to the ring R1, all the carbon atoms possess negative charge, except C14 in all the conformers. The magnitude of charges on O24 is negative for all the conformers and maximum in C-I which is 0.298 a.u. The value of positive charge on H is the highest for H25 in all the conformer due to attachment with electronegative O16 atom. Its value is increased by 0.181, 0.100 and 0.182 a.u. in comparison to C-II, C-III and CIV conformers respectively.

Vibrational assignments The molecule HBT is a 25 atomic molecule having 69 normal modes of vibration shown in Table 1 (supplementary material). The computed IR and Raman spectra for the four conformers of HBT are shown in Figs. 3A–3D. The calculated fundamental wavenumbers along with their IR intensities, Raman activities, depolarization ratio for the Raman bands and the proposed vibrational assignments are collected in Table 4 for the C-I conformer. The computed frequencies for all the four conformers have been compared in Table 5. The experimentally observed IR and Raman

66

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

Table 3 Calculated APT and Mulliken Charges of the four conformers of HBT. Atoms

C1 C2 C3 C4 C5 C6 C7 H8 H9 H10 H11 N12 S13 C14 C15 C16 C17 H18 C19 C20 H21 H22 H23 O24 H25

C-I

C-II

C-III

C-IV

APT charges

Mulliken charges

APT charges

Mulliken charges

APT charges

Mulliken charges

APT charges

Mulliken charges

0.009 0.161 0.019 0.094 0.078 0.015 0.768 0.056 0.034 0.035 0.050 0.634 0.159 0.382 0.139 0.470 0.273 0.053 0.089 0.069 0.033 0.051 0.036 0.705 0.416

0.802 0.188 0.537 0.228 0.241 0.331 0.267 0.162 0.182 0.164 0.188 0.087 0.528 0.796 0.538 0.136 0.598 0.096 0.331 0.134 0.175 0.183 0.164 0.298 0.446

0.027 0.122 0.034 0.104 -0.081 0.123 0.585 0.051 0.128 0.029 0.043 0.542 0.058 0.148 0.027 0.440 0.167 0.092 0.108 0.015 0.031 0.026 0.037 0.632 0.282

1.732 1.379 0.644 0.371 0.006 0.182 0.170 0.179 0.166 0.165 0.170 0.204 0.381 1.407 0.542 0.613 0.426 0.186 0.296 0.106 0.170 0.127 0.165 0.184 0.265

0.018 0.149 0.014 0.079 0.069 0.004 0.682 0.056 0.033 0.034 0.050 0.573 0.193 0.255 0.060 0.511 0.196 0.071 0.130 0.059 0.030 0.052 0.036 0.680 0.325

1.348 0.607 0.687 0.230 0.240 0.184 0.380 0.179 0.179 0.166 0.182 0.213 0.543 1.179 0.555 0.359 0.375 0.197 0.570 0.280 0.175 0.189 0.163 0.266 0.346

0.023 0.128 0.024 0.092 0.063 0.008 0.615 0.054 0.029 0.029 0.045 0.530 0.180 0.147 0.051 0.467 0.187 0.060 0.114 0.037 0.032 0.022 0.036 0.604 0.273

0.738 0.197 0.373 0.094 0.323 0.450 0.197 0.185 0.179 0.167 0.182 0.014 0.581 0.556 0.468 0.188 0.206 0.146 0.341 0.089 0.178 0.134 0.166 0.162 0.264

Fig. 3A. Calculated IR and Raman spectra of HBT (C-I conformer).

Fig. 3B. Calculated IR and Raman spectra of HBT (C-II conformer).

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

67

Fig. 3D. Calculated IR and Raman spectrum of HBT (C-IV conformer). Fig. 3C. Calculated IR and Raman spectra of HBT (C-III conformer).

wavenumbers for the C-I conformer are also included in the Table 4. With the help of computed PEDs the normal mode assignments have been proposed. The vibrational assignments for the fundamentals of the HBT molecule could be discussed under the following sections: O-H modes (3 modes) There are m(OAH), a(CAOAH) and s(CAOH) modes due to a one OAH group attached to the ring R3. The modes m27, m51, m69 in the CI conformer; m8, m44, m69 in the C-II conformer; m12, m51, m69 in the CIII conformer and m9, m44, m69 in the C-IV conformer correspond to these modes respectively. The OAH stretching vibration lies in the region 3200– 3600 cm1. Bogatyreva [34] reported m(OAH) vibration at 3620 cm1 for phenol. The strong band at 3314 cm1 in the C-I conformer, 3662 cm1 in the C-II conformer, 3577 cm1 in the C-III conformer and 3659 cm1 in the C-IV conformer correspond to this vibration according to the present calculation. The region for angle bending of (CAOAH) mode lies in the range 1150– 1450 cm1 [35]. In the present calculation, the wavenumbers 1378 cm1 in C-I, 1136 cm1 in C-II, 1302 cm1 in C-III and 1139 cm1 in C-IV correspond to the a(CAOAH) mode. These vibrations are coupled with many other modes. The frequencies 781 cm1 in C-I, 266 cm1 in C-II, 437 cm1 in C-III and 323 cm1 in C-IV correspond to s(C16AOH) mode. The

transition mode is not a pure mode. It couple with many other modes, e.g. in C-I it is coupled with U(R2); In C-II, this mode is coupled with c(C16AO24) while in C-III and C-IV this mode is coupled with U(R1), Bf(R1–R2) and U(R1), U(R2) respectively. CAH modes (24) There are 8m(CAH), 8b(CAH), 8c(CAH) due to 8CAH bonds of benzene ring (R1) and phenol ring (R3). The modes m61 to m68 in all the conformers correspond to the stretching modes. The modes m40, m41, m42, m43, m44, m46, m48 and m52 in C-I correspond to the inplane bending mode. These modes in C-II, C-III and C-IV correspond to (m40, m41, m42, m43, m47, m48, m49 and m53), (m40, v41, m42, m43, m47, m48, m51 and m54) and (m40, m41, m42, m43, m46, m48, m51 and m53) respectively. The m(CAH) vibrations lie in the region 3000–3100 cm1 and are not affected much due to the nature of substituents [36]. Also the CAH stretching vibrations are pure CAH modes and are highly localized. The bands at 3025, 3029, 3031, 3040, 3048, 3049, 3054 and 3055 cm1 in C-I conformer; 3009, 3025, 3034, 3036, 3046, 3048, 3053 and 3068 cm1 in C-II conformer; 3028, 3029, 3040, 3044, 3049, 3053, 3056 and 3061 cm1 in C-III conformer and 3007, 3026, 3031, 3037, 3039, 3047, 3047, 3054 and 3055 cm1 in C-IV conformer correspond to CAH stretching vibrations respectively. The bands corresponding to the out-of-plane and in-plane CAH deformations fall in the regions 600–1000 cm1 and 1000– 1400 cm1 [37,38] respectively. The present calculation places the 8 b(CAH) modes at 1092, 1096, 1129, 1132, 1181, 1209, 1225 and 1400 cm1 in the C-I conformer; 1064, 1094, 1130, 1131, 1220,

68

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

Table 4 Calculated and observed vibrational frequencies and assignments for the most stable conformer C-1 of HBT. S. no

Calculatedd Unscaled freq (cm

Observed 1

)

c

Scaled freq (cm

1

)

IR

a

Potential energy distributions Raman

m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17 m18 m19 m20 m21 m22 m23 m24 m25 m26

44(0,0)0.75 61(1,0)0.75 119(1,0)0.10 147(0,4)0.75 193(1,1)0.75 240(0,1)0.75 266(2,3)0.1 297(5,2)0.72 318(0,0)0.75 404(0,3)0.69 429(2,0)0.75 457(7,0)0.75 505(4,5)0.19 510(0,15)0.24 524(4,0)0.75 545(2,4)0.72 563(0,0)0.75 590(6,9)0.74 626(0,1)0.75 676(2,8)0.16 700(35,4)0.16 720(1,28)0.12 722(17,0)0.75 743(0,1)0.75 758(77,1)0.75 764(66,1)0.75

43 60 117 144 189 235 260 291 311 395 420 447 494 499 513 533 551 577 613 662 685 705 707 727 742 748

– – – – – – – – – – – – – – – – – – – – – – – – – –

– – 118 – – – 266 293 – – – – – – 505 537 – 580 – 663 – – 715 – – –

m27 m28 m29 m30 m31 m32 m33 m34 m35 m36 m37 m38 m39 m40 m41 m42 m43 m44 m45 m46 m47 m48 m49

798(63,1)0.75 832(16,32)0.17 859(2,0)0.75 865(1,0)0.75 884(3,24)0.13 947(1,0)0.75 949(1,0)0.75 978((43,1)0.11 984(0,0)0.75 985(0,0)0.75 1039(7,66)0.10 1057(10,48)0.10 1082(5,84)0.25 1143(3,25)0.34 1148(5,68)0.26 1182(28,17)0.33 1185(1,12)0.66 1237(118,75)0.23 1266(41,404)0.27 1283(12,23)0.21 1300(39,10)0.63 1317(1,45)0.34 1347(14,29)0.36

781 814 841 847 865 927 929 957 963 964 992 1009 1033 1092 1096 1129 1132 1181 1209 1225 1242 1258 1286

– – – – – – – – – – – 1017 1035 1071 – 1122 1128 1155 – 1217 1242 1254 1272

– 818 – – 870 – – – – 970 – – – 1071 – 1126 A 1156 – – 1243 – 1271

m50

1351(14,223)0.28

1290

–

–

m51 m52 m53 m54 m55

1443(31,7)0.29 1466(58,702)0.37 1481(14,403)0.34 1490(59,894)0.33 1522(132,241)0.33

1378 1400 1414 1423 1454

– 1406 – 1441 1460

– – – – 1464

m56 m57 m58

1542(66,1019)0.31 1598(5,62)0.54 1617(76,5)0.75

1473 1526 1544

1483 1504 –

– 1508 –

m59 m60 m61 m62 m63 m64 m65

1630(5,180)0.43 1660(71,430)0.39 3167(1,20)0.52 3172(1,43)0.75 3174(9,102)0.70 3183(5,141)0.71 3192(12,111)0.46

1557 1585 3025 3029 3031 3040 3048

1561 1588 – – – – –

– – – – – – –

b

s(C7AC14) (62) + s(O24AH25) (16) Bf(R1AR2) (22) + c(C7AC14) (21) + c(C7AC14) (19) + U(R2) (15) b(C7AC14) (52) + b(C7AC14) (21) U(R3) (28) + U(R2) (15) + s(O24AH25) (14) Bf(R1AR2) (60) + U(R2) (18) + U (R1) (13) U(R3) (36) + c(C7AC14) (17) m(C7AC14) (25) + a(R3) (24) + m(C7AS13) (10) b(C7AC14) (45) + m(C7AS13) (10) U(R1) (20) + U(R2) (18) + U(R3) (15) + U(R3) (14) b(C7AC14) (21) + a(R1) (19) + m(C1AS13) (17) + a(R2) (11) U(R1) (41) + Bf(R1AR2) (21) + U(R1) (15) U (R3) (26) + c(C7AC14) (19) + U(R3) (15) + U(R2) (12) + U(R1) (10) b(C16AO24) (58) + b(C7AC14) (11) a(R2) (50) + m(C1AS13) (14) + a(R1) (11) c(C16AO24) (23) + U(R3) (15) a(R3) (36) + a(R3) (15) a(R1) (12) U(R3) (12) + U(R3) (12) + U(R1) (11)+U(R2) (10) a(R3) (31) + a(R1) (31) U(R2) (41) + Bf(R1AR2) (24) + c(C7AC14) (16) a(R3) (29) + a(R1) (22) + a(R2) (16) m(C7AS13) (29) + b(C7AC14) (13) + b(C7AC14) (12) + a(R1) (12) a(R1) (32) + m(C1AS13) (20) + m(C7AS13) (10) U(R1) (49) + U(R2) (13) + U(R2) (12) + c(C4AH9) (10) U(R3) (51) + c(C16AO24) (21) + c(C7AC14) (16) c(C17AH21) (37) + c(C15AH18) (21) + c(C20AH23) (16) c(C5AH10) (31) + U(R1) (13) + c(C6AH11) (12) + c(C3AH8) (12) + c(C4AH9) (11) s(C16AO24AH25) (77) + U(R2) (5) m(C14AC16) (17) + m(C16AO24) (16) + m(C16AC19) (12) + a(R3) (12) c(C6AH11) (34) + c(C3AH8) (33) + c(C4AH9) (13) c(C19AC22) (36) + c(C15AH18) (19) + U(R3) (13) + c(C16AO24) (10) a(R1) (24) + a(R3) (19) + m(C2AN12) (11) c(C15AH18) (41) + c(C17AH21) (27) + c(C19AC22) (14) c(C6AH11) (28) + c(C3AH8) (28) + c(C5AH10) (22) a(R3) (29) + a(R2) (23) + a(R1) (13) + m(C7A S13) (11) c(C4AH9) (27) + c(C5AH10) (19) + c(C20AH23) (16) c(C20AH23) (35) + c(C19AH22) (12) + c(C4AH9) (12) + c(C17AH21) (11) m(C4AC5) (44) + m(C5AC6) (15) + m(C3AC4) (14) + b(C3AH8) (11) m(C17AC20) (47) + b(C19AH22) (13) + m(C19AC20) (13) + m(C15AC17) (8) a(R1) (35) + m(C1AS13) (21) + m(C1AC2) (12) b(C20AH23) (19) + mC15AC17) (16) b(C3AH8) (23) + b(C4AH9) (21) + m(C3AC4) (13) b(C17AH21) (27) + b(C20AH23) (18) + b(C19AH22) (13) b(C5AH10) (33) + b(C4AH9) (15) + b(C6AH11) (10) m(C16AC19) (20) + m(C7AC14) (15) + b(C19AH22) (10) m(C2AN12) (22) + b(C15AH18) (15) + m(C14AC15) (13) b(C6AH11) (17) + b(C15AH18) (15) + b(C3AH8) (11) + m(C14AC15) (10) m(C16AO24) (28) + a(R3) m(C2AN12) (17) + m(C7AC14) (11) + b(C3AH8)(9) m(C2AC3) (14) + m(C1AC6) (14) + m(C4AC5) (13) + m(C1AC2) (13) + m(C5AC6) (10) m(C14AC16) (19) + m(C19AC20) (14) + m(C15AC17) (10) + m(C16AO24) (10) + b(C19AH22) (7)+m(C14AC15) (6) a(C16AO24AH25) (32) + b(C20AH23) (17) + b(C15AH18) (11) b(C6AH11) (22) + b(C3AH8) (15) b(C4AH9) (10) + m(C7AN12) (9) + m(C5AC6) (8) b(C5AH10) (15) + b(C4AH9) (11) + m(C1AC6) (8) b(C15AH18) (13) + b(C19AH22) (13) + m(C7AN12) (13) + m(C17AC20) (11) + a(C16AO24AH25) (10) m(C7AN12) (29) + m(C7AC14) (12) m(C4AC5) (27) + m(C1AC2) (14) m(C19AC20) (17) + m(C17AC20) (16) + m(C14AC16) (13) + a(C16AO24AH25) + b(C20AH23) m(C3AC4) (17) + m(C2AC3) (16) + m(C5AC6) (13) + m(C1AC6) (11) m(C16AC19) (18) + m(C15AC17) (17) + m(C14AC15) (10) m(C15AH18) (68) + m(C17AH21) (17) + m(C20AH23) (12) m(C5AH10) (47) + m(C4AH9) (35) + m(C6AH11) (12) m(C20AH23) (66) + m(C15AH18) (22) m(C4AH9) (34) + m(C6AH11) (33) + m(C3AH8) (20) + m(C5AH10) (13) m(C6AH11) (44) + m(C3AH8) (32) + m(C5AH10) (22)

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

m66 m67 m68 m69

3193(10,113)0.69 3198(14,267)0.12 3199(9,397)0.12 3314(493,137)0.26

3049 3054 3055 3165

– – – –

– – – –

69

m(C17AH21) (51) + m(C19AH22) (43) m(C3AH8) (42) + m(C4AH9) (30) + m(C5AH10) (17) + m(C6AH11) (10) m(C19AH22) (46) + m(C17AH21) (29) + m(C20AH23) (20) m(O24AH25) (100)

The no. before the modes are the % potential energy calculated using normal coordinate analysis. The abbreviations are – m = stretching c = out-of-plane deformation, b = inplane deformation, a = planar ring deformation, U = non-planar ring deformation, Bf = butterfly mode. a From Ref. [53]. b From Ref. [20]. c Calculated wave numbers below 1000 cm1 were scaled by the scale factor 0.9786 and those above 1000 cm1 by the scale factor 0.9550 for larger wave numbers. d Number outside bracket is frequency in cm1 unit, numbers within the bracket are IR intensity and Raman activity and number outside bracket is depolarization ratio.

1255, 1270 and 1414 cm1 in the C-II conformer; 1066, 1094, 1126, 1131, 1148, 1216, 1419 and cm1 in the C-III conformer and 1074, 1092, 1131, 1132, 1213, 1251, 1298 and 1408 cm1 in the C-IV conformer. These vibrations are coupled with the other bending and stretching modes. The frequencies 748, 841, 847, 929, 963, 964, 927 and 742 cm1 in the C-I conformer; 746, 751, 837, 841, 930, 940, 961 and 976 cm1 in the C-II conformer; 754, 756, 848, 856, 935, 950, 968 and 971 cm1 in the C-III conformer and 744, 753, 837, 846, 931, 934, 963 and 966 cm1 in the C-IV conformer respectively correspond to the out-of-plane deformation modes. These are in accordance with IR absorption due to non-planer CAH deformation modes for the aromatic compounds [37]. All the out-of-plane bending modes are coupled with many other vibrations. CAOH modes (3 modes) The CAO(H) group attached to the benzene ring(R3) has 3 normal modes as-a m{CAO(H)}, a b{CAO(H)}and a c{CAO(H)}modes. The modes m47, m13 and m15 in the C-I conformer; m46, m14 and m16 in the C-II conformer; m45, m14 and m16 in the C-III conformer and m47, m14 and m16 in the C-IV conformer correspond to these vibrations respectively. The CAO stretching mode in alcohols and phenols produces a band in the region 1000–1300 cm1. Bogatyreva [34] reported m(CAO) vibration at 1262 cm1 for phenol. In the present calculation the bands at 1242 cm1 in C-I, 1208 cm1 in C-II, 1187 cm1 in C-III and 1225 cm1 in C-IV correspond to the m(C16AO24) vibration, which is in good agreement with the earlier result [20]. The m(C16AO24) and c(C16AO24) modes in the present case correspond to 494 and 513 cm1 in the C-I conformer; 467 and 521 cm1 in the C-II conformer; 475 and 520 cm1 in the C-III conformer and 471 and 522 cm1 in the C-IV conformer respectively. These modes are also coupled with many other modes. C7AC14 modes (6 modes) There are 1m(C7AC14), 2b(C7AC14), 2c(C7AC14), 1s(C7AC14) modes due to C7AC14 bond, connecting the benzothiazole (R1R2) and phenol (R3) rings to each other. In the present calculation m7 with wavenumber 260 cm1 in the C-I conformer, m9 with 270 cm1 in the C-II conformer, m7 with 248 cm1 in the C-III conformer and m7 with 246 cm1 correspond to the m(C7AC14) mode. The planar bending mode b(C7AC14) in the present calculation corresponds to 117 and 291 cm1 in the C-I conformer; 116 and 244 cm1 in the C-II conformer; 96 and 275 cm1 in the C-III conformer and 82 and 280 cm1 in the C-IV conformer. The wavenumbers 60 cm1(m2) and 235 cm1(m6) in C-I; 59 cm1(m2) and 453 cm1(m13) in C-II, 60 cm1 (m2) and 213 cm1(m6) in C-III and 58 cm1(m2) and 204 cm1(m6) in C-IV are assigned to the c(C7AC14) modes. The frequencies corresponding to m1 in all the conformers correspond to the s(C7AC14) mode which are calculated to be 43, 29, 38 and 28 cm1 in C-I, C-II, C-III and C-IV respectively.

Ring modes (33 modes) Ring modes can be divided into 4 categories as: (a) phenyl ring modes of benzothiazole moiety (R1) (12 modes); (b) Modes of thiazole ring (R2) (8 modes); (c) Modes of phenyl ring (R3) (12 modes) and (d) Butterfly mode (R1-R2) (1 mode)

Phenyl ring modes of benzothiazole moiety (R1) (12 modes) The 12 modes of vibrations due to the R1 ring are the 6 stretching modes – m37, m49, m53, m54, m57, and m59 in the C-I conformer; m37, m50, m52, m54, m57 and m59 in the C-II conformer; m37, m50, m52, m53, m57 and m59 in the C-III conformer and m37, m50, m52, m54, m57 and m59 in the C-IV conformer; the 3 in-plane ring deformation modes - m10, m19 and m39 in the C-I conformer; m11, m18 and m39 in the C-II conformer; m10, m18 and m39 in the C-III conformer and m11, m18 and m39 in the C-IV conformer and the 3 out-of-plane ring deformation modes – m11, m17 and m23 in the C-I conformer; m10, m12 and m24 in the C-II conformer; m11, m13 and m24 in the C-III conformer; m12, m18 and m24 in C-IV. The highest ring R1 stretching mode is m59 and it corresponds to the C@C ring stretching mode which is calculated to be 1557 cm1 corresponding to m(C2AC3) in the C-I conformer, 1561 cm1 in the C-II conformer, 1561 cm1 in the C-III conformer and 1561 cm1 in the C-IV conformer. Our calculation places the other ring stretching modes at 1526, 1423, 1414, 1286 and 992 cm1 in the C-I conformer; 1520, 1419, 1394, 1286 and 989 cm1 in the C-II conformer; 1525, 1419, 1397, 1284 and 990 cm1 in the C-III conformer and 1521, 1419, 1395, 1284 and 990 cm1 in the C-IV conformer respectively. The frequencies corresponding to the m37 and m38 modes, in all the conformers represent the ring breathing modes for the R1 and R2 rings respectively. This mode is strongly coupled with the b (C3AH8) in all the conformers. The 3 in-plane-deformation modes are calculated to be 395, 577 and 1033 cm1 in the C-I conformer; 388, 582 and 1038 cm1 in the C-II conformer; 378, 580 and 1032 cm1 in the C-III conformer and 383, 578 and 1032 cm1 in the C-IV conformer. The three outof-plane ring deformation modes are calculated to be 420, 551, 707 cm1 for the C-I conformer; 315, 425 and 712 cm1 for the C-II conformer; 414, 454 and 718 cm1 for the C-III conformer; 425, 558 and 718 cm1 for the C-IV conformer.

Modes of thiazole ring (R2) (8 modes) The 8 modes of vibration due to the ring R2 are the 4 stretching modes - m(C1AS13), m(C7AS13), m(C2AN12) and m(C7AN12) mode; two in-plane ring deformation modes a(R2) and 2 out-of-plane ring deformation modes U(R2). The CAS bond is highly polarisable and hence produces stronger spectral activity. The CAS stretching vibration is expected in the region 710–685 cm1 [39]. In the present calculation the m(C1AS13) and m(C7AS13) modes are observed respectively at 705 and 685 cm1 in the C-I conformer; 704 and 672 cm1 in the C-II

70

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

Table 5 Calculated vibrational characteristics for the four conformers of HBT. Mode

C-Ia Scaled freq (cm1)

C-IIa Scaled freq (cm1)

C-IIIa Scaled freq (cm1)

C-IVa Scaled freq (cm1)

Species

Normal mode assignment

m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17 m18 m19 m20 m21 m22 m23 m24 m25 m26 m27 m28 m29 m30 m31 m32 m33 m34 m35 m36 m37 m38 m39 m40 m41 m42 m43 m44 m45 m46 m47 m48 m49 m50 m51 m52 m53 m54 m55 m56 m57 m58 m59 m60 m61 m62 m63 m64 m65 m66 m67 m68 m69

43 (0,0)0.75 60 (1,0)0.75 117 (1,0)0.10 144 (0,4)0.75 189 (1,1)0.75 235 (0,1)0.75 260 (2,3)0.1 291 (5,2)0.72 311 (0,0)0.75 395 (0,3)0.69 420 (2,0)0.75 447 (7,0)0.75 494 (4,5)0.19 499 (0,15)0.24 513 (4,0)0.75 533 (2,4)0.72 551 (0,0)0.75 577 (6,9)0.74 613 (0,1)0.75 662 (2,8)0.16 685 (35,4)0.16 705 (1,28)0.12 707 (17,0)0.75 727 (0,1)0.75 742 (77,1)0.75 748 (66,1)0.75 781 (63,1)0.75 814 (16,32)0.17 841 (2,0)0.75 847 (1,0)0.75 865 (3,24)0.13 927 (1,0.0)0.75 929 (1,0)0.75 957 ((43,1)0.11 963 (0,0)0.75 964 (0,0)0.75 992 (7,66)0.10 1009 (10,48)0.10 1033 (5,84)0.25 1092 (34,25)0.34 1096 (5,68)0.26 1129 (28,17)0.33 1132 (1,12)0.66 1181(118,75)0.23 1209(41,404)0.27 1225 (12,23)0.21 1242 (39,10)0.63 1258 (1,45)0.34 1286 (14,29)0.36 1290 (14,223)0.28 1378 (31,7)0.29 1400 (58,702)0.37 1414 (14,403)0.34 1423 (59,894)0.33 1454 (132,25)0.33 1473(66,1019)0.31 1526 (5,62)0.54 1544 (76,5)0.75 1557 (5,180)0.43 1585 (71,430)0.39 3025 (1,20)0.52 3029 (1,43)0.75 3031 (9,102)0.70 3040 (5,141)0.71 3048 (12,111)0.46 3049 (10,113)0.69 3054 (14,267)0.12 3055 (9,397)0.12 3165(493,137)0.26

29 (0,0)0.75 59 (4,0)0.75 116 (3,1)0.75 550 (0, 0)0.75 193 (2,1)0.75 453 (7, 0)0.75 270 (1, 3)0.13 244 (2,0)0.36 236 (6,1)0.75 388 (4, 2)0.70 315 (1, 0)0.75 139 (1,5)0.75 467 (9, 0)0.74 502 (0,16)0.28 521 (5, 0)0.75 541 (8,11)0.49 425 (2, 0)0.75 582 (3, 2)0.13 615 (1, 1)0.75 654 (5,10)0.18 672 (14,7)0.20 704 (0,22)0.10 712 (14,0)0.75 728 (0,2)0.75 746 (116,0)0.75 751 (21,2)0.75 266 (90,2)0.75 812 (4,20)0.23 841 (1, 0)0.75 837 (0, 1)0.75 864 (7,42)0.19 976 (1, 0)0.75 930 (2, 0)0.75 953 (52,2)0.70 961 (0, 0)0.75 940 (1, 0)0.75 989(7,57)0.10 1019(2,84)0.14 1038 (2,91)0.28 1064 (80,1)0.42 1094 (2,106)0.27 1414 (70,104)0.32 1113 (4,4)0.29 1130 (11,32)0.52 1191 (9,604)0.26 1270 (71,135)0.34 1208 (19,89)0.21 1255 (13,47)0.27 1286 (16,25)0.64 1299 (42,15)0.74 1136 (34,59)0.19 1220 (16,91)0.27 1394 (24,686)0.37 1419 (3,105)0.34 1464 (42,1317)0.33 1444 (33,811)0.31 1520 (4,71)0.55 1549(14,101)0.31 1561 (19,62)0.53 1570 (19,819)0.39 3068 (5,73)0.14 3025 (1,48)0.74 3034 (6,92)0.64 3036 (6,148)0.69 3046 (18,152)0.33 3048 (14,270)0.22 3053 (14,281)0.12 3009 (12,116)0.38 3662 (89,170)0.23

38 (1,7)0.54 60 (1,1)0.67 96 (1,3)0.71 553 (2,2) 0.75 190 (4,3)0.46 213 (2,1)0.63 248 (1,3)0.14 275 (4,3)0.27 160 (0,3)0.68 378 (3,1)0.67 414 (51,1)0.55 320 (2,2)0.57 475 (6,4)0.67 497 (2,17)0.22 520 (5,7)0.53 542 (12,11)0.34 457 (9,0)0.69 580 (6,1)0.40 621 (4,8)0.29 652 (13,13)0.10 660 (17,3)0.27 701 (3,17)0.9 718 (20,0)0.65 737 (2,2)0.75 751 (21,2)0.75 754 (55,1)0.69 437 (23,2)0.50 810 (6,12)0.21 935 (2,0)0.71 856 (1,11)0.17 863 (2,12)0.10 950 (10,1)0.58 837 (0,1)0.75 940 (52,1)0.71 968 (0,0)0.69 971 (1,1)0.55 990(6,54)0.09 1013 (5,64)0.10 1032 (6,44)0.27 1066 (4,3)0.16 1094 (5,77)0.26 1422 (51,20)0.30 1131 (1,4)0.72 1126 (22,32)0.32 1187 (86,193)0.31 1302 (16,13)0.71 1203 (30,463)0.25 1252 (14,75)0.29 1284 (16,9)0.52 1272 (41,10)0.71 1148 (61,7)0.05 1216 (24,3)0.26 1397 (28,218)0.40 1419 (4,72)0.35 1440 (42,16)0.28 1480 (91,1876)0.33 1525 (4,47)0.59 1539 (36,109)0.31 1561 (12,10)0.70 1576 (50,435)0.41 3061 (7,165)0.12 3028 (1,49)0.74 3029 (2,66)0.72 3040 (5,147)0.66 3049 (12,134)0.34 3044 (9,145)0.56 3056 (10,284)0.12 3053 (7,184)0.20 3577 (173,178)0.20

28 (0,10)0.54 58 (3,2)0.64 82 (2,3)0.71 155 (0,3)0.70 188 (1,2)0.33 204 (0,1)0.71 246 (0,3)0.14 280 (9,1)0.75 332 (46,2)0.48 383 (2,5)0.45 425 (7,1)0.36 453 (5,3)0.48 471 (9,8)0.18 498 (0,13)0.25 522 (4,2)0.67 530 (3,3)0.36 558 (1,1)0.57 578 (2,7)0.58 621 (8,11)0.29 656 (1,3)0.22 668 (26,9)0.06 701 (3,18)0.09 718 (22,0)0.74 737 (3,9)0.26 744 (71,7)0.34 753 (52,1)0.68 320 (2,2)0.57 807 (20,1)0.17 846 (1,0)0.45 837 (1,2)0.51 854 (4,19)0.14 935 (2,0)0.71 940 (52,1)0.71 950 (10,1)0.58 966 (0,0)0.71 963 (0,1)0.32 990(6,49)0.08 1019 (5,62)0.10 1032 (8,56)0.24 1074 (50,5)0.27 1092 (5,77)0.25 1408 (41,25)0.26 1131 (2,4)0.74 11851132 (8)0.53 1184 (59,451)0.24 1298 (41,10)0.58 1225 (29,29)0.17 1251 (27,58)0.37 1284 (16,11)0.46 1265 (46,61)0.26 1139 (25,3)0.29 1213 (17,27)0.35 1395 (20,197)0.40 1419 (8,85)0.32 1460 (47,7)0.65 1497 (48,1593)0.32 1521 (4,32)0.65 1549 (22,10)0.26 1561 (3,118)0.48 1572 (40,533)0.39 3031 (1,76)0.53 3026 (1,52)0.75 3039 (12,89)0.52 3037 (7,146)0.66 3047 (16,151)0.29 3054 (13,258)0.16 3055 (10,276)0.12 3007 (12,112)0.38 3659 (71,148)0.23

a00 a00 a0 a00 a00 a00 a0 a0 a00 a0 a00 a00 a0 a0 a00 a0 a00 a0 a00 a0 a0 a0 a00 a00 a00 a00 a00 a0 a00 a00 a0 a00 a00 a0 a00 a00 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0 a0

s(C7AC14) c(C7AC14) b(C7AC14) U(R2) Bf(R1AR2) c(C7AC14) m(C7AC14) b(C7AC14) U(R3) a(R1) U(R1) U(R3) b(C16AO24) a(R2) c(C16AO24) a(R3) U(R1) a(R1) U(R2) a(R3) m(C7AS13) m(C1AS13) U(R1) U(R3) c(C17AH21) c(C5AH10) s(C16AO24AH25) m(ring) (R3) c(C6AH11) c(C19AC22) a(R3) c(C15AH18) c(C3AH8) a(R2) c(C4AH9) c(C20AH23) m(ring) (R1) m(ring) (R3) a(R1) b(C20AH23) b(C4AH9) b(C17AH21) b(C5AH10) b(C19AH22) m(C2AN12) b(C15AH18) m(C16AO24) b(C3AH8) m(ring) (R1) m(ring) (R3) a(C16AO24AH25) b(C6AH11) m(ring) (R1) m(ring) (R1) m(ring) (R3) m(C7AN12) m(ring) (R1) m(ring) (R3) m(ring) (R1) m(ring) (R3) m(C15AH18) m(C5AH10) m(C20AH23) m(C4AH9) m(C6AH11) m(C17AH21) m(C3AH8) m(C19AH22) m(O24AH25)

a Number outside bracket is frequency in cm1 unit, numbers within the bracket are IR intensity and Raman activity and number outside bracket is depolarization ratio (from left to right).

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

conformer; 701 and 660 cm1 in the C-III conformer; and 701 and 68 cm1 in the C-IV conformer. Primary aromatic amines with N directly attached to the ring absorbs strongly in the range 1330–1260 cm1 due to the phenyl CAN bond [40]. For 2-mercaptobenzothiazole, the CAN stretching mode was observed at 1332 cm1. In the present calculation m(C2AN12) and m(C7AN12) correspond to 1258 and 1473 cm1 in the C-I conformer, 1191 and 1444 cm1 in the C-II conformer, 1203 and 1480 cm1 in the C-III conformer and 1184 and 1497 cm1 in the C-IV conformer.

71

C-I conformer while this is the mode m44 in the C-II, C-III and CIV conformers. The IR intensity for this mode increases by a factor of 2 in the C-I and C-II in comparison to the C-III while Raman activity is the highest for the C-II conformer. Moreover, Raman bands are weakly polarized for the C-II and C-III conformer in comparison to the C-I and C-IV conformers. The torsion about the O24AH25 bond i.e. s(C16AO24AH25) correspond to m27 in the C-I conformer while this mode is present at m8, m12 and m9 in C-II, CIII and C-IV respectively. The frequency corresponding to this vibration is 781 cm1 in the C-I conformer which is the highest among the four conformers.

Modes of phenyl ring (R3) (12 modes) CAH modes The 12 modes of vibration of the R3 ring are 6 stretching modes – m28, m38, m50, m55, m58 and m60 in the C-I conformer; m28, m38, m51, m56, m58 and m60 in the C-II conformer; m28, m38, m49, m55, m58 and m60 in the C-III conformer and m28, m38, m49, m55, m58 and m60 in the C-IV conformer; 3 in-plane ring deformation modes- m16, m20 and m31 in the C-I conformer, m17, m21 and m31 in the C-II conformer, m17, m21 and m31 in the C-III conformer and m13, m21 and m31 in the C-IV conformer and 3 out- of- plane ring deformation modes- m9, m12 and m24 in the C-I conformer; m4, m6 and m25 in the C-II conformer; m4, m9 and m25 in the C-III conformer; m10, m13 and m25 in the C-IV conformer. The ring stretching modes have calculated to have frequencies 814, 1009, 1290, 1454, 1544 and 1585 cm1 for the C-I conformer; 812, 1019, 1299, 1464, 1549 and 1570 cm1 for the C-II conformer; 810, 1013, 1272, 1440, 1539 and 1576 cm1 for the C-III conformer; 807, 1019, 1265, 1460, 1549 and 1572 cm1 for the C-IV conformer. The frequencies 1009, 1019, 1013 and 1019 cm1 correspond to the ring breathing mode in the C-I, C-II, C-III and C-IV conformers respectively. The 3 in-plane ring deformation modes correspond to the frequencies respectively 533, 662 and 865 cm1 in the C-I conformer; 541, 654 and 864 cm1 in the C-II conformer; 542, 652 and 863 cm1 in the C-III conformer and 530, 656 and 854 cm1 in the C-IV conformer. The three out-ofplane ring deformation modes were found to be 311, 447 and 727 cm1 in the C-I conformer; 141, 236 and 728 cm1 in the C-II conformer; 160, 320 and 737 cm1 in the C-III conformer and 332, 453 and 737 cm1 in the C-IV conformer. Butterfly mode (R1-R2) (1 mode) There is a single butterfly mode in each conformer of HBT. This mode corresponds to m5 in all the conformers. The frequencies corresponding to this vibration are 189, 193, 190 and 188 cm1 in C-I, C-II, C-III and C-IV respectively. Comparison of C-I, C-II, C-III and C-IV In the following, the changes noticed in the vibrational characteristics of the four conformers (C-I, C-II, C-III and C-IV) of the HBT molecule are discussed. Most of the vibrational frequencies have nearly the same magnitude for the four conformers; however, significant changes are found in their intensities, Raman activities and depolarization ratios of the Raman bands. OAH modes The O24AH25 stretching frequency in all the conformers correspond to the mode m69. The magnitude of frequency corresponding to this vibration is the lowest in the C-I conformer in the comparison to the C-II, C-III and C-IV conformers. The IR intensity for this mode is the highest for the C-I conformer whereas it is the lowest in the C-IV conformer. In the present calculation the O24AH25 inplane bending mode i.e. a (C16AO24AH25) is the mode m51 in the

The modes due to m(C19AH22) are calculated to be 3055, 3009, 3053 and 3007 cm1 in the C-I, C-II, C-III and C-IV conformers respectively. The vibrational characteristics for stretching of this mode is almost same for the C-II and C-IV conformers, however, Raman activity increases by a factor of 2 and Raman band is strongly polarized for the C-I in comparison to the C-III conformer. The m(C17AH21) mode in the C-I, C-II, C-III and C-IV conformers is calculated to be 3049, 3048, 3044 and 3054 cm1 respectively. Raman activity increases by a factor2.5 in going from C-I to C-II, whereas the IR intensity decreases by a factor of 1.75 in going from C-II to C-III. The Raman band is highly polarized in the C-I conformer for this mode. The vibrational characteristic for CAH bending mode of the C6AH11 bond is the highest in the C-I conformer (1400 cm1) while C-II and C-III it differs by 4 cm1. The IR intensity, Raman activity and polarization ratio are the highest for C-I. The b(C15AH18) mode corresponds to the frequencies 1225, 1270, 1302 and 1298 cm1 in the C-I, C-II, C-III and C-IV conformers. The IR intensity and Raman activity for this mode is comparatively high in the C-II conformer. The mode b(C19AH22) in the C-I conformer has the highest frequency (1181 cm1) while the mode b(C17AH21) in the C-III has the highest frequency (1422 cm1). The out-of-plane bending c(C6AH11) frequency is calculated to be nearly the same for the C-I, C-II and C-IV conformers whereas it is the highest for the C-III conformer. CAOH modes The stretching frequency for the m(C16AO24) mode is the highest for the C-I conformer. The IR intensity for C-I conformer and Raman activity for the C-III conformer are the highest and the Raman bands in the C-I conformer is the highly polarized among all. The in plane bending mode for C16AO24 correspond to m14 for C-I, C-II and C-III while in C-I it is m13. The frequency corresponding to this mode is nearly the same for C-II, C-III and C-IV while it is higher for C-I. Similarly, c(C16AO24) correspond to m16 in C-II, C-III and C-IV while it is m15 in C-I. C7AC14 modes There is not very much difference for the modes corresponding to the C7AC14 bond. The one of the out-of-plane bending mode, c(C7AC14) correspond to m6 in the C-I, C-III and C-IV conformers while in the C-II conformer it is m13. The frequency corresponding to this mode is the highest in the C-II conformer. Ring modes The stretching frequency corresponding to the C1AC6 bond is the same in the C-II, C-III and C-IV conformers while it is slightly higher in the C-I conformer. Similarly, the stretching frequency for the bond C5AC6 is calculated to be 1414 cm1 for the C-I conformer which is higher than the C-II, C-III and C-IV conformers,

72

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

having nearly the same frequency. The frequency corresponding to the m(C1AC2) bond is same in the C-III and C-IV and C-I and C-II conformers. Similarly, the frequency corresponding to the ring breathing modes are nearly the same in the C-II, C-III and C-IV conformers and is higher in the C-I conformer. The stretching frequency corresponding to the C14AC15 bond corresponds to m49 in both the C-III and C-IV conformers while it is m51 in the C-II conformer and m50 in the C-I conformer. The frequency corresponding to this mode is the highest in the C-II conformer and the lowest in the C-IV conformer. The IR intensity is the highest for the C-IV conformer, while, the Raman activity is the highest for the C-I conformer. This mode is the most polarized in the C-II conformer. The stretching frequency corresponding to the (C17AC20) bond is m55 in the C-I, C-III and C-IV conformers while m56 in the C-II conformer. The frequencies corresponding to this mode is the lowest (1508 cm1) in the C-III conformer. The IR intensity is the highest in the C-I conformer, while, the Raman activity is the largest in the C-II conformer. NBO analysis The most accurate possible natural Lewis structure picture of orbits provided by NBO analysis since all the details of orbitals are mathematically chosen to include the highest possible percentage of the electron density. Natural orbitals are used to calculate the electron density distribution in atoms and bonds between the atoms. NBO method gives information about the interactions in both field and virtual orbital spaces thus analysis of intra- and inter molecular interactions can be enhanced. The NBO analysis of all the conformers are shown in Table 3 (supplementary material). It is observed from Table 3 (supplementary material) that BD(1)C1AS13 orbital having 1.97665 electrons has 52.59% C1 character in a sp3.05 hybrid and 47.41% S13 character in a sp4.43 hybrid. The sp3.05 hybrid on C has 7.519% p character and the sp4.43 hybrid on S has 81.11% p character. The BD(1)C2AN12 orbital with 1.98147 electrons has 40.28% C2 character in a sp2.50 hybrid and 59.72% N12 character in a sp1.91 hybrid. The sp2.50 hybrid on C has 71.34% p character and sp1.91 hybrid on N has 65.58% p character. Similarly BD(1)C16AO24 orbital with 1.99390 electrons has 33.83% C16 character in a sp2.88 hybrid and 66.17% O24 character in a sp1.95 hybrid. The sp2.88 hybrid on C has 74.05% p character and sp1.95 hybrid on O has 66.03% p character. An ideal sp3 hybrid has 75% p character. The coefficients written before hybridization like 0.6347, 0.7728, 0.5817, etc. are polarization coefficients. The

size of these coefficients tells about the role of the two hybrids in the formation of the bond. N has larger percentage of NBO, which yields the larger polarization coefficient because of its higher electronegativity. Lone pairs which are expected to attain the Lewis structure are shown in the end of the table. HOMO–LUMO analysis The most important orbitals in a molecule that determine the way the molecule interacts with the other species are, the frontier molecular orbitals (FMOs), called highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). They are the main orbital part that take part in chemical stability [41]. They are the key parameter in determining molecular properties, molecular electrical transport properties [42,43]. The Eigen value of HOMO characterizes the ability of donating electron and the Eigen value of LUMO characterizes the ability of accepting electrons. The energy gap between HOMO and LUMO reflects the chemical stability and they are responsible for chemical and spectroscopic properties of the molecule [44,45]. The orbitals HOMO– LUMO and their properties such as their energy are very useful for physicists and chemists. This is also used by the frontier electron density for predicting the most reactive position in p-electron system and also explains several type of reaction in conjugated system [46]. In conjugated molecules there is a small separation between HOMO–LUMO which is the result of a significant degree of intermolecular charge transfer from the end-capping electron donor groups to the efficient electron acceptor groups through pconjugated path [47]. Energy difference between HOMO–LUMO orbital is called as energy gap which is important for stability for structures [48]. An electronic system with larger HOMO–LUMO gap is less reactive than one having smaller gap [49]. If energy gap is larger, kinetic stability will be greater and chemical reactivity will be lower because it is energetically unfavorable to add electrons to a high lying HOMO and to remove electrons from a low lying LUMO and hence to form the activated complex of any potential reaction [50]. This gap between HOMO and LUMO has been used recently to prove the bioactivity from intramolecular charge transfer [51,52]. The HOMO and LUMO energy calculated by B3LYP/6-311++G level for the four conformers are shown in Table 2 (supplementary material). The plots of HOMO and LUMO for the most stable C-I conformer in Fig. 4 and for the conformers C-II, C-III and C-IV in Fig. 1(a–c) (supplementary material) are shown.

Fig. 4. The atomic orbital compositions of the frontier MO of the C-I conformer of HBT.

U. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 61–73

For C-I conformer HOMO–LUMO energy gap is 0.153 a.u., for C-II conformer HOMO–LUMO energy gap is 0.160 a.u., for C-III conformer HOMO–LUMO energy gap is 0.164 a.u, and for C-IV conformer HOMO–LUMO energy gap is 0.170 a.u. Conclusions For the first time DFT calculations of the molecular structures, vibrational spectra, NBO, HOMO–LUMO analysis have been carried out for the four conformers of HBT. The equilibrium geometry by B3LYP/6-311++g level for both the bond lengths and bond angles is performed better. The computed vibrational frequencies have been correlated to the experimentally observed frequencies on the basis of PEDs. The present calculation predicts that among the four conformers of HBT two possess planar and two possess nonplanar structures with the Cs and C1 point groups. The C-I is found to be the most stable conformer of HBT. The intramolecular hydrogen bonding is expected between the N atom of the thiazole ring and H of the OH group in the C-I conformer only. The lowering of the HOMO–LUMO gap supports bioactive property of the molecule. 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.2014.02.091. References [1] A. Achson, An Introduction to the Chemistry of Heterocyclic Compounds, third ed., Willy-Intersciences, India, 2009. [2] S. Gupta, N. Ajmera, N. Gautam, R. Sharma, D. Gauatam, Ind. J. Chem. 48B (2009) 853–858. [3] R.M. Kumbhare, V.N. Ingle, Ind. J. Chem. 48B (2009) 996–1000. [4] Y. Murthi, D. Pathak, J. Pharm. Res. 7 (3) (2008) 153–155. [5] B. Rajeeva, N. Srinivasulu, S. Shantakumar, E-J. Chem. 6 (3) (2009) 775–779. [6] M. Maharan, S. William, F. Ramzy, A. Sembel, Molecules 12 (2007) 622–633. [7] S. Kini, S. Swain, A. Gandhi, Ind. J. Pharm. Sci. (2007) 46–50. January–February. [8] H.L.K. Stanton, R. Gambari, H.C. Chung, C.O.T. Johny, C. Filly, S.C.C. Albert, Bioorg. Med. Chem. 16 (2008) 3626–3631. [9] M. Wang, M. Gao, B. Mock, K. Miller, G. Sledge, G. Hutchins, Q. Zheng, Bioorg. Med. Chem. 14 (2006) 8599–8607. [10] I. Hutchinson, M.S. Chua, H.L. Browne, V. Trapani, T.D. Bradshaw, A.D. Westwell, J. Med. Chem. 44 (2001) 1446–1449. [11] M. Sreenivasa, E. jaychand, B. Shivakumar, K. Jayrajkumar, J. Vijaykumar, Arch. Pharm. Sci. Res. 1 (2) (2009) 150–157. [12] S. Pattan, C. Suresh, V. Pujar, V. Reddy, V. Rasal, B. Koti, Ind. J. Chem. 44B (2005) 2404–2408. [13] P.S. Yadav, Devprakash, G.P. Senthilkumar, IJPSDR 3 (1) (2011) 1–7. January– March. [14] Anna. Pla-Dalmau, J. Org. Chem. 60 (1995) 5468–5473. [15] Per. Stenson, Acta Chem. Scand. 24 (1970) 3729–3738. [16] Pradipta Purkayastha, Nitin Chattopadhyay, Int. J. Mol. Sci. 4 (2003) 335–361. [17] Regina de Vivie-Riedle, Vincent De Waele, Lukas Kurtz, Eberhard Riedle, J. Phys. Chem. A 107 (2003) 10591–10599. [18] Fei Liu, Lijun Wang, Xia Li, Jinting Tang, Wu Qi, Xuwu Yang, J. Chem. Eng. Data 55 (2010) 3364–3368. [19] M. Pfeiffer, K. Lenz, A. Lau, T. Elsaesser, J. Raman Spectrosc. 26 (1995) 607–615. [20] M. Pfeiffer, K. Lenz, A. Lau, T. Elsaesser, J. Raman Spectrosc. 28 (1997) 61–72.

73

[21] F. Larmer, T. Elsaesser, W. Kaiser, Chem. Phys. Lett. 148 (1988) 119–124. [22] M.A. Rıos, M.C. Rıos, J. Phys. Chem. A 99 (1995) 12456–12460. [23] A. Douhal, F. Lahmani, A. Zehnacker-Rentien, F. Amat-Guerri, J. Phys. Chem. A 98 (1994) 12198–12205. [24] M. Brauer, M. Mosquera, J.L. Peres-Lustres, F. Rodriguez-Prieto, J. Phys. Chem. A 102 (1998) 10736–10745. [25] V. Gualler, M. Moreno, J.M. Lluch, F. Amat-Guerri, A. Douhal, J. Phys. Chem. A 100 (1996) 19789–19794. [26] L. Premvardhan, L. Peteanu, Chem. Phys. Lett. 296 (1998) 521–529. [27] D. Le Gourrierec, V.A. Kharlanov, R.G. Brown, W. Rettig, J. Photochem. Photobiol. A 130 (2000) 101–111. [28] C.A.S. Potter, R.G. Brown, F. Vollmer, W. Rettig, J. Chem. Soc. Faraday Trans. 1 (90) (1994) 59–67. [29] M.C.R. Rodriguez, F. Rodriguez-Prieto, M. Mosquera, Phys. Chem. Chem. Phys. 1 (1999) 253–260. [30] S. Santra, G. Krishnamoorthy, S.K. Dogra, Chem. Phys. Lett. 327 (2000) 230– 237. [31] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.1, Gaussian Inc., Wallingford CT, 2009. [32] M.M.C. Ferreira, E. Suto, J. Phys. Chem. 96 (1992) 8844–8849. [33] V.K. Rastogi, M.A. Palafox, L. Mittal, N. Peica, W. Kiefer, k. Lang, P. Ohja, J. Raman Spectrosc. 38 (2007) 1227–1241. [34] I.K. Bogatyreva, J. Appl. Spectrosc. 34 (4) (1981) 677–680. [35] R.A. Yadav, M. Kumar, R. Singh, P. Singh, S. Jaiswal, G. Srivastav, R.L. Prasad, Spectrochim. Acta A 71 (2008) 1565–1570. [36] M. Silverstein, G. Clayton Basseler, C. Morill, Spectometric Identification of Organic Compounds, Wiley, New York, 1981. [37] G. Socrates, Infrared Characteristic Group Frequencies, Wiely, New York, 1980. [38] G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, 1969. [39] J. Coates, Interpretation of Infrared Spectra, a Practical Approach, John Wiley and Sons Ltd., Chichester, 2000. [40] R. Saxena, L.D. Kauedpal, G.N. Mathur, J. Polym. Sci. A: Polym. Chem. 40 (2002) 3559–3592. [41] J.R. During, T.S. Little, T.K. Gounev, J.K. Gardnerjr, J.F. Sullivan, J. Mol. Struct. 375 (1996) 83–84. [42] M. Amalanathan, V.K. Rastogi, I.H. Joe, M.A. Palafox, R. Tomar, Spectrochim. Acta A 78 (2011) 1437–1444. [43] K. Fukui, Science 218 (1982) 747–754. [44] P.W. Atkins, Physical Chemistry, Oxford University Press, Oxford, 2001. [45] D. Mahadevan, S. Periandy, M. Karabacak, S. Ramalingam, Spectrochim. Acta A 82 (2011) 481–492. [46] C.H. Choi, J. Phys. Chem. A 101 (1997) 3823–3831. [47] S. Gunasekaran, R. Arun Balaji, S. Kumaresan, G. Anand, S. Srinivasan, Can. J. Anal. Sci. Spectrosc. 53 (2008) 149–162. [48] L. Padmaja, C. Ravi Kumar, D. Sajan, I.H. Joe, V.S. Jayakumar, G.R. Pettit, O.F. Nielsen, J. Raman Spectrosc. 40 (2009) 419–428. [49] R. Kurtaran, O. Sinem, A. Akin, Polyhedron 26 (2007) 5069–5074. [50] D.F. Manolopoulos, J.C. May, S.E. Down, Chem. Phys. Lett. 181 (1991) 105–111. [51] C. Ravi Kumar, I.H. Joe, V.S. Jayakumar, Chem. Phys. Lett. 460 (2008) 552–558. [52] A. Pirnau, V. Chis, O. Oniga, N. Leopold, L. Szabo, M. Baias, O. Cozar, Vib. Spectrosc. 48 (2008) 289–296. [53] M. Rini, J. Dreyer, E.T.J. Nibbering, T. Elsaesser, Chem. Phys. Lett. 374 (2003) 13–19.