Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

Contents lists available at ScienceDirect

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

Characterization of 1,5-dimethoxynaphthalene by vibrational spectroscopy (FT-IR and FT-Raman) and density functional theory calculations M. Kandasamy a,⇑, G. Velraj b, S. Kalaichelvan c, G. Mariappan d a

Department of Physics, Arignar Anna Govt. Arts College, Namakkal 637 002, India Department of Physics, Periyar University, Salem 636 011, India Tamilnadu Teachers Education University, Lady Willington College Campus, Chennai 600 005, India d Department of Physics (Engg.), Annamalai University, Annamalai Nagar 608 002, Chidambaram, Tamil Nadu, India b c

h i g h l i g h t s

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

 The FTIR and FT-Raman spectra of

1,5-dimethoxynaphthalene were recorded.  The vibrational frequencies were calculated by DFT method and compared.  HOMO and LUMO energies were calculated and analyzed.  NBO and NPA analysis were also carried out.

a r t i c l e

i n f o

Article history: Received 29 March 2014 Received in revised form 9 June 2014 Accepted 12 June 2014 Available online 21 June 2014 Keywords: 1,5-Dimethoxynaphthalene DFT Vibrational spectra NBO Electronic properties Ionization potential

a b s t r a c t In this work, we reported a combined experimental and theoretical study on molecular structure, vibrational spectra and natural bond orbital (NBO) analysis of 1,5-dimethoxynaphthalene. The optimized molecular structure, atomic charges, vibrational frequencies and natural bond orbital analysis of 1,5dimethoxynaphthalene have been studied by performing DFT/B3LYP/6-31G(d,p) level of theory. The FTIR, FT-Raman spectra were recorded in the region of 4000–400 cm1 and 3500–50 cm1 respectively. The scaled wavenumbers are compared with the experimental values. The difference between the observed and scaled wavenumber values of the most fundamentals is very small. The formation of hydrogen bond was investigated in terms of the charge density by the NBO analysis. Natural Population Analysis (NPA) was used for charge determination in the title molecule. Besides, molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis were investigated using theoretical calculations. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Naphthalene is the simplest and the most important member of this class of arenas, in which two benzene rings are fused in ortho ⇑ Corresponding author. Tel.: +91 4286 220132. E-mail address: [email protected] (M. Kandasamy). http://dx.doi.org/10.1016/j.saa.2014.06.092 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

positions. The naphthalene and its derivatives are the most important class of organic compounds. Because of their spectroscopic properties and chemical significance, naphthalene and its derivatives were studied extensively by spectroscopic and theoretical methods. It is also used in the production of dye and plastics. Several naphthalene containing drugs are available, such as nafacillin, naftifine, tolnaftate, and terbinafine. It is widely recognized that

Computational details In the present work, the density functional theory (DFT/B3LYP) at the 6-31G(d,p) basis set level was adopted to calculate the optimized parameters and vibrational wavenumbers of the normal modes of the title molecule. All the theoretical calculations were

1079

1500

761

1585

3098

3000

2000

765

1259

1500

1047

1591 1508

2500

1394

864

603

1923 1837 1749

2330

2115

2596 2512

1250

1389

3500

2935 2830

IR intensity (arb units)

2896 3072

Experimental

1000

500

Wavenumber (cm -1)

1

Fig. 1. Comparison of experimental and theoretical (B3LYP/6-31G(d,p)) FTIR spectra for 1,5-dimethoxynaphthalene.

1365

2500

2000

1500

200

668

1000

500 97

1387

3000

1444

3108

2896

1574

322

B3LYP/6-31G(d,p)

3000

2000

1500

1000

334 265 223

535

853

1200 1148 1081

1466

2836

2500

685

1582

Experimental

3083 3007

The compound 1,5-dimethoxynaphthalene was purchased from Sigma-Aldrich Chemical Company with a stated purity of greater than 98% and it was used as such without further purification. The FTIR spectrum of the sample was carried out between 4000 cm1 and 400 cm1 on an IFS 66 V spectrometer using the KBr pellet technique. The room temperature, FT-Raman spectrum was recorded using a Thermo Electron Corporation model Nexus 670 spectrophotometer equipped with FT-Raman module accessory. The 1064 nm line of an Nd-YAG laser was used as excitation wavelength in the region of 3500–50 cm1. The spectral resolution was set to 4 cm1 in a back scattering mode. A liquid nitrogen cooled Ge detector was used to collect 50 scans for a good Raman spectrum. The laser output was kept at 150 mW for the solid samples. The experimental FTIR and FT-Raman spectra along with the theoretically predicted IR and Raman spectra using DFT/B3LYP/631G(d,p) level of calculations are shown in Figs. 1 and 2.

B3LYP/6-34G(d,p)

2961

Experimental details

Transmittance (%)

polycyclic aromatic hydrocarbons and their metabolites are among the most toxic, carcinogenic and mutagenic atmospheric contaminants [1–4]. Naphthalene and its derivatives are widely used as the chemical intermediate, wetting agent in many industrial applications, to study heat transfer with mass sublimation in engineering field, household fumigants such as mothballs and fumigant pesticides. The molecule 1,5-dimethoxynaphthalene is a compound having two methoxy groups are substituted to naphthalene ring system. There are positional isomers differing by the location of the methoxy group. The different positions provide various chemical structures which offer important roles to each characteristic. Librando and Alparone [5,6] investigated methyl naphthalene isomers based on quantum mechanical approach and the electronic polarizability of dimethylnaphthalenes. Buchanan et al., [7] have been studied solid-state structure of 2,3-dimethoxynaphthalene via X-ray crystallography and solid-phase 13C nuclear magnetic resonance spectroscopy. Das et al. [8] reported the infrared spectra of dimethylnaphthalenes in the gas phase. The vibrational analysis using DFT method of naphthoic acid, 2-naphthoic acid, bromo naphthoic acid, 1-naphthaldehyde, 1,5-dinitronaphthalene and 1hydroxynaphthalene have been extensively studied and analyzed [9–11]. Recently, Nagabalasubramanian and Periandy reported a scaled quantum mechanical vibrational analysis on 1,5-methylnaphthalene using FTIR and FT-Raman spectra [12]. Molecular structure, anharmonic vibrational frequencies and NBO analysis of naphthalene acetic acid by DFT calculations were carried out by Kavitha et al. [13]. Xavier et al. [14] have been investigated the 1-methoxynapthalene by using Wilson’s F-G matrix method. Most recently, Govindarajan et al. [15] investigated the FTIR and FT-Raman spectra of 1-methoxynapthalene. Spectroscopic (FTIR and FT Raman) analysis and vibrational study on 2,3-dimethylnaphthalene were made by Prabhu et al. [16]. To the best of our knowledge, neither quantum chemical calculation, nor the vibrational spectra of 1,5dimethoxynaphthalene have been reported. Therefore, the present work aims to provide a complete description on the molecular geometry, molecular vibrations and electronic features of the 1,5dimethoxynaphthalene molecule.

588

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

Raman intensities (arb units)

192

500

Wavenumber (cm -1) Fig. 2. Comparison of experimental and theoretical (B3LYP/6-31G(d,p)) FT-Raman spectra for 1,5-dimethoxynaphthalene.

performed using the Gaussian 03W program package [17] with the default convergence criteria, without any constraint on the geometry [18]. The equilibrium geometry corresponding to the true minimum on the potential energy surface (PES) was effectively obtained by solving self-consistent field equation. The vibrational spectra of the 1,5-dimethoxynaphthalene were obtained by taking the second derivative the energy, computed analytically. The optimized structural parameters were used in the vibrational frequency calculations at DFT levels to characterize all stationary points as minima using the GAUSSVIEW animation program [19]. Vibrational frequencies were computed at DFT level which had reliable one-to-

193

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

one correspondence to experimental IR and Raman frequencies [20]. The vibrational assignments of the normal modes were made on the basis of the potential energy distribution (PED) calculated by using the VEDA 4 program [21]. Subsequent potential energy distribution to each observed frequencies, predicts well the purity of the fundamental modes and shows the reliability and accuracy of the spectral analysis. In the present study, we have used the following scaling factor of 0.9608 for B3LYP/6-31G(d,p) method [22]. A comparison of the frequencies calculated with the experimental values revealed that the 6-31G(d,p) basis set result shows very good agreement with the experimental observations. The Raman activities (SRa) calculated with Gaussian 03 program [17] converted to relative Raman intensities (IRa) using the following relationship derived from the intensity theory of Raman scattering [23,24]

Ii ¼

f ðmo  mi Þ4 Si mi ½1  exp ðhcmi =ktÞ

ð1Þ

where m0 is the laser exciting wavenumber in cm1 (in this work, we have used the excitation wavenumber m0 = 9398.5 cm1, which corresponds to the wavelength of 1064 nm of a Nd-YAG laser), mi the vibrational wavenumber of the ith normal mode (cm1) where as Si is the Raman scattering activity of the normal mode mi f (is a constant equal to 1012) is a suitably chosen common normalization factor for all peak intensities. h, k, c and T are Planck and Boltzmann constants, speed of light and temperature in Kelvin, respectively. Results and discussion Molecular geometry The most relevant structural parameters, bond lengths, bond angles and dihedral angles of 1,5-dimethoxynaphthalene determined by Density functional theoretical calculations with 6-31G(d,p) basis set which are given in Table 1. Geometry optimization was carried out, without any symmetry constraints;

including polarization functions to correctly take into account intramolecular H-bonding in the molecule. The atom numbering of 1,5dimethoxynaphthalene molecule used in this paper is reported in Fig. 3. To the best of our knowledge the experimental data on the geometric structure of the title molecule is not available till now in the literature. Due to the absence of experimental data, some of the essential structural parameters are now compared with similar systems for which the crystal structures have been solved. The optimized structure of 1,5-dimethoxynaphthalene and experimental structure of closely related molecule 2,3-dimethoxynaphthalene available in the literature [7] were compared. The agreement between the optimized and experimental crystal structure is quite good showing that the geometry optimization almost exactly reproduces the experimental conformation. The molecular structure of 1,5-dimethoxynaphthalene belongs to C2 point group symmetry. An account of bond length, bond angles and dihedral angles the left part of the naphthalene ring almost reproduced the right part as like as a mirror image, it is evident from Fig. 3 and Table 1. The title compound was planar, as indicated by C5–C6–O11–C12 and C10–C1–O13–C14 torsional angles of 180.0° and 180.0° respectively. It is well correlated with the experimental data of available literature value of 177.6° and 176.7° [7] respectively. The methyl C–H bond distances longer than the aromatic C–H bond distances. An interesting fact that occur both the methoxy group, among the three methyl C–H bonds, one bond in the ring of the molecule is shorter than the other two non-planar C–H bonds by an angle of 0.087 Å. As it is evident from the bond lengths of C1–C2 and C2–C10 is 1.381 and 1.430 Å, show small deviation when compared with C10–C9 and C9–C8 of 1.420 and 1.374 Å, and symmetry of naphthalene ring is distorted, yielding ring angles smaller or larger than the normal value of 120° exactly at the substitution as shown in Table 1. The C2–C1–C10 angle is 120.8° and C8–C9–C10 angle is 119.8° (120.6° and 119.8° on [7]). There is an elongation calculated in the C–O bond length when it connected with methyl group carbon atom than the ring carbon which has the values of 1.418 and 1.418 Å respectively. The experimental data also correlated well with these values which are observed at 1.432 and 1.436 Å [7] respectively. The

Table 1 Comparison of experimental and calculated optimized geometrical values of the 1,5-dimethoxynaphthalene (bond length in (Å), angles in (°)).

a

Bond length

B3LYP

C1–C2 C1–C10 C1–O13 C2–C3 C2–H15 C3–C4 C3–16 C4–C5 C4–H17 C5–C6 C5–C10 C6–C7 C6–O11 C7–C8 C7–H18 C8–C9 C8–H19 C9–C10 C9–H20 O11–C12 C12–H21 C12–H22 C12–H23 O13–C14 C14–H24 C14–H25 C14–H26

1.381 1.433 1.366 1.415 1.083 1.374 1.086 1.420 1.083 1.433 1.428 1.381 1.366 1.415 1.083 1.374 1.086 1.420 1.083 1.418 1.091 1.098 1.098 1.418 1.091 1.098 1.098

Taken from Ref. [7].

a

XRD

1.432

1.436

Bond angle

B3LYP

C5–C6–C7 C5–C6–O11 C7–C6–O11 C6–C7–C8 C6–C7–H18 C8–C7–H18 C7–C8–C9 C7–C8–H19 C9–C8–H19 C8–C9–C10 C8–C9–H20 C10–C9–H20 C1–C10–C9 C5–C10–C9 C6–O11–C12 O11–C12–H22 O11–C12–H23 H21–C12–H22 H21–C12–H23 C1–O13–C14 O13–C14–H25 O13–C14–H26 H24–C14–H25 H24–C14–H26 C2–C1–C10 C2–C1–O13 C10–C1–O13

120.8 114.8 124.4 119.9 120.9 119.2 121.3 118.7 120.0 119.8 121.1 119.1 121.8 120.1 118.3 111.6 111.6 109.3 109.3 118.3 111.6 111.6 109.3 109.3 120.8 124.4 114.8

a

XRD

119.8

120.6

Dihedral angle

B3LYP

C7–C8–C9–H20 H19–C8–C9–C10 H19–C8–C9–H20 C8–C9–C10–C1 C8–C9–C10–C5 H20–C9–C10–C1 C10–C1–O13–C14 C5–C6–O11–C12 C6–O11–C12–H22 C1–O13–C14–H25 C1–O13–C14–H26 C6–O11–C12–H23 C1–O13–C14–H24

180.0 180.0 0.0 180.0 0.0 0.0 180.0 180.0 61.1 61.1 61.1 61.1 180.0

a

XRD

176.7 177.6

194

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

where n is the number of the experimental or calculated data. The RMS errors of the observed IR and Raman bands are found to be 18.29 and 17.94, respectively.

Fig. 3. Optimized molecular dimethoxynaphthalene.

structure

and

atomic

numbering

of

1,5-

values in the parenthesis are mentioned as experimental data available in literature. Vibrational assignments The observed (FTIR and FT-Raman) wavenumbers, calculated IR and Raman intensities and assigned wavenumbers of vibrational modes calculated at the B3LYP level using basis set 6-31G(d,p) along with their PED of 1,5-dimethoxynaphthalene are depicted in Table 2. The vibrational spectrum is mainly performed by the modes of the free molecule observed at higher wavenumbers together with the lattice modes in the low wavenumbers region. The aim of the vibrational analysis is to find vibrational modes connected with specific molecular structures of calculated compound. The comparison of experimental and theoretical FTIR and FTRaman spectra are shown in Figs. 1 and 2. The DFT method predicts vibrational spectra with high accuracy and is applicable to a large number of compounds, except for the cases where the effect of dispersion forces is significant. Precise vibrational frequency assignment for aromatic and other conjugated system is necessary for characterization of compound. It should be noted that the calculations were made for a free molecule in vacuum, while experiments were performed for solid samples. With the assumed structural model, the molecule belongs to C1 point group symmetry and has 26 atoms with 72 normal modes of vibrations. In order to facilitate assignment of the observed peaks, we have analyzed the vibrational wavenumbers and compared our calculated values with the experimental results. The experimental and calculated data refer to m(exp) and m(cal), respectively. The agreement between the theoretical and experimental results has been expressed by root mean square (RMS) deviation using the following expression.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u 1 X  calc 2 RMS ¼ t v i  v exp i n1 i

ð2Þ

C–H vibrations The aromatic C–H stretching vibrations are normally found between 3100 and 3000 cm1 due to aromatic C–H stretching vibrations [25]. The wavenumbers calculated in the range 3107– 3062 cm1 by B3LYP method is assigned to C–H stretching vibrations. A strong band appeared at 3127 and 3055 cm1 in FTIR spectrum and 3082 cm1 in FT-Raman spectrum are assigned to C–H ring stretching vibrations. As evident from the PED column, they are pure stretching vibrations almost contributing to above 90%. The C–H in-plane ring bending vibrations are normally occurred as a number of strong to weak intensity bands in the region 1300–1000 cm1 [26]. In the present study, the C–H in-plane bending vibrations of the compound is computed in the range 1390– 932 cm1 by B3LYP method. The wavenumbers observed at 1391 cm1 in FTIR spectrum and 1386 cm1 in FT-Raman spectrum showed excellent agreement with a predicted wavenumber 1390 cm1. Substitution patterns on the ring can be judged from the out-of-plane bending of the ring C–H bonds in the region 960–675 cm1 and these bands are highly informative [27]. The C–H out-of-plane bending vibrations are normally observed in the region 1000–809 cm1 [26,28–31]. The C–H out-of-plane bending vibrations are observed in FTIR spectrum at 963, 931, 862 and 712 cm1 and FT-Raman at 853 cm1. The experimental and theoretical (1013–712 cm1 by B3LYP method) out-of-plane bending vibrational wavenumbers are found to be well within their characteristic region. –OCH3 group vibrations Electronic effects such as back-donation and induction, mainly caused by the presence of oxygen atom adjacent to CH3 group, can shift the position of CH stretching and bending modes [32–35]. Methyl group vibrations are generally referred to as electron-donating substituent in the aromatic rings system, the antisymmetric C–H stretching mode of CH3 is expected around 2980 cm1 whereas symmetric stretching is at 2870 cm1 [36,37]. For the assignments of CH3 group one can expect that nine fundamentals can be associated to each CH3 group, namely the symmetrical stretching in CH3 (CH3 symmetric stretch) and asymmetrical stretching (CH3 asymmetric stretch), in-plane stretching modes (i.e. in-plane hydrogen stretching mode), the symmetrical (CH3 symmetric deform) and asymmetrical (CH3 asymmetric deform) deformation modes, the in-plane rocking (CH3 ipr), out-of-plane rocking (CH3 opr) and twisting (tCH3) bending modes. For the methyl group compound [38], the asymmetric stretching mode appears in the range 2825–2870 cm1, lower in magnitude compared to its value in CH3 (compounds) (2860–2935 cm1) whereas the asymmetric stretching modes for both type of compounds lie in the same region 2925–2985 cm1. The weak bands observed at 2984, 2941 cm1 in FTIR spectrum and 3008, 2960 and 2937 cm1 in FT-Raman spectrum could be attributed to CH3 asymmetric stretching vibrations. The same vibrations are computed at 3024, 2957 cm1 by B3LYP method show good agreement with experimental observations. The bands observed at 2880 cm1 in FTIR spectrum and 2851 cm1 in FTRaman spectrum could be attributed to CH3 symmetric stretching vibration. The theoretically computed value for symmetric stretching vibration at 2898 cm1 and 2897 cm1 by B3LYP method shows excellent agreement with experimental observation. For methyl substituted benzene derivatives, the antisymmetric and symmetric deformation vibrations of methyl group normally appear in the region 1465–1440 cm1 and 1390–1370 cm1, respectively [39–41] while the rocking mode appears in the region of 1050–990 cm1 predicted by the DFT calculation, the series of

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

195

Table 2 Comparison of the experimental and calculated vibrational spectra and proposed assignments of 1,5-dimethoxynaphthalene. Mode nos.

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

Experimental wavenumbers (cm1)

Theoretical wavenumbers (cm1)/B3LYP/6-31G(d,p)

FTIR

Unscaled

Scaled

a

3234 3233 3224 3224 3187 3187 3147 3147 3077 3077 3016 3015 1678 1649 1638 1562 1522 1519 1512 1504 1504 1497 1484 1460 1447 1420 1383 1301 1295 1249 1225 1216 1208 1204 1186 1179 1179 1124 1112 1093 1055 970 969 891 880 879 870 825 793 790 741 695 686 611 602 546 534 524 478 478 357 335 315 281 256 254 208 183 173 118 95 61

3107 3107 3098 3098 3062 3062 3024 3024 2957 2957 2898 2897 1612 1585 1574 1501 1463 1460 1453 1445 1445 1438 1426 1403 1390 1365 1329 1250 1244 1200 1177 1168 1160 1157 1139 1133 1133 1080 1068 1050 1013 932 931 856 845 844 836 793 762 759 712 668 659 587 578 524 513 503 460 459 343 322 303 270 246 244 200 176 166 114 91 59

0.00 13.12 27.95 0.00 0.00 30.30 0.00 46.97 82.93 0.00 0.00 130.46 0.00 83.43 0.00 131.98 0.00 74.69 0.00 10.59 0.00 11.64 0.00 0.00 268.72 0.00 7.68 302.43 0.00 33.94 0.00 34.36 0.00 13.30 0.00 1.66 0.00 203.20 0.00 58.39 0.00 0.00 1.28 16.16 0.00 1.93 0.00 0.00 89.27 13.08 0.00 0.00 3.33 18.92 1.06 0.00 0.00 6.54 0.00 0.00 1.72 0.00 0.00 0.00 0.00 0.28 0.00 0.13 4.04 7.52 0.00 3.85

3127s

FT-Raman

3082s

3055s 2984s

3008s

2941s

2960w 2937w

2880w 2851w 1611w 1597w 1541w 1495w 1470w

2835w 1583vs

1466w 1453w

1446w

1414vs 1391w 1344m 1321w 1274s 1244vs

1386vs

1173m

1149w 1138vs 1102w 1080m 1040vs 963w 931w 862s

853m

839s

754s 712s 685s 655vs

535m

478m 456w 334s

265w 223m

145w 96vs

IIR

Vibrational assignments with PED (P10%)

b

IRA

69.22 0 0 61.97 65.87 0 69.84 0 0 42.95 100 0 16.24 0 127.5 0 34.85 0 39.02 0.02 81.88 0 6.43 118.84 0 474.6 0 0 4.7 0 12.92 0 27.49 0 23.07 0 21.72 0 31.18 0 17.45 3.86 0 0 15.37 0 26.67 9.23 0 0 24.2 119.38 0 0 0 35.9 23.09 0 9.05 59.55 0 175.46 0 8.2 50.85 0 98.9 0 0 0 30.45 0

tCH(93) tCH(91) tCH(93) tCH(91) tCH(92) tCH(92) tCH(91)methyl tCH(91)methyl tCH(98)methyl tCH(88)methyl tCH(81)methyl tCH(90)methyl tCC(62) + dCCC(11) tCC(61) tCC(64) tCC(33) qHCH(49) cCHOH(11) tCC(15) + cCHOH(14) + tOC(11) cCHOH(13) cCHOH(13) cCHOH(20) cCHOH(27) sHCCC(57) + qHCH(31) sHCCC(38) + qHCH(43) tCC(72) tCC(19) tOC(30) tOC(49) + cCHOH(14) tCC(53) cCOCH(29) cCHOH(32) + tCC(12) cCHOH(26) tCC(16) + cCHOH(11) tCC(11) cCHOH(85) cCHOH(85) tOC(57) tCC(23) + tOC(18) tCC(57) tOC(72) sHCCC(81) sHCCC(64) tOC(25) + dCCC(12) + tCC(11) sHCCC(71) sHCCO(65) + sHCCC(10) tCC(31) sCCCC(43) + sHCCC(22) sHCCO(18) + sHCCC(52) dCCC(70)

sHCCC(49) + sCCCC(18) tCC(44) sCCCC(76) dCOC(77)

sCCCC(79) dCCC(26) + tCC(13)

sCCCC(83) dCCC(61) dCCC(60) + tCC(11) sCCCC(58) dCCC(62) dCOC(46) + tCC(11) + dCCC(11) sCCCO(42) + sHCOC(23) + sCCCC(10) sHCOC(75) + sCCCC(11) dCOC(85) sHCOC(62) + sCCCO(22) sCCCC(51) + sHCOC(22) sCCCC(53) dCOC(80) sCCCO(72) sCCOC(89) sCCCC(70)

m-Stretching; d-in-plane-bending; c-out-of-plane bending; s-torsion; q-rocking; w-weak; s-strong; vs-very strong; m-medium. a b

IIR-IR intensity (K mmol1). IRa-Raman intensity (Arb units) (intensity normalized to 100%).

196

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

bands appearing in the 1400–1500 cm1 region are mainly due to the methyl deformation coupling with the ring C–C–H bending and C–C stretching motions, to different extents and in different ways. In the case of title molecule, the bands observed at 1470 and 1466 cm1 in FTIR and FT-Raman spectra respectively correspond to CH3 deformation which are correlated with the calculated wavenumbers at 1463 cm1 by B3LYP method. Ring vibrations Naphthalene ring stretching vibrations are expected in the region 1620–1390 cm1. Naphthalene ring vibrations are found to make a major contribution in the IR and Raman spectra [42,43]. The wavenumbers observed in FTIR spectrum at 1611, 1597, 1541, 1495, 1344 and 1321 cm1 and in FT-Raman spectrum at 1583 and 1453 cm1 are assigned to C–C stretching vibrations. The theoretically predicted harmonic wavenumbers at 1612, 1585, 1574, 1501, 1453, 1365 and 1329 cm1 (mode nos. 13–16, 19, 26–27) by B3LYP/6-31G(d,p) level show a good agreement with experimental data. These vibrations are mixed up with C–H inplane bending vibrations as shown in Table 2. The PED corresponds to these vibrations are mixed modes as evident from Table 2. C–O–C vibrations The (O–C) and (C–O) stretching vibrations have already been reported by Druzbicki et al. [44] and Dolega et al. [45] at 1270 cm1 and 1040 cm1. In the present study, the stretching bands are observed at 1274 and 1244 cm1 in FTIR and 1080 cm1 in FT-Raman. The calculated m(O–Cnaphthalene) frequency in methoxynaphthalene group are at 1250 and 1244 cm1. On the other hand, the stretching vibrations of m(Cmethyl–O) bands assigned at 1080 and 1014 cm1. The PED corresponds to these vibrations are mixed modes as evident from Table 2. Natural bond orbital (NBO) analysis NBO analysis has been performed on the molecule 1,5-dimethoxynaphthalene at the DFT/B3LYP/6-31G(d,p) level in order to elucidate the intramolecular, rehybridization and delocalization of electron density within the molecule. Natural bond orbital analysis provides an efficient method for studying intra-and

intermolecular bonding and interaction among bonds, and also provides a convenient basis for investigating charge transfer or conjugative interaction in molecular systems [46]. The larger E(2) value, the more intensive is the interaction between electron donors and electron acceptors, i.e., the more donating tendency from electron donors to electron acceptors of the whole system. Delocalization of electron density between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (antibond or Rydgberg) non-Lewis NBO orbitals correspond to a stabilizing donor-acceptor interaction. The second-order Fock matrix was carried out to evaluate the donor-acceptor interactions in the NBO analysis [47]. The interactions result is a loss of occupancy from the localized NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor(i) and acceptor(j), the stabilization energy E(2) associated with the delocalization i ? j is estimated as 2

Eð2Þ ¼ DEij ¼ qi

Fði; jÞ ej  ei

ð3Þ

where qi is the donor orbital occupancy, ei and ej are diagonal elements and F(i, j) is the off diagonal NBO Fock matrix element. The intramolecular interaction are formed by the orbital overlap between r(C–C) and rC–C); p(C–C) and p(C–C) bond orbital which results intramolecular charge transfer (ICT) causing stabilization of the system. The strong intramolecular hyperconjugation interaction of the r and p electrons of C–C and C–H to anti C–C, C– H and C–O bond in the ring leads to stabilization of some part of the ring as evident from Table 3. Particularly in the naphthalene ring system, the intramolecular hyperconjugative interaction of the p(C5–C10) conjugate with antibonding orbital of [p(C1–C2), p(C6–C7)] and [p(C3–C4), p(C8–C9)] which leads to charge delocalization of 18.97 and 15.88 kJ/mol respectively. On the other hand, Among the title molecule, a strong intramolecular hyperconjugative interaction of p-electrons with the greater energy contributions from C1–C2 ? C3–C4 (18.68 kJ mol1), C5–C10 (13.72 kJ mol1); C3–C4 ? C1–C2 (15.53 kJ mol1), C5–C10 (16.81 kJ mol1); C6–C7 ? C5–C10 (13.72 kJ mol1), C8–C9 (18.68 kJ mol1); C8–C9 ? C5–C10 (16.81 kJ mol1), C6–C7 (15.53 kJ mol1) for naphthalene ring of the molecule, Furthermore, the most interaction energy, related

Table 3 Second order perturbation theory analysis of Fock matrix in NBO basis for 1,5-dimethoxynaphthalene. Donor (i)

ED (i) (e)

p(C1–C2)

1.739

p(C3–C4)

1.756

p(C5–C10)

1.585

p(C6–C7)

1.739

p(C8–C9)

1.756

r(C2–C3) r(C2–H15) r(C4–H17)

1.975 1.978 1.980

LP(2)O11 LP(2)O13 p(C1–C2) p(C6–C7)

1.840 1.840 0.330 0.330

Acceptor (j) 

p (C3–C4) p(C5–C10) p(C1–C2) p(C5–C10) p(C1–C2) p(C3–C4) p(C6–C7) p(C8–C9) p(C5–C10) p(C8–C9) p(C5–C10) p(C6–C7) r(C1–O13) r(C1–C10) r(C2–C3) r(C5–C10) p(C6–C7) p(C1–C2) p(C3–C4) p(C8–C9)

ED means electron density. d E(2) means energy of hyper conjugative interactions. e Energy difference between donor and acceptor i and j NBO orbitals. f F(i, j) is the Fock matrix element between i and j NBO orbitals.

ED (j) (e)

E(2)d KJ mol1

E(j)–E(i)e a.u

F(i, j)f a.u

0.265 0.469 0.330 0.469 0.330 0.265 0.330 0.265 0.469 0.265 0.469 0.330 0.028 0.029 0.016 0.030 0.330 0.330 0.265 0.265

18.68 13.72 15.53 16.81 18.97 15.88 18.97 15.88 13.72 18.68 16.81 15.53 5.21 4.55 4.06 4.26 31.12 31.12 172.2 172.2

0.31 0.3 0.28 0.29 0.27 0.28 0.27 0.28 0.3 0.31 0.29 0.28 1.04 1.06 1.07 1.06 0.35 0.35 0.01 0.01

0.068 0.06 0.06 0.066 0.065 0.062 0.065 0.062 0.06 0.068 0.066 0.06 0.066 0.062 0.059 0.06 0.097 0.097 0.076 0.076

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

to the resonance in the molecule, is electron donating from the nO11(2) and nO13(2) to the antibonding acceptor p(C6–C7) and p(C1–C2) leads to moderate stabilization energy of 31.12 kJ/mol respectively. The p(C6–C7) and p(C1–C2) of the NBO conjugated with p(C8–C9) and p(C3–C4) resulting to a greater stabilization energy of 171.2 kJ/mol respectively for the title molecule.

197

dimethoxynaphthalene. The atomic orbital compositions of the frontier molecular orbital are sketched in Fig. 4.

HOMO energy ¼ 5:0986 eV LUMO energy ¼ 0:4757 eV HOMO  LUMO energy gap ¼ 4:6229 eV

Frontier molecular orbital (FMO) analysis The electronic absorption corresponds to the transition from the ground to the first excited state and is mainly described by one electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO represents the ability to donate an electron, LUMO as an electron acceptor represents the ability to obtain an electron. Both HOMO and LUMO are the main orbitals that take part in chemical stability [48]. The energy values of LUMO and HOMO and their energy gap reflect the chemical activity of the molecule. The decrease in the HOMO and LUMO energy explains the intramolecular charge transfer (ICT) interaction taking place within the molecule which is responsible for the activity of the molecule. The HOMO–LUMO energy separation has served as a simple measure of kinetic stability. A molecule with a small or no HOMO–LUMO gap is a chemically reactive [49]. Pearson showed that the HOMO–LUMO gap represents the chemical hardness of the molecule [50,51]. The HOMO–LUMO energy gap of 1,5-dimethoxynaphthalene was calculated at the B3LYP/6-31G(d,p) level and their values shown below reveals that the energy gap reflect the chemical activity of the molecule. The HOMO is located over the naphthalene part of the molecule, the HOMO ? LUMO transition implies an electron density transfer to oxygen and conjugated bond of ring system from naphthalene part. Moreover, these orbital significantly overlap in their position for 1,5-

Ionization potential By using HOMO and LUMO energy values for a molecule, the ionization potential and chemical hardness of the molecule were calculated using Koopmans’ theorem [52] and are given by

g ¼ ðIP  EA Þ=2

ð4Þ

where,

IP  EðHOMOÞ

ð5Þ

EA  EðLUMOÞ

ð6Þ

IP = ionization potential (eV), EA = electron affinity (eV). The ionization potential and electron affinity of the title molecule in gas phase is calculated at 5.0986 eV and 0.4757 eV respectively. Considering the chemical hardness, large HOMO–LUMO gap means a hard molecule and small HOMO–LUMO gap means a soft molecule. One can also relate the stability of molecule to hardness, which means that the molecule with least HOMO–LUMO gap, it is more reactive. Molecular electrostatic potential (MEP) Molecular electrostatic potential (MEP) at a point in the space around a molecule gives an indication of the net electrostatic effect produced at that point by the total charge distribution (electron + nuclei) of the molecule and correlates with dipole moments,

HOMO Plot (Ground state)

HOMO Energy = -5.0986 eV

Energy gap = 4.6230 eV

LUMO Energy = -0.4757 eV

LUMO Plot (First excited state)

Fig. 4. The atomic orbital compositions of the frontier molecular orbital for 1,5-dimethoxynaphthalene.

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199

Natural charge

198

Atom position Fig. 5. The histogram of calculated natural charges for 1,5-dimethoxynaphthalene.

electronegativity, partial charges and chemical reactivity of the molecules. The MEP is related to the electronic density and is a very useful descriptor for determining sites for electrophilic attack and nucleophilic reactions as well as hydrogen bonding interaction [53,54]. It provides a visual method to understand the relative polarity of the molecule. The different values of the electrostatic potential represented by different colors; red represents the regions of the most negative electrostatic potential, white represents the regions of the most positive electrostatic potential and blue represents the region of zero potential. Potential increases in the order red < yellow < green < violet < blue. To predict reactive sites for electrophilic and nucleophilic attack for the title molecule, MEP was calculated at the B3LYP/6-31G(d,p) OMOHOklkkjds optimized geometry. The negative (red) regions of MEP were related to electrophilic reactivity and the positive (blue) regions to nucleophilic reactivity shown in Supplementary data S1. As easily can be seen in figure this molecule has two possible sites for electrophilic attack over O11 and O13. The negative electrostatic potential corresponds to an attraction of the proton by the aggregate electron density in the molecule (shades of red), while the positive electrostatic potential corresponds to the repulsion of the proton by the atomic nuclei (shades of blue). According to these calculated results, the MEP map shows that the negative potential sites are on oxygen atoms as well as the positive potential sites are around the hydrogen atoms of methoxy group. These sites give information concerning the region from where the compound can have metallic bonding and intermolecular interactions. Natural atomic charges In order to determine the electron population of each atom of the title molecule, the natural atomic charges of 1,5-dimethoxynaphthalene calculated by DFT/B3LYP method using 6-31G (d,p) basis set. Illustration of atomic charges plotted is shown in Fig. 5. The charge distribution of 1,5-dimethoxynaphthalene shows that the carbon atom attached with hydrogen atoms is negative, whereas the remaining carbon atoms are positively charged. The oxygen atoms have more negative charges whereas all the hydrogen atoms have positive charges. The maximum positive atomic charge is obtained for carbon atoms (C1 and C6) when compared with all other atoms. This is due to the attachment of negatively charged oxygen atoms. Conclusion In this present study, we have investigated to clarify the complete characterization of 1,5-dimethoxynaphthalene by using of computational methods along with the experimental FTIR and

FT-Raman spectra of 1,5-dimethoxynaphthalene. The molecular geometry and wavenumber have been calculated using DFT/ B3LYP with 6-31G(d,p) basis set and the normal modes are assigned based on TED values. The equilibrium geometries, atomic charges and harmonic wavenumbers calculations of 1,5-dimethoxynaphthalene have been carried out for the first time at DFT level. According to the geometry results the left part of the naphthalene ring almost reproduced the right part as like as a mirror image. The vibrational frequencies of the fundamental modes of the compound have been precisely assigned and analyzed and the theoretical results were compared with the experimental vibrations. The theoretically constructed IR and Raman spectra exactly coincide with experimentally observed counterparts. The NBO result reflects the charge transfer mainly due to methoxy group. HUMO and LUMO orbitals have been visualized. It has been conclude that the lowest singlet excited state of the 1,5-dimethoxynaphthalene molecule is mainly derived from the HOMO ? LUMO (p ? p) electron transition. Acknowledgement Authors are very grateful to Professor N. Sundaraganesan for providing the Gaussian Software 03 package of computer program to run all the above calculations. 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.2014.06.092. References [1] D. Hoffmann, E.L. Wynder, A.C. Stern (Eds.), Air Pollution, vol. 2, Academic Press, New York, 1977, pp. 361–455. [2] M. Nordquist, D.R. Thakker, H. Yagi, R.E. Lehr, A.W. Wood, W. Levin, A.H. Conney, D.M. Jerina, R.S. Bhatnagar (Eds.), Molecular Basis of Environmental Toxicity, Ann Arbor Science Publisher, Ann Arbor, MI, 1980, pp. 329–357. [3] R.G. Harvey, Polycyclic Aromatic Hydrocarbons: Chemistry and Carcinogenicity, Cambridge University Press, Cambridge, UK, 1991. [4] S. Motta, C. Federico, S. Saccone, V. Librando, P. Mossesso, Mutat. Res. 561 (2004) 45–52. [5] V. Librando, A. Alparone, Environ. Sci. Technol. 41 (2007) 1646–1652. [6] V. Librando, A. Alparone, Polycycl. Aromat. Compd. 27 (2007) 65–94. [7] G.W. Buchanan, M. Gerzain, C. Bensimon, J. Mol. Struct. 354 (1995) 227–231. [8] P. Das, E. Arunan, P.K. Das, Vib. Spectrosc. 47 (2008) 1–9. [9] S. Chandra, H. Saleem, N. Sundaraganesan, S. Sebastian, Spectrochim. Acta A 74 (2009) 704–713. [10] V. Krishnakumar, R. Mathammal, S. Muthunatesan, Spectrochim. Acta A 70 (2008) 201–209. [11] M. Arivazhagan, V. Krishnakumar, R.J. Xavier, G. Ilango, V. Balachandran, Spectrochim. Acta A 72 (2009) 941–946. [12] P.B. Nagabalasubramanian, S. Periandy, Spectrochim. Acta A 77 (2010) 1099– 1107.

M. Kandasamy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 191–199 [13] E. Kavitha, N. Sundaraganesan, S. Sebastian, M. Kurt, Spectrochim. Acta A 77 (2010) 612–619. [14] R.J. Xavier, V. Balachandran, M. Arivazhagan, G. Ilango, Ind. J. Pure Appl. Phys. 48 (2010) 245–250. [15] M. Govindarajan, K. Ganasan, S. Periandy, M. Karabacak, Spectrochim. Acta 79A (2011) 646–653. [16] T. Prabhu, S. Periandy, S. Mohan, Spectrochim. Acta 78A (2011) 566–574. [17] Gaussian 03 program, Gaussian Inc., Wallingford CT, 2004. [18] H.B. Schlegel, J. Comput. Chem. 3 (1982) 214–218. [19] A. Frisch, A.B. Nielson, A.J. Holder, GAUSSVIEW User Manual, Gaussian Inc., Pittsburgh, PA, 2000. [20] V. Krishnakumar, R. Mathammal, S. Muthunatesan, Spectrochim. Acta 70A (2008) 210–216. [21] M.H. Jamroz, Vibrational Energy Distribution Analysis, VEDA 4 Computer Program, Poland, 2004. [22] National Institute of Standards and Technology. Vibrational Frequency Scaling Factors on the Web. (accessed 24.09.07). [23] G. Kereztury, S. Holly, J. Varga, G. Besenyei, A.Y. Wang, J.R. Durig, Specrochim. Acta 49A (1993). 2007–2017, 2019–2026. [24] G. Kereztury, Raman spectroscopy: theory, in: J.M. Chalmers, P.R. Griffith (Eds.), Hand Book of Vibrational Spectroscopy, vol. 1, John Wiley & Sons Ltd., New York, 2002. [25] G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, 1969. [26] N. Sundaraganesan, H. Saleem, S. Mohan, M. Ramalingam, V. Sethuraman, Spectrochim. Acta 62A (2005) 740–751. [27] P.S. Kalsi, Spectroscopy of Organic Compounds, Wiley Eastern Ltd., New Delhi, 1993, p. 116. [28] G. Mariappan, N. Sundaraganesan, J. Mol. Struct. 1063 (2014) 192–202. [29] A. Altun, K. Gölcük, M. Kumru, J. Mol. Struct. (Theochem.) 637 (2003) 155–169.

199

[30] Y.X. Sun, Q.L. Hao, Z.X. Yu, W.J. Jiang, L.D. Lu, X. Wang, Spectrochim. Acta 73A (2009) 892–901. [31] N. Sundaraganesan, B.D. Joshua, T. Radjakoumar, Ind. J. Pure Appl. Phys. 47 (2009) 248–258. [32] B. Smith, Infrared Spectral Interpretation, A Systematic Approach, CRC Press, Washington, DC, 1999. [33] M. Gussoni, C. Castiglioni, J. Mol. Struct. 521 (2000) 1. [34] J. Palomar, J.L.G. De Paz, J. Catalan, Chem. Phys. 246 (1999) 167. [35] M. Rumi, G. Zerbi, J. Mol. Struct. 509 (1999) 11. [36] D. Sajan, I. Hubert Joe, V.S. Jayakumar, J. Raman Spectrosc. 37 (2005) 508–519. [37] M. Gussoni, C. Castiglioni, M.N. Ramos, M.C. Rui, G. Zerbi, J. Mol. Struct. 224 (1990) 445–470. [38] D.N. Singh, I.D. Singh, R.A. Yadav, Ind. J. Phys. 76B (3) (2002) 307–318. [39] B.V. Reddy, G.R. Rao, Vib. Spectrosc. 6 (1994) 231–250. [40] J.F. Areanas, I.L. Tocn, J.C. Otero, J.I. Marcos, J. Mol. Struct. 410 (1997) 443–446. [41] D.A. Long, W.O. Jeorge, Spectrochim. Acta 19 (1963) 1777–1790. [42] C. Surisseau, P. Marvel, J. Raman Spectrosc. 25 (1994) 447–451. [43] A.J. Barnes, M.A. Majid, M.A. Stuckey, P. Gregory, C.V. Stead, Spectrochim. Acta 41A (1985) 629–635. [44] K. Druzbicki, E. Mikuli, M.D.O. Chrusciel, Vib. Spectrosc. 52 (2010) 54–62. [45] D. Dolega, A.M. Mikuli, J. Chrusciel, J. Mol. Struct. 933 (2009) 30–37. [46] M. Snehalatha, C. Ravikumar, I. Hubert Joe, N. Sekar, V.S. Jayakumar, Spectrochim. Acta A 72 (2009) 654–662. [47] M. Szafran, A. Komasa, E.B. Adamska, J. Mol. Struct. (Theochem.) 827 (2007) 101–107. [48] R.G. Pearson, J. Org. Chem. 54 (1989) 1423–1430. [49] M.D. Diener, J.M. Alford, Nature (London) 393 (1998) 668–671. [50] R.G. Pearson, Proc. Natl. Acad. Sci. USA 83 (1986) 8440–8441. [51] R.G. Pearson, J. Am. Chem. Soc. 110 (1988) 2092–2097. [52] T.A. Koopmans, Physica 1 (1934) 104–113. [53] E. Scrocco, J. Tomasi, Adv. Quant. Chem. 11 (1978) 115–193. [54] N. Okulik, A.H. Jubert, Int. Electron. J. Mol. Des. 4 (2005) 17–30.

Characterization of 1,5-dimethoxynaphthalene by vibrational spectroscopy (FT-IR and FT-Raman) and density functional theory calculations.

In this work, we reported a combined experimental and theoretical study on molecular structure, vibrational spectra and natural bond orbital (NBO) ana...
949KB Sizes 0 Downloads 4 Views