Density functional theory, comparative vibrational spectroscopic studies, HOMO–LUMO, first hyperpolarizability analyses of 2-fluoro 5-nitrotoluene and 2-bromo 5-nitrotoluene

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 77–86

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Density functional theory, comparative vibrational spectroscopic studies, HOMO–LUMO, first hyperpolarizability analyses of 2-fluoro 5-nitrotoluene and 2-bromo 5-nitrotoluene V. Krishnakumar a,⇑, R. Sangeetha b, R. Mathammal c, D. Barathi d a

Department of Physics, Periyar University, Salem 636 011, India Department of Physics, Nehru Memorial College, Puthanampatti 621 007, India Department of Physics, Sri Sarada College for Women (Autonomous), Salem 636 016, India d Department of Physics, N.K.R. Govt. Arts College (W), Namakkal 637 001, 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

" Simulated and observed vibrational

In this work, we reported a combined experimental and theoretical study on molecular structure, vibrational spectra, hyperpolarizability, HOMO, LUMO, NMR analysis of 2-fluoro 5-nitro toluene and 2-bromo 5-nitro toluene.

spectra are agreed well. 1 13 " H and C NMR chemical shifts have been compared with experimental values. " Polarizability, hyperpolarizability and total dipole moment have been calculated. " The HOMO–LUMO energy gap of FNT and BNT are 0.182 a.u and 0.172 a.u.

a r t i c l e

i n f o

Article history: Received 17 May 2012 Received in revised form 23 September 2012 Accepted 4 October 2012 Available online 15 November 2012 Keywords: 2-Fluoro 5-nitrotoluene 2-Bromo 5-nitrotoluene DFT FTIR FT-Raman NMR

a b s t r a c t This work deals with the vibrational spectra of 2-fluoro 5-nitrotoluene and 2-bromo 5-nitrotoluene by quantum chemical calculations. The solid phase FTIR and FT-Raman spectra of the title compounds were recorded in the regions 4000–400 cm1 and 4000–50 cm1 respectively. The spectra were interpreted with the aid of normal co-ordinate analysis based on density functional theory (DFT) using standard B3LYP/6-31G basis set for the most optimized geometry. The vibrational frequencies are calculated and scaled values are compared with experimental FTIR and FT-Raman spectra. The scaled theoretical wave numbers showed very good agreement with the experimental ones. The complete vibrational assignments are performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanical (SQM) method. 13C and 1H NMR chemical shifts results are compared with the experimental values. Ó 2012 Elsevier B.V. All rights reserved.

Introduction

⇑ Corresponding author. Tel.: +91 427 2345766x246; fax: +91 427 2345565. E-mail address: [email protected] (V. Krishnakumar). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.10.002

In the last decade, quantum mechanical ab initio and DFT(density functional theory) computational methods have gained great popularity in vibrational spectroscopy as well as in other areas of

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structural investigations. Vibrational spectroscopy and normal mode calculations for molecules and similar chemical species have become standard features of many quantum chemical program packages. In recent time, toluene and substituted toluene have become very important on account of their wide range of applications in medicine and industry. Density functional theory calculations are reported to provide excellent vibrational frequencies of organic compounds if the calculated frequencies are scaled to compensate for the approximate treatment of electron correlation, for basis set deficiencies and for the anharmonicity. Toluene is an organic compound with the chemical formula C7H8. It is a water-insoluble liquid with the typical smell of paint thinners [1]. It is mono-substituted benzene derivative. It is an aromatic hydrocarbon that is widely used as an industrial feedstock. Toluene is also capable of dissolving a number of notable inorganic chemicals such as sulfur [2]. In addition, it is used as a solvent to create a solution of carbon nanotubes. Toluene can be used to break open red blood cells in order to extracts hemoglobin in biochemistry experiments [3,4]. The methyl group makes it around 25 times more reactive than benzene in such reactions. It undergoes nitration to give ortho and para nitrotoluene isomers. Toluene is another group of fuels that have been recently used as components for jet fuel surrogate blends [5]. It had also been used in the process of removing the cocaine from coca leaves in the production of Coca-Cola syrup [6]. The inclusion of a substituent group in toluene leads to the variation of charge distribution in molecules and consequently this affects the structural, electronic and vibrational parameters. The methyl groups are generally referred to as electron donating substituent in aromatic ring systems [7]. The CH3 interacts with nearby p-system via hyper conjugation, this imply electronic delocalization is taken into account by the molecular orbital approach [8,9]. The assignments of band in the vibrational spectra of molecules are an essential step in the application of vibrational spectroscopy for solving various structural chemical problems. The vibrational spectral studies of the molecules can provide deeper knowledge about the relationships between molecular structure, on linear response and hyperpolarizability. In order to understand the vibrational properties and structural characteristics of the compounds 2-fluoro 5-nitrotoluene and 2bromo 5-nitrotoluene, the density functional theory (DFT) calculation with B3LYP/6-31G combination has been carried out and the observed bands are assigned based on the results of normal coordinate analysis [10–13]. In addition, the gauge-including atomic orbital (GIAO) 13C and 1H NMR chemical shifts calculations of the title compounds have been carried out by using B3LYP method with 6-31G.

Experimental Pure chemical compound of 2-fluoro 5-nitrotoluene and 2-bromo 5-nitrotoluene were obtained from Sigma–Aldrich chemical company, USA and was used as such without further purification to record FTIR and FT-Raman spectra. The room temperature FTIR spectra of the title compounds are measured with KBr pellet technique in the 4000–400 cm1 region at a resolution of ±1 cm1 using BRUKER IFS 66 V FTIR spectrometer equipped with a cooled MCT detector for the mid-IR range. The FT-Raman spectra are recorded using BRUKER IFS-66V model interferometer equipped with an FRA-106 FT-Raman accessory. The spectra are recorded in the 4000–50 cm1 stokes region using 1064 nm line of an Nd:YAG laser for excitation operating at 200 mW power. The reported frequencies are believed to be accurate within ±1 cm1. 1H and 13C NMR spectra are taken in DMSO

solutions and all signals are referenced to TMS on a BRUKER TPX-400 FTNMR spectrometer. All NMR spectra are measured at room temperature. Computational details The entire quantum chemical calculations have been performed at DFT (B3LYP) method with 6-31G basis set using the Gaussian 03 program [14,15]. The optimized structural parameters have been evaluated for the calculations of vibrational frequencies at different level of theories and a variety of basis sets by assuming Cs point group symmetry. At the optimized geometry for the title molecules, no imaginary frequency modes were obtained, therefore a true minimum on the potential energy surface was found. As a result, the unscaled calculated frequencies, reduced masses, force constants, infrared intensities, Raman activities, and depolarization ratios were obtained. In order to improve the calculated values in agreement with the experimental values, it is necessary to scale down the calculated harmonic frequencies. After scaled with the scaling factor, the deviation from the experiments is less than 10 cm1 with a few exceptions. The assignments of the calculated normal modes have been made on the basis of the corresponding TEDs. The TEDs were computed from quantum chemically calculated vibrational frequencies using SQM method [16,17]. Gauss view program [18] has been considered to get visual animation and also for the verification of the normal modes assignment. For NMR calculations 13C and 1H NMR chemical shifts (dH and dC) are calculated using the GIAO method [19] at B3LYP with 631G basis set. Relative chemical shifts are then estimated by using the corresponding TMS shielding calculated in advance at the same theoretical level as the reference. 13C and 1H isotropic magnetic shielding (IMS) of any X carbon (or hydrogen) atoms is made according to the value 13C IMS of TMS: CSX = IMSTMs–IMSX (1H IMS of TMS: HSX = IMSTMs–IMSX). The experimental values for 1H and 13C isotropic chemical shifts for TMS were 13.84 and 188.1 ppm, respectively [20]. Prediction of Raman intensities The Raman activities (Si) calculated with the Gaussian-03 program and adjusted during the scaling procedure with Molvib was subsequently converted to relative Raman intensities (Ii) using the following relationship derived from the intensity theory of Raman scattering [21–23]:

Ii ¼

ti

h

f ðto  ti Þ4 Si  i ti 1  exp hc kT

where to is the laser exciting wave number in cm1 (in this work, we have used the excitation wave number to = 9398.5 cm1, which corresponds to the wavelength of 1064 nm of a Nd:YAG laser), ti the vibrational wave number of the ith normal mode (cm1), while Si is the Raman scattering activity of the normal mode ti. h, k, c and T are Planck, Boltzmann constants, speed of light and temperature in Kelvin, respectively. Results and discussion Geometrical structures The molecular structure along with numbering of atoms of FNT, BNT is obtained from Gaussian 03 and GAUSSVIEW programs which are shown in Fig. 1a and b. The global minimum energy obtained by DFT/B3LYP structure optimization using 6-31G basis set for the title molecules are 3047.172 Hartrees and 575.294

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79

resonance structure, in which the electronic charge is concentrated at this site. This is the reason for the shortening of bond lengths between nitrogen and oxygen. Vibrational force constants

Fig. 1. (a) Molecular model of FNT along with numbering of atoms and (b) molecular model of BNT along with numbering of atoms.

Table 1 Optimized geometrical parameters of FNT and BNT obtained by B3LYP/6-31G density functional calculations. Bond length

C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C1–C7 C7–H8 C7–H9 C7–H10 C2–F11(Br11) C3–H12 C4–H13 C5–N14 N14–O15 N14–O16 C6–H17

Value(A°)

Bond angle

FNT

BNT

1.400 1.390 1.391 1.393 1.396 1.507 1.093 1.096 1.096 1.347 1.085 1,.231 1.468 1.231 1.231 1.083

1.405 1.395 1.391 1.392 1.392 1.506 1.095 1.093 1.095 1.908 1.084 1.082 1.471 1.230 1.230 1.083

C1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C2–C1–C7 C1–C7–H8 C1–C7–H9 C1–C7–H10 C3–C2–F11(Br11) C4–C3–H12 C5–C4–H13 C6–C5–N14 C5–N15–O15 C5–N15–O16 C1–C6–H17

The output of the quantum chemical calculations contains the force constant matrix in Cartesian coordinates in Hartree/Bohr2 units. These force constants are transformed to the force fields in the internal local-symmetry coordinates. The local-symmetry coordinates, defined in terms of the internal valence coordinates, are given in Tables ST1–ST4 for FNT and BNT, respectively. The force fields determined were used to calculate the vibrational total energy distribution (TED) among the normal coordinates. The most important diagonal stretching force constants of FNT and BNT are listed in Table ST5. The bonding properties of FNT and BNT were influenced by the rearrangements of electrons during substitutions and addition reactions. The values of stretching force constants between carbon and fluorine atoms are higher compared with carbon and bromine atom because fluorine atom is highly electronegative. Molecular vibrations and simulated spectra

Value (°) FNT

BNT

123.631 118.881 118.398 122.211 120.402 110.552 111.329 111.329 118.487 121.683 119.841 118.848 117.673 117.753 120.776

122.596 119.700 118.219 122.113 122.395 111.273 110.468 111.273 117.946 120.472 120.075 118.879 117.574 117.724 120.513

The atom indicated in the parenthesis belongs to BNT; for numbering of atoms refer Fig. 1a and b.

Hartrees, respectively. This energy difference is clearly understandable, since the molecules are under different environments. The optimized values of bond lengths and bond angles are reported in Table 1. The title compounds FNT and BNT belong to Cs point group symmetry with 17 atoms composing each structure. The optimized molecular structure of FNT and BNT reveals that para-substituted group is on the same plane with the benzene ring in Fig. 1a and b. The optimized bond lengths of C–C in FNT fall in the range:1.400–1.39 Å at B3LYP/6-31G. While the introduction of the substituent group causes slight difference between them. The breakdown of hexagonal symmetry of the benzene ring is obvious from the elongation of C1–C2 (1.400 Å) from the remaining C–C bond lengths are (1.39 Å).The asymmetry of the benzene ring is also evident from the negative deviation of C3–C4–C5 (118°) and positive deviation of C1–C2–C3 (122°) from the normal value of 120° this may be due to electron donating nature of CH3 group. The small discrepancy found between the experimental and theoretically calculated values, this is due to that the theoretical calculations belong to isolated molecules in gaseous phase and the experimental results belong to molecule in the solid state. In both molecules, the NO2 group is of strong electron-withdrawing nature and expected to increase the contribution of the

The compounds of FNT and BNT belong to CS point symmetry and their 45 fundamentals are distributed among the symmetry species as:

Cvib ¼ 31A0 ðin-planeÞ þ 14A00 ðout-of-planeÞ All the vibrations are active in both Raman scattering and infrared absorption. In the Raman spectrum the in-plane vibrations (A0 ) give rise to polarized bands while the out-of-plane ones (A00 ) to depolarized band. The observed and calculated infrared and Raman spectra of FNT and BNT are produced in common frequency scales in Figs. 2–5. The assignments of the normal modes of vibrations of the investigated molecules along with the observed fundamentals, unscaled frequencies obtained by B3LYP/6-31G calculations and scaled frequencies as well as the TED descriptions are reported in Tables 2 and 3 for FNT and BNT, respectively. Root mean square (RMS) values are obtained in this study using the following expression,

RMS ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 Xn calc 2 ðti  texp i Þ i ðn  1Þ

Deviations between the unscaled and experimental frequencies for all modes are found to be 10.3 cm1 and 13.2 cm1 for FNT and BNT respectively. This is quite obvious; since the frequencies calculated on the basis of quantum mechanical force fields usually differ appreciably from the observed frequencies. This is partly due to the neglect of anharmonicity and partly due to the approximate nature of the quantum mechanical methods. In order to reduce the overall deviation between the unscaled and observed fundamental frequencies, scale factors are applied in the normal coordinate analysis and the subsequent least square fit refinement resulted in a very close agreement between the observed fundamentals and the scaled frequencies. Refinement of the scaling factors applied in this study achieved a weighted mean deviation of 6.6 cm1 and 9.9 cm1 between the experimental and SQM frequencies of the title compounds. Due to the low symmetry of the molecule, several internal coordinates contribute to each normal mode of the title compounds. The detailed vibrational assignments of fundamental modes of FNT and FNT by normal mode analysis based on scaled quantum mechanical force field calculations are listed in Tables 2 and 3.

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Fig. 2. FTIR spectra of FNT (a) observed and (b) calculated with B3LYP/6-31G.

Fig. 4. FTIR spectra of BNT (a) observed and (b) calculated with B3LYP/6-31G.

Fig. 3. FT-Raman spectra of FNT (a) observed and (b) calculated with B3LYP/6-31G.

Carbon–hydrogen vibrations Most of the aromatic compounds have infrared peaks in the region 3100–3000 cm1 due to ring C–H stretching bonds. The calculated frequencies which lie in the range 3119, 3104 and 3090 cm1 are assigned to the three C–H stretching vibrations of the title compound in FNT and their experimental counterpart appear in the range 3126, 3087 cm1 of the IR spectrum and 3101 cm1 of the Raman spectrum. But in the case of BNT, it was observed at 3157, 3140, 3136 cm1 of IR spectrum. In the halogen atom, the C–H stretching wavenumber shifts to higher due to the inductive effect of the halogen atom. The in-plane C–H bending vibrations appear in the range 1000–1300 cm1 in the substituted benzenes and the out-of-plane bending vibrations occurs in the frequency range 750–1000 cm1 [24–27]. Accordingly, the calculated frequencies assigned to three C–H in-plane bending vibrations are

Fig. 5. FT-Raman spectra of BNT (a) observed and (b) calculated with B3LYP/6-31G.

1251, 1119, 1096 cm1 for FNT, while the observed IR frequencies are 1245, 1122, 1092 cm1. The calculated and observed frequencies assigned to three C–H out of-plane bending vibrations are 940, 906, 827 cm1 and 940, 905, 829 cm1 for FNT. In the BNT molecule the frequencies are calculated at 968, 911, 838 cm1 are assigned to three C–H out of-plane bending vibrations The slight deviation in low frequency bands are due to the interaction between NO2 and C–H out-of-plane bending frequencies, which is also confirmed by TED output. Nitro group vibrations For molecules with an NO2 group, the NO2 asymmetric stretching vibration band range is 1625–1540 cm1 and that of the

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Table 2 Assignments of fundamental vibrations of 2-fluoro 5-nitro toluene by normal mode analysis based on SQM force field calculations using selectively scaled B3LYP/6-31G force field. Sl. 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

Symmetry species (Cs)

0

A A0 A0 A0 A0 A0 0 A0 A0 A0 A0 A0 A0 0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 0 A0 A0 0 A0 A00 A00 A0 A0 A0 0 A00 A0 A0 A00 A0 A00 A0 A0 A0 0 A0 A0 A0 0 A00 A0 0 A0 0

Observed frequencies (cm1)

Calculated frequencies (cm1) with B3LYP/6-31G force field

Infrared

Raman

Unscaled

Scaled

IRa (Ai)

Ramanb(Ii)

3126 – 3087 3052 2962 – – – 1586 1524 – 1494 1448 1418 1383 1344 1279 1245 1194 1122 1092 1070 1010 940 – 905 829 813 762 747 698 637 560 540 – 443 – – – – – – – – –

– 3101 – – – 2932 1631 1588

3258 3243 3229 3140 3116 3058 1681 1648 1625 1541 1516 1513 1459 1444 1397 1378 1297 1287 1216 1149 1114 1077 1024 970 948 932 845 822 782 754 700 647 573 547 547 453 421 358 332 281 208 198 117 88 58

3119 3104 3090 3053 2970 2923 1635 1599 1579 1518 1500 1497 1439 1422 1377 1343 1284 1251 1196 1119 1096 1067 1006 940 933 906 827 808 768 751 695 622 561 540 533 448 409 349 319 266 196 188 113 84 57

0.006 0.041 4.068 0.650 1.713 3.499 1.395 0.022 3.988 1.463 1.426 0.015 0.241 15.490 4.802 23.266 11.752 16.046 18.065 13.845 9.584 0.557 2.873 2.403 59.868 4.966 5.512 12.537 73.231 10.559 393.677 0.555 0.108 15.189 48.453 23.920 107.544 100.134 43.824 15.397 10.929 12.618 1.618 1.603 2.342

0.072 0.374 0.041 2.301 0.291 0.351 1.943 2.900 4.145 0.083 1.021 0.382 6.341 2.445 0.321 1.913 19.587 4.747 3.101 1.151 7.572 1.452 2.109 0.840 4.919 5.751 5.605 0.793 20.101 1.665 203.664 10.229 26.644 15.647 6.141 6.192 26.436 53.259 7.774 181.565 93.606 48.978 97.906 51.163 75.634

– 1510 – – – – – – – – – – – – – 926 – – – – – – – – – 530 – 420 350 320 260 200 180 110 80 50

TED(%)among type of internal coordinates

tCH(99) tCH(99) tCH(99) CH3ips(93) CH3ss(56), CH3ops(44) CH3ops(53), CH3ss(40) tCC(53), tNO(17) tCC(45), tNO(23) tNO(50), tCC(33) CH3ipr(64), CH3opr(12) tCC(32), bCH(25), CH3ipr(15) CH3opr(76) CH3sb(23), CH3ipb(15), CH3ipr (14), CH3opb(13), CC(10) tCC(31), CH3opr(14), CH3opb(12), CH3sb(11), bCH(10) tNO(33), bONO(29), bONC(25), tCN(11) tCC(88) tCC(36), tCF(29), bCH(18) bCH(76) tCCm(24), bring(22), tCF(15), tCC(12), bCH(11) bCH(52), tCC(20), tCN(12) bCH(37), tCC(22), tCN(11) CH3opb(45), CH3sb(41) CH3ipb(47), CH3sb(17), tCC(10) gCH(82), tring(13) tCN(26), bring(17), bONC(16), tCC(15), bONO(10) gCH(72), tring(9) gCH(69), tring(10), gCF(9), bONO(51), bONC(37) tCF(48), tCC(17), bring(15), CCm (12) tring(39), tNO2(26), gCN(22), gCF(55), tring(22) bring(33), bONO(25), bONC(24) bCF(36), bCN(16), bONC(14) tring(43), gCN(29), gCF(16) bONC(38), bring(36) tring(63), gCCm(19) bring(27), bONC(26), bCF(19) bring(51), tCN(16), bONO(12) gCCm(43), tring(22), gCN(13) bCCm(47), bCF(26), bCN(10) bCN(57), bCCm(24) tNO2(28), tring(23), gCN(20), gCCm(17) gCN(45), tring(35), gCH(11) tCH3(82) tNO2(99)

Abbreviations used: R, ring; b, bending; t, torsion; t, stretching. Contributions larger than 10% are given. a Relative absorption intensities normalized with highest peak absorption equal to 1.0. b Relative Raman intensities calculated by Eq. (2) and normalized to 100.

symmetric stretching vibration is 1400–1360 cm1 [28]. The IR bands observed at 1599 cm1 in FNT and Raman bands observed at 1577 cm1 in BNT with medium and strong intensity were within NO2 asymmetric stretching vibrations. The Raman bands observed at 1360 cm1 in FNT and IR bands at 1377 cm1 in BNT were within NO2 symmetric stretching vibrations respectively. Aromatic nitro compounds have a band of weak-to-medium intensity in the region 590–500 cm1 [29] due to the in-plane deformation mode of the NO2 group. This was observed at 530 cm1 in IR for FNT and it was shifted to 743 cm1 in IR for FNT due to steric effect. The deformation vibrations of NO2 group (rocking, wagging and twisting) contribute to several normal modes in the low frequency region [30]. These bands were also found well within the characteristic region 510 cm1 in Raman spectrum for FNT and 200 cm1 for BNT. The NO2 torsion vibrations were observed in the Raman spectrum at 50 cm1 for FNT and 20 cm1 for BNT. As the torsion vibrations are very anharmonic, its frequency is difficult to reproduce within the harmonic approach.

Methyl group vibrations For the assignments of CH3 group frequencies, basically nine fundamentals can be associated to each CH3 group namely CH3 ss – symmetric stretch; CH3 ips – in-plane stretch (i.e. in-plane hydrogen stretching modes); CH3 ipb – in-plane bending (i.e. inplane hydrogen deformation modes); CH3 sb – symmetric bending; CH3 ipr – in-plane rocking; CH3 opr – out-of-plane rocking; tCH3 – twisting hydrogen bending modes. In addition to that, CH3 ops – out-of-plane-stretch and CH3 opb – out-of-plane bending modes of CH3 group would be expected to be depolarized for A00 symmetry species. The CH3 ss frequencies are established at 2962 cm1 and 2957 cm1 in IR for FNT and BNT compounds respectively. These assignments are also supported by literature [31] in addition to TED output. The four in-plane methyl hydrogen deformation modes are also well established. The symmetrical methyl deformation modes CH3

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Table 3 Assignments of fundamental vibrations of 2-bromo 5-nitro toluene by normal mode analysis based on SQM force field calculations using selectively scaled B3LYP/6-31G force field. Sl. 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

Symmetry species (Cs)

0

A A0 A0 A0 A0 0 A0 A0 A0 A0 A0 A0 0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 0 A0 A0 A0 0 A0 A00 A0 0 A0 A0 0 A0 0 A0 A0 A0 A00 A0 A00 A0 A0 0 A0 A0 A0 0 A0 A0 0 A0 0 A0 0

Observed frequencies (cm1)

Calculated frequencies (cm1) with B3LYP/6-31G force field

Infrared

Raman

Unscaled

Scaled

IRa (Ai)

Ramanb (Ii)

3157 3140 3130 3094 – 2956 – 1615 1608 1515 1510 1475 1438 – 1380 1343 1269 – 1151 1114 1075 1041 1027 964 918 – 834 – 742 712 690 – 530 – 482 – – – – – – – – – –

– – – – 3090 – 1660 – – – – – – 1430 – – – 1210 – – – – – – – 905 – 795 – – – 560 – 527 – 444 380 285 240 220 190 165 150 80 50

3257 3244 3230 3138 3118 3057 1672 1634 1614 1522 1515 1501 1448 1440 1396 1355 1296 1237 1161 1128 1081 1046 1029 994 938 935 859 814 758 719 699 566 540 537 484 454 388 290 253 231 199 173 155 81 57

3155 3142 3129 3110 3071 2957 1661 1621 1603 1511 1509 1480 1439 1421 1380 1348 1273 1208 1146 1113 1073 1042 1024 968 913 911 838 794 743 705 688 561 531 526 473 451 380 283 249 225 192 172 150 81 55

2.102 1.262 0.861 11.826 12.618 7.948 63.343 94.102 90.505 20.735 9.624 27.069 5.743 2.741 363.498 19.047 1.855 15.910 3.476 22.249 3.027 52.571 18.239 0.194 47.185 9.733 15.448 33.628 11.609 11.111 1.168 1.289 4.994 0.080 9.975 2.467 0.712 4.337 0.186 1.032 0.001 2.554 0.274 2.721 0.100

88.977 63.523 56.285 57.499 83.614 118.280 5.274 110.716 43.210 3.161 16.686 0.896 20.836 3.672 331.529 7.197 0.211 72.115 7.400 19.590 0.773 7.199 6.747 1.339 4.308 1.777 2.763 10.881 0.568 1.492 7.135 1.939 9.430 0.004 0.373 0.070 1.224 1.512 2.254 0.386 1.932 0.569 0.289 0.288 0.152

TED (%) among type of internal coordinates

tCH(99) tCH(99) tCH(99) CH3ips(55), CH3ops(44) CH3ops(52), CH3ips(39) CH3ss(92) tNO(40), tCC(33), bONC(10) tCC(51), tNO(21), bCH(12) tCC(46), tNO(36) CH3ipb(33), CH3ipr(21), CH3opr(20), bCH(13), CC(10) CH3opr(86) tCC(36), bCH(34) CH3ipr(44), CH3opb(20), CH3sb(15) tCC(39), CH3ipr(22), bCH(16) tNO(35), bONO(30), bONC(24) tCC(87) bCH (73), tCC(10) tCCm(33), tCC(29), bCH(15), bring(13) bCH(59), tCC(28) bCH(36), tCC(30), tCN(15) CH3opb(41), CH3sb(40) CH3sb(32), bring(19), tCC(15), CH3ipb(10) tCC(32), CH3ipb(23), bring(17), CH3sb(10) gCH(80), tring(15) tCN(29), bONC(21), bONO(14), bring(11), CCm (11) gCH(69), tring(10) gCH(76) bONC(48), bONO(40) tring(39), tNO2(26), gCN(21) tring(47), tNO2(15), gCBr(13), gCCm(12) bring(40), bONO(19), bONC(19) bring(35), bONC(33) bring(19), bCN(17), bCBr(16), tCC(12), CCm(11) gCCm(41), gCN(30), tring(11), gCBr(10) bONO(26), tCBr(22), tCN(18), bring(11) tring(59), gCBr(12), gCCm(11) bCCm(34), bONC(26), bCBr(12) gCBr(33), gCN(27), gCCm(12), tring(12) tCBr(33), bring(29), tCN(11) bCN(32), bCBr(26), bCCm(16), bring(12) gCN(35), tNO2(24), tring(20) bCBr (50), bCN(39) tCH3(82) tNO2(33), tring(30), gCN(19) tNO2(99)

Abbreviations used: R, ring; b, bending; d, deformation; x, xagging; t, torsion; t, stretching. Contributions larger than 10% are given. a Relative absorption intensities normalized with highest peak absorption equal to 1.0. b Relative Raman intensities calculated by Eq. (2) and normalized to 100.

sb appeared at 1448 cm1 in for FNT and 1041 cm1 in IR for BNT In-plane methyl deformation modes CH3 ipb are observed at 1010 cm1 and 1515 cm1 in IR. The bands at 2932, 3091 cm1 and 1070, 1075 cm1 in IR are attributed to CH3 ops and CH3 opb respectively in the title compounds. The methyl deformation modes are mainly coupled with in-plane bending vibrations. The ipr and opr modes of CH3 group are found at 1518 cm1, 1497 cm1 for FNT and 1497 cm1, 1509 cm1 for BNT, respectively. The tCH3 (methyl twisting mode) vibrations are assigned within the characteristic region and reported in Tables 2 and 3.

Carbon–nitrogen vibrations The identification of C–N vibrations is a very difficult task, since the mixing of several bands is possible in the region. Nitro aromatic compounds show a C–N stretching vibrations near 870 cm1 [32].

The observed frequency at 926 cm1 in Raman for FNT and 918 cm1 in IR for BNT are attributed to C–N aromatic stretching mode. For our title compounds, it was good agreement with TED output. The C–N bending vibrations assigned to 200 and 220 cm1 in Raman for FNT and BNT, respectively. These assignments are good agreement with the literature values [33].

Ring vibrations Many ring modes are affected by the substitution in the aromatic ring. In the present study, the bands absorbed at 1631, 1588, 1510, 1418, 1344, 1194, 1615, 1608, 1475, 1343, 1027, 1210 cm1 and 320, 260, 527, 380 cm1 for FNT and BNT have been designated to ring stretching and bending modes respectively. For most of the remaining ring vibrations, the overall agreement is satisfactory. Small changes in frequencies observed for these modes

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V. Krishnakumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 77–86 Table 4 Atomic charges for optimized geometry of FNT and BNT. Atomsa

C1 C2 C3 C4 C5 C6 C7 H8 H9 H10 F11(Br11) H12 H13 N14 O15 O16 H17

B3LYP/6-31+G Mulliken FNT

BNT

0.090771 0.352896 0. 205271 0.153364 0.280355 0.209988 0.523640 0.171563 0.167078 0.186356 0.280825 0.164433 0.189596 0.379123 0.396280 0.396575 0.183773

0.148297 0.043609 0.158760 0.150117 0.285264 0.211410 0.526248 0.175934 0.166856 0.190202 0.102257 0.170369 0.189868 0.379883 0.392481 0.393899 0.184890

a The atoms indicated in the parenthesis belongs to BNT; for numbering of atoms refer Fig. 1a and b.

Table 5 Theoretically computed energies (a.u), zero-point vibrational energies (kcal/ mol), rotational constants (GHz), entropies (cal/mol K) and dipole moment (Debye). Parameters

Total energy Zero-point energy Rotational constant

Entropy Total Translational Rotational Vibrational Dipole moment

B3LYP/6-31G FNT

BNT

575.3034 77.2188 2.44565 0.71248 0.55365

3047.173 76.14919 2.26217 0.37055 0.31903

96.254 41.025 30.189 25.040 3.7718

100.206 41.999 31.464 26.743 3.5069

are due to the changes in force constants/reduced mass ratio resulting mainly due to the extent of mixing between ring and substituent group. C–X (F, Br) vibrations Vibrations belong to C–X (F, Br) bonds which are formed between the ring and the halogen atoms are interesting since mixing of vibrations are possible due to the lowering of the molecules symmetry and the presence of heavy atoms [34,35] The assignments of C–X (C–F and C–Br) vibrations have been made by comparison with halogen substituted benzene derivatives. C–F, C–Br stretching vibrations appear in the lower range of frequencies i.e.1000–1400 cm1 and 760–505 cm1 and C–F and C–Br deformation occur in the range 700–500 cm1 and 300–140 cm1 respectively [36,37]. The bands of observed spectra identified the stretching and bending vibrations arising at 1279, 240 cm1 and 560, 165 cm1 for FNT and BNT respectively. Other molecular properties Charge analysis Atomic charges of the title compounds computed by Mulliken method and at the B3LYP/6-31G level of calculation, are illus-

Table 6 Nonlinear optical properties of FNT and BNT calculated using B3LYP/6-31G basis set. NLO behavior

FNT

BNT

Dipole moment (l) Mean polarizablity (a) Anisotropy of the polarizability (Da) First hyper polarizability (bo)

1.6633 Debye 4.230  1024 esu 85.02

1.0521 Debye 5.0097  1024 esu 103.33

0.1947  1030 esu

2.3156  1030 esu

Table 7 HOMO–LUMO energy value calculated by B3LYP/6-31G. Parameters (a.u)

HOMO LUMO HOMO–LUMO

B3LYP/6-31G FNT

BNT

0.270 0.088 0.182

0.087 0.009 0.078

trated in Table 4. The magnitudes of the carbon atomic charges for the compounds were found to be both positive and negative. These magnitudes were obtained to change between 0.523640 and 0.090771 for 2-fluoro 5-nitrotoluene; 0.526248 and 0.148297 for 2bromo 5-nitrotoluene; respectively. The maximum atomic charge was obtained for C2 atom in FNT&C5 atom in BNT molecule due to the effect of negatively charged carbon atoms. The magnitude of N atom was computed to be about 0.379123 and 0.379883 for FNT and BNT respectively. As for oxygen atoms, while the magnitudes of the oxygen atomic charges for the FNT and BNT were calculated to be only negative. The magnitudes of the hydrogen atomic charges were noted to be only positive values, indicating the charge transfer from hydrogen to carbon atom. Additionally the magnitudes of the fluorine and bromine atomic charges were noted to be only negative values, indicating the charge transfer from carbon atom to fluorine and bromine atom. Thermodynamic properties Several thermo dynamical parameters have been calculated by using DFT and 6-31G basis set has been given in Table 5. Scale factors have been recommended [38] for an accurate prediction determining the zero-point vibrational energies for DFT calculation. The total energy of the molecule is the sum of the translational, rotational, vibrational and electronic energies. Dipole moment Dipole moment reflects the molecular charge distribution and is given as a vector in three dimensions. Therefore, it can be used as descriptor to depict the charge movement across the molecule. Direction of the dipole moment vector in a molecule depends on the centers of positive and negative charges. Dipole moments are strictly determined for neutral molecules. Dipole moment values of the title compounds are shown in Table 5. Polarizability and hyperpolarizability The polarizabilities and hyperpolarizabilities characterize the response of a system in an applied electric field. The potential application of the title molecule in the field of nonlinear optics demands the investigation of its structural and bonding features contributing to the hyperpolarizability enhancement, by analyzing the vibrational modes using IR and Raman spectroscopy. Many organic molecules, containing conjugated p electrons are characterized by large values of molecular first hyperpolarizabilities, were analyzed by means of vibrational spectroscopy [39,40]. The first hyperpolarizability b of this novel molecular system of FNT and BNT are

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magnitude of total static dipole moment l, the mean polarizabilty atot, the anisotropy of the polarizabilty Da, and the mean first polarizability bo, using the x, y, z components from Gaussian 03W output is as follows.

ltot ¼ ðl2x þ l2y þ l2z Þ1=2 atot ¼ 1=3½ðaxx þ ayy þ azz Þ pffiffiffi Da ¼ 1= 2½ðaxx  ayy Þ2 þ ðayy  azz Þ2 þ ðazz  axx Þ2 þ 6a2xz þ 6a2xy þ 6a2yz 1=2 bo ¼ ½ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2 1=2 Fig. 6. HOMO and LUMO plot of FNT.

The polarizability and hyperpolarizability are reported in atomic units (a.u), the calculated values have been converted into electrostatic units (e.s.u) (for a:1 a.u. = 0.1482  1024 esu, for b:1 a.u. = 8.6393  1033 esu). Theoretically calculated values of first hyperpolarizability and dipole moment are as shown in Table 6. The large value of hyperpolarizability, b which is a function of the non-linear optical activity of the molecular system, is associated with the intramolecular charge transfer, resulting from the electron cloud movement through p conjugated frame work from electron donor to electron acceptor groups. The physical properties of these conjugated molecules are governed by the high degree of electronic charge delocalization along the charge transfer axis and by the low band gaps. So we conclude that the title molecule is an attractive object for future studies of nonlinear optical properties.

HOMO–LUMO analysis

Fig. 7. HOMO and LUMO plot of BNT.

Table 8 Theoretical and experimental 1H NMR and 13C NMR spectra of FNT and BNT (with respect to TMS, all values in ppm) for B3LYP/6-31G. Atoms

C1 C2 C3 C4 C5 C6 C7 H8 H9 H10 H14 H15 H17

FNT

BNT

Exp

Cal

Exp

Cal

126.93 167.47 116.11 123.63 144.17 127.29 14.64 7.12 8.06 8.14 2.37 2.37 2.37

132.32 170.35 120.02 128.97 147.42 131.76 27.74 8.5 8.48 7.17 2.82 2.82 2.29

139.84 132.25 133.23 122.09 147.07 125.25 23.11 7.697 7.878 8.081 2.503 2.503 2.503

144.69 155.16 136.64 126.98 150.33 130.60 33.95 7.706 8.346 8.453 2.880 2.426 2.880

For numbering of atoms refer Fig. 1a and b.

calculated using the ab initio quantum mechanical method, based on the finite-field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field. First hyperpolarizability (b) is a third rank tensor that can be described by a 3  3  3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry [41]. The first hyperpolarizability b, dipole moment l and polarizability a is calculated using 6-31G basis set on the basis of the finite-field approach. The complete equations for calculating the

The total energy, energy gap and dipole moment have influence on the stability of a molecule. The HOMO–LUMO energy gap of FNT and FNT have been calculated at the B3LYP/6-31G level and are shown in Table 7 which reveals that the energy gap reflect the chemical activity of the molecules. HOMO and LUMO are very important parameters for chemical reaction which are examined the molecular orbital (MO) for the title molecules. The HOMO is the orbital that primarily acts as an electron donor (Highest occupied MO) and the LUMO is the orbital that largely acts as the electron acceptor (Lowest unoccupied MO) and the gap between HOMO and LUMO characterizes the molecular chemical stability. The energy gap between the highest occupied and the lowest unoccupied molecular orbital, is a critical parameter in determining molecular electrical transport properties because it is a measure of electron conductivity. The energy values of HOMO are computed 0.270, and 0.087 eV and LUMO are 0.266 and 0.094 eV, and the energy gap values are 0.182, and 0.172 eV in FNT and BNT molecule, respectively. Surfaces for the frontier orbital were drawn to understand the bonding scheme of present compound. The positive phase is red and the negative one is green. According to Fig. 6, the HOMO of FNT presents a charge density localized over the benzene ring, F atom, NO2 group and CH3 group, however, LUMO is characterized by a charge distribution on all the molecule expect of hydrogen atoms in CH3 group. The HOMO ? LUMO transition implies an electron density transfer. Moreover lower in the HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. The atomic orbital compositions of the frontier molecular orbital are shown in Figs. 6 and 7.

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160 140 120 100 80 60

(b)

8

Calculated chemical shift/ppm

Calculated chemical shift/ppm

9

(a)

180

7 6 5 4 3

40

2

20 0

20

40

60

80

100

120

140

160

2

180

3

6

7

8

9

C (a) and 1H (b), NMR chemical shifts for FNT.

9

(a) Calculated chemical shift/ppm

Calculated chemical shift/ppm

5

13

Fig. 8. The linear regression between experimental and theoretical

160

4

Experimental chemical shift/ppm

Experimental chemical shift/ppm

140 120 100 80 60

(b)

8 7 6 5 4 3

40

2 20 20

40

60

80

100

120

140

160

2

3

Fig. 9. The linear regression between experimental and theoretical

NMR spectra The molecular structure of the title compounds is optimized. Then, gauge –including atomic orbital (GIAO) 13C NMR and 1H NMR chemical shifts calculations of the title compounds have been carried out by using B3LYP/functional with 6-31G basis set. The GIAO [42,43] method is one of the most common approaches for calculating isotropic nuclear magnetic shielding tensors. The NMR spectra calculations were performed by using the Gaussian 03 program package. Experimental and theoretical chemical shifts of FNT and BNT in 13 C NMR and 1H NMR spectra were recorded and the obtained data are presented in Table 8. The linear correlations between calculated and experimental data of 13C NMR and 1H NMR spectra are noted. Correlation coefficients of 13C NMR and 1H NMR are determined as 1.0 for FNT and 0.99838 and 0.90515 for BNT. The data show a good correlation between predicted and observed proton and carbon chemical shifts. The correlations of NMR spectra are presented in Figs. 8 and 9 for FNT and BNT. The agreement between the experimental and calculated data is satisfactory for carbon 13 and slightly worse for proton shifts. The protons are located on the periphery of the molecule and therefore are supposed to be more susceptible to molecular (solute – solvent) effects than carbons.

4

5

6

7

8

9

Experimental chemical shift/ppm

Experimental chemical shift/ppm 13

C (a) and 1H (b), NMR chemical shifts for BNT.

For this reason the agreement between the experimental and calculated data for protons is worse than that of carbon 13 [44]. The range of the 13C NMR chemical shifts for a typical organic molecule usually is greater than 100 ppm [45,46] and the accuracy ensures reliable interpretation of spectroscopic parameters. In the present study, the 13C NMR chemical shifts in the ring for FNT and BNT are >100 ppm, as they would be expected. The C7 and atoms in FNT and BNT to methyl shows determined 13C NMR shifts very low.

Conclusion FTIR and FT-Raman spectra of FNT and BNT were recorded and the detailed vibrational assignments were obtained. The molecular geometry, vibrational frequencies, infrared intensities and Raman scattering activities of the molecules were calculated by using DFT (B3LYP) method with 6–31G basis set. The influence of halogen atoms in the title compounds and their role in the vibrational spectral data were also discussed. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. 1H and 13C NMR chemical shifts were compared with experimental values. As a result, all the vibration frequencies were calculated and scaled values (with 6-31G basis set) were compared with

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experimental FTIR and FT-Raman spectra. The observed and the calculated frequencies are in good agreement. The difference between the corresponding wave numbers (observed and calculated) is very small for most of fundamentals. Therefore, the results presented in this work for FNT and BNT indicate that this method is reliable for prediction of both infrared and Raman spectra of the title compounds. Furthermore, the nonlinear optical, first-order hyperpolarizabilities and total dipole moment properties of the molecules show that the title molecules are an attractive object for future studies of nonlinear optical properties. Acknowledgements

[15] [16] [17] [18] [19] [20] [21] [22]

The authors are thankful to Sophisticated Analytical Instrumentation Facility (SAIF), IIT Madras, Chennai, and Nehru Memorial College, Puthanampatti, Trichirappalli, India for providing spectral measurements.

[23]

Appendix A. Supplementary material

[26]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.10.002.

[24] [25]

[27] [28] [29]

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