First order molecular hyperpolarizabilities and intramolecular charge transfer from vibrational spectra of NLO material: 2,6-dichloro-4-nitroaniline

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1 Indian Journal of Pure & Applied Physics Vol. 51, September 2013, pp First order molecular hyperpolarizabilities and intramolecular charge transfer from vibrational spectra of NLO material: 2,6-dichloro-4-nitroaniline A Janaki a, V Balachandran b, *& A Lakshmi c a Department of Physics, Govt Arts College for Women (Autonomous), Pudukkottai , India b Department of Physics, Arignar Anna Govt Arts College, Musiri , India c Department of Physics, Government Arts College, Tiruchirappalli , India * brsbala@rediffmail.com Received 3 December 2012; revised 17 March 2013; accepted 5 July 2013 Vibrational spectral analysis of the non-linear optical (NLO) material, 2,6-dichloro-4-nitroaniline (DCNA) has been carried out by using FT-IR and FT-Raman spectroscopic techniques. The Hartree-Fock(HF) and Density Functional Theoretical (DFT) computations have been performed at G** level to derive equilibrium geometry, vibrational wavenumbers, intensities and first order hyperpolarizability. The optimized geometry, various bonding features and harmonic vibrational frequencies of DCNA have been investigated. Vibrational bands to the various structural groups and their significance were investigated by analyzing the vibrational spectra. Vibrational analysis reveals that simultaneous IR and Raman activation to the phenyl ring modes also provide evidence for the charge transfer interaction between the donors and acceptor can make the molecule polarized and the intramolecular charge transfer interaction must be responsible for the NLO properties of DCNA. The Mulliken atomic charge analysis was also made in the present study. The dipole moment (µ), polarizability (α) and the hyperpolarizability ( ) values of the investigated molecule have been computed using HF and B3LYP methods. Keywords: FT-IR, FT-Raman, HF, DFT, 2,6-dichloro-4-nitroaniline 1 Introduction Non-linear optical (NLO) materials capable of generating the second harmonic generation (SHG) play a significant role in the domain of optoelectronic and laser technology. There is much interest for non-linear optical materials because of their applications in telecommunications optical computing, optical data storage and optical information processing 1-3. Organic NLO materials have good non-linear optical susceptibilities but low laser damage threshold than inorganic counterparts. Conjugated organic systems have some advantages over inorganic materials; they possess relatively low dielectric constant, faster responses times, small refractive indices and they crystallize in non-centro symmetric system. Overlapping of -bonds inorganic materials causes delocalization of charges and their leads to increased non-linearity 4. Vibrational spectroscopy has been recently employed to investigate the structural features responsible for these NLO properties 5, which seem to be related to the existence of a larger intramolecular charge transfer at relatively low energy values, which in turn depends on the efficiency of the -electron delocalization through the donor and the acceptor groups. Aniline based compounds play a very important role in designing organic materials for molecular electronics. The high second order hyperpolarizabilities of many nitroaniline molecules have lead to describe them as photo types for second harmonic generation. Aniline and its derivatives have also been widely used as starting materials in a vast amount of pharmaceutical, electro-optical and many other industrial processes. The understanding of their molecular properties as well as nature of reaction mechanisms they undergo has great importance. The inclusion of a substituent group in aniline leads to the variation of charge distribution in the molecule, and consequently, greatly affects the structural, electronic and vibrational parameters 6, 7. Many researchers studied the vibrational spectra and non-linear optical properties of aniline derivaties. Ravikumar et al 8. established the NLO property and vibrational spectra of 4-methoxy-2-nitroaniline. Krishnakumar et al 7. studied the FT-IR and FT-Raman spectral analysis of 2,6-dibromo-4- nitroaniline and 2-(methylthio) aniline based on density functional theory. More recently 9,10, the experimental vibrational spectra of 3-chloro-4-

2 602 INDIAN J PURE & APPL PHYS, VOL 51, SEPTEMBER 2013 methylaniline, 2,4-dichloroaniline and 3,4-dimethoxy aniline have been investigated in comparison with ab-initio and DFT values. The evolution of DFT that includes electron correlation in an alternative way affording opportunities of performing vibrational analysis of moderately large organic molecules 11. The results from HF and DFT theory with results obtained from experiments have shown that the methods using B3LYP are the most promising in providing correct vibrational wave numbers. Systematic studies on the vibrational spectra and structure of the title compound with the aid of HF/B3LYP have been carried out in the present paper. 2 Experimental Details The fine sample of DCNA was obtained from Lancaster Chemical Company, UK and used as such without any further purification. The room temperature Fourier transform infrared spectra of the title compound was measured in the region cm 1 at a resolution of ±1 cm 1, using BRUKER IFS 66V vacuum Fourier transform spectrometer, equipped with an MCT detector, a KBr beam spitter and globar source. The FT-Raman spectra were also recorded on the same instrument with FRA 106 Raman accessories in the region cm 1, Nd:YAG Laser operating at 200 mw power with 1064 nm excitation was used as source. 3 Computational Methods The molecular structure optimization of the title compound and corresponding vibrational harmonic frequencies were calculated using HF and DFT with Beckee-3-Lee-Yang-Parr (B3LYP) with G** basis set using Gaussian 09 program 12 package without any constraint on the geometry. Geometries have been first optimized with full relaxation on the potential energy surfaces at HF/ G** basis set and then reoptimized at B3LYP/ G** level. The optimized geometrical parameters, true rotational constants, fundamental vibrational frequencies, IR intensity, Raman activity and dipole moment were calculated by using the Gaussian 09 program package. By combining the results of the GAUSSVIEW 13 program with symmetry considerations, vibrational frequency assignments were made with a high degree of accuracy. However, the defined coordinates forms complete set and matches quite well with the motions observed using GAUSSVIEW program. The symmetry of the molecule was also helpful in making vibrational assignments. The symmetry of the vibrational modes was determined by using standard procedure 14,15 of decomposition the traces of the symmetry operations into the irreducible representations. The Raman activities (S i ) calculated by the Gaussian-09 program were converted to relative Raman intensities (I i ) using the following relationship derived from the basic theory of Raman scattering 16,17. 4 f ( υ0 υi ) si Ii = υ [1 exp( hcυ )] / kt i i where ν 0 is the exciting frequency (in cm 1 ), ν i is the vibrational wave number of the i th normal mode, h, c and k are the universal constants and f is the suitably chosen common normalization factor for all the peak intensities. The simulated FT-Raman and FT-IR spectra plotted from the calculated intensity values using pure normal Lorentzian band shape with a band width of ±10 cm 1. The analysis for the vibrational modes of DCNA was presented in some detail in order to better describe the basis for the assignments. All the parameters were allowed to relax and all the calculations converged to an optimized geometry which corresponds to a true energy minimum, as revealed by the lack of imaginary values with wave number calculations. The Cartesian representation of the theoretical force constant has been computed at the fully optimized geometry by assuming the molecule belongs to C s point group symmetry. The transformation force field from Cartesian to internal local symmetry coordinates, scaling the subsequent normal coordinate analysis (NCA), calculations of potential energy distribution (PED) were done on a PC with the version V7.0-G77 of the MOLVIB program written by Sundius 18,19. 4 Results and Discussion 4.1 Molecular geometry The molecular structure along with numbering of atoms of DCNA is as shown in the Fig. 1. The energy was obtained by the HF and B3LYP are as and hartrees, respectively. The most optimized structural parameters (bond length, bond angle and dihedral angle) were also calculated by HF and B3LYP with G** basis sets and were compared with the experimental data as presented in Table 1. The C1 C2 and C1 C6 bonds occupy the high bond length values as compared to other C C bond.

3 JANAKI et al.: MOLECULAR HYPERPOLARIZABILITIES AND INTRAMOLECULAR CHARGE TRANSFER 603 Fig. 1 Optimized molecular structure of 2,6-dichloro-4- nitroaniline. This may be due to the effect of substitution of amino group in C1 atom. The dihedral angle of C2 C1 C6 Cl16, C1 C2 C3 H11, Cl10 C2 C3 C4, H11 C3 C4 C5, N12 C4 C5 C6, and C3 C4 N12 O13 is 179 by HF and B3LYP methods C2 C3 C4 N12 and C3 C4 C5 H15 are coplanar angle. The calculated values are (both method HF/B3LYP) compared with aniline 20,21 p-methylaniline 22 which the crystal structure has been solved. The calculated values of bond length and bond angle are slightly longer (or) shorter than the experimental values, these variations may be due to the substitution of NH 2, NO 2 and Cl groups Vibrational assignments A detailed description of vibrational modes can be given by means of normal coordinate analysis. For this purpose, the full set of 54 standard internal coordinates containing 12 redundancies were defined as given in Table 2. From these, a non-redundant set of local symmetry coordinates were constructed by suitable linear combinations of internal coordinates following the recommendation by Rauhut and Pulay et al 15. as presented in Table 3. The theoretically calculated HF and DFT force fields were transformed to this later set of vibrational coordinares and used in all subsequent calculations. The observed and calculated wave numbers and normal mode descriptions for the title compound are presented in Table 4. When using computational methods to predict normal vibrations for relatively complex polyatomic molecules, scaling strategies are used to bring computed wave numbers. The vibrational frequencies obtained from HF and B3LYP were suitably scaled using the various scale factors for stretching, in-plane bending, out-of-plane bending and ring vibrations. The vibrational assignments in the present work are based on the B3LYP frequencies, infrared intensities, Raman activities as well as characteristic group frequencies. In agreement with Cs symmetry, all the 42 vibrations are distributed as 16 stretching vibrations, 13 in-plane and 13 out-of-plane vibrations of same symmetry species. Assignments were made through visualization of the atomic displacement representations for each vibrations, viewed through GAUSSVIEW 13 and matching the predicted normal wave numbers and intensities with experimental data. The harmonic vibrational frequencies calculated for DCNA at HF and B3LYP levels using the triple split valence basis set along with the diffuse and polarization function, G** observed FT-IR and FT-Raman frequencies for various modes of vibrations have been presented in Table 4. The comparison of frequencies calculated at HF and B3LYP with the experimental values reveal the over estimation of the calculated vibrational modes due to the neglect of a harmonicity in real system. Inclusion of electron correlation in the density functional theory to certain extend makes the frequency values smaller in comparison with the HF frequency data. Reduction in the computed harmonic vibrations, although basis set sensitive in only marginal as observed in the B3LYP values using G**. Any way notwithstanding the level of calculations, it is customary to scale down the calculated harmonic frequencies in order to develop the agreement with the experiment. The scaled calculated frequencies minimize the root-mean square difference between calculated and experimental frequencies for bands with definite identifications. The descriptions concerning the assignment have also been indicated in Table 4. The computed vibrational spectral IR intensities, Raman activities of the title molecule for corresponding frequencies by HF and B3LYP methods with G** basis set have also been collected in Table 4. For visual comparison, the observed and simulated FT-IR and FT-Raman spectra are shown in Figs 2 and 3, respectively. It is convenient to discuss the vibrational spectra of DCNA in terms of characteristic spectral regions.

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5 JANAKI et al.: MOLECULAR HYPERPOLARIZABILITIES AND INTRAMOLECULAR CHARGE TRANSFER 605 No. Symbol Type Definition Table 2 Definition of internal coordinates of 2,6-dichloro-4-nitroaniline Stretching 1-6 ri C C C1 C2, C2 C3, C3 C4, C4 C5, C5 C6, C6 C1 7-8 Ri C N C1 N7, C4 N Pi C H C3 H11, C5 H Qi C Cl C6 Cl16, C2 Cl qi N H N7 H8, N7 H Ti N O N12 O13 Bending(in plane) i Ring C1 C2 C3, C2 C3 C4, C3 C4 C5, C4 C5 C6, C5 C 6C1, C6 C1 C i CCH C4 C3 H11, C4 C5 H15, C2 C3 H11, C6 C5 H i CCl C3 C2 Cl10, C1 C2 Cl10, C5 C6 Cl16, C1 C6 Cl i CCN C6 C1 N7, C2 C1 N7, C3 C4 N12, C5 C4 N i CNO C4 N12 O13, C4 N12 O14 37 i O N O O13 N12 O i CNH C1 N7 H8, C1 N7 H9 40 i H N H H8 N7 H9 Out of plane bending i CH H11 C3 C4 C2, H15 C5 C4 C i CCl Cl10 C2 C3 C1, Cl16 C6 C5 C1 45 i NO 2 C4 N12 O13 O i CN N7 C1 C2 C6, N12 C4 C3 C5 48 i NH C1 N7 H8 H9 Torsion i Ring C1 C2 C3 C4, C2 C3 C4 C5, C3 C4 C5 C6, C4 C5 C6 C1, C5 C6 C2 C1, C6 C1-C2 C3 Table 3 Definition of local symmetry coordinates of 2,6-dichloro-4-nitroaniline No. Symbol Definition Scale factors used 1-6 CC r1, r2, r3, r4, r5, r CN(amino) r7, r CH P9, P CCl Q11, Q NH 2 ss (q13+q14)/ NH 2 ass (q13-q14)/ NO 2 ss (T15+T16)/ 2,(T17+T18)/ NO 2 ass (T15-T16)/ 2,(T17-T18)/ Rtrigd ( )/ Rsymd ( )/ Rasyd ( )/ b CH ( 23-24)/ 2, ( 25-26)/ 2, b CCl ( 27-28)/ 2,( 29-30)/ b CN ( 31-32)/, ( 33-34)/ NO 2 rock ( 35-36)/ NO 2 twist ( )/ NO 2 sciss ( )/ NH 2 rock ( 38-38)/ NH 2 twist ( )/ NH 2 sciss ( )/ CH 41, CCl 43, NO 2 wag CN 46, NH 2 wag t Rtrig ( )/ t Rsym ( )/ t Rasy ( )/

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7 JANAKI et al.: MOLECULAR HYPERPOLARIZABILITIES AND INTRAMOLECULAR CHARGE TRANSFER 607 Fig. 2 Observed and simulated FT-IR spectra of 2,6-dichloro- 4-nitroaniline. (a) Observed (b) HF/ G** (c) B3LYP/ G** Amino group vibrations The molecule under consideration possesses one NH 2 group and hence six internal modes of vibrations are possible such as; the symmetric stretching, antisymmetric stretching, scissoring, rocking, wagging and the twisting modes. Subash Chandra Bose et al 23. assigned the cm 1 region for the NH 2 stretching vibration of NH 2 group. The anti-symmetric stretching mode calculated at higher frequencies 3492, 3483 cm 1 than the symmetric stretching one at 3383, 3365 cm 1 in HF and B3LYP and was related to the FT-IR bands observed experimentally at 3480, 3360 cm 1, and FT-Raman band observed at 3362 cm 1. So the strong intensities of the bands corresponding to the deformation vibration of NH 2 in the range cm 1 reported for aniline by Thomson 24. Based on this, the internal deformation vibration is known NH 2 as scissoring frequency observed at 1606 cm 1 in FT-Raman spectrum as a medium strong band and it is also found at 1629 cm 1 in HF and 1611 cm 1 in B3LYP method. The B3LYP method is closer to the observed value. The calculated values 895 cm 1, 890 cm 1 belong to NH 2 rocking vibration by HF and B3LYP, respectively. The observed strong band value at 889 cm 1 (FT-IR) is assigned to NH 2 rocking mode, which is in agreement with the calculated frequency of B3LYP method. In this study, the observed FT-Raman band at 292 cm 1 is assigned to wagging of NH 2 vibration, while the NH 2 wagging mode calculated at 302 cm 1 in HF and 292 cm 1 in B3LYP method. The B3LYP method value coincides with experimental value. The weak Raman band observed at 381 cm 1 is assigned to NH 2 twisting mode, which is in agreement with theoretical values at 390 in HF and B3LYP method at 383 cm 1. Fig. 3 Observed and simulated FT-Raman spectra of 2,6-dichloro-4-nitroaniline. (a) Observed (b) HF/ G** (c) B3LYP/ G** C H vibrations The assignments of the carbon hydrogen stretching modes are straight forward on the basis of the HF/ G** and B3LYP/ G** predicted wave numbers. The DCNA molecule gives rise to two C H stretching, two C H in-plane bending and two C H out-of-plane bending vibrations. Aromatic compounds commonly exhibit multiple weak bands in the region cm 1 due to aromatic C H stretching vibration 25. The aromatic C H stretching vibrations are observed at 3067 cm 1 in IR and 3091 cm 1 in Raman which are found to be in agreement with calculated values by B3LYP/ G** at 3066 and 3093 cm 1.

8 608 INDIAN J PURE & APPL PHYS, VOL 51, SEPTEMBER 2013 The C H in-plane bending vibrations usually occur in the region cm 1 and very useful for characterization purposes 26,27. In the present work, the bands due to C H in-plane ring vibration interacting somewhat with C C stretching vibration are observed as a number of medium strong intensity sharp bands in the region cm 1. The values observed at 1163,922 cm 1 in IR spectrum and 1153,924 cm 1 in Raman spectrum are assigned to C H in-plane bending vibrations. These values at 1186 and 933 cm 1 are calculated by HF/ G** method and 1160 cm 1, 925 cm 1 are calculated by B3LYP/ G** are assigned to C H in-plane bending vibrations. The C H out-of-plane bending vibrations 28 are strongly coupled vibrations and occur in the region cm 1. In the present work, the peaks observed at 822 cm 1 in FT-IR and the FT-Raman bands at 864 and 818 cm 1 are assigned to C H outof-plane bending vibrations. The calculated C H outof-plane bending vibrations occur in the range at 873 cm 1, 837 cm 1 by HF/ G** and 864 cm 1, 820 cm 1 by B3LYP/ G** method and the calculated values (B3LYP) are nearly equal to the observed values. All these vibrational frequencies show good agreement with literature data 29, C Cl vibrations The vibrations belonging to the bond between the ring and the halogen atoms are worth to discuss here, since mixing of vibrations is possible due to the presence of heavy atoms on the periphery of the molecule 31, 32. In DCNA, the C Cl stretching vibrations appeared at 744 and 722 cm 1 in FT-IR and 727 cm 1 in Raman spectrum. The C Cl in-plane bending vibrations were found at 364 and 315 cm 1 in FT-Raman spectrum. The C Cl out-of-plane bending mode is recorded at 122 cm 1 in FT-Raman. The calculated values of C Cl stretching, in-plane and out-of-plane bending modes are found to be at 748, 736, 371, 327, 126, 88 cm 1 in HF/ G** and 744, 725, 365, 316, 121, 86 cm 1 in B3LYP/ G** C N vibrations The C N stretching frequency is rather difficult task since there are problems in identifying these frequencies from other vibrations 33. The assigned C N stretching vibrations in the present work, a IR band is observed at 1063 cm 1 and 1313 cm 1, 1061 cm 1 in Raman due to the stretching vibration between carbon and nitrogen atoms which is in line with the literature 33,34. The bands observed at 556 and 480 cm 1 in IR spectrum are assigned to C N in-plane bending vibrations and the C N out-of-plane bending vibrations have been assigned at 307 cm 1 and 236 cm 1, respectively. This implies that the C N vibrations are not much influenced by other substitutions in the ring and also this vibration not influenced by amine. It should be highlighted that the frequency calculated with the B3LYP/ G** method for the C N in-plane bending mode at 480 cm 1 is found to be in agreement with experimental value at 480 cm C C vibration In our present study, the frequencies observed in the FT-IR spectrum at 1563, 1488, 1276 cm 1 and, 1576, 1485, 1335, 1274 cm 1 in FT-Raman are assigned to C C stretching vibrations. The theoretically calculated C C stretching vibrations by B3LYP/ G** method at cm 1 show good agreement with recorded spectrum as well as the literature data 35. Several ring modes were affected by the substitution to the aromatic ring of aniline. The bands ascribed at 544 and 511 cm 1 in FT-IR and 515, 472, 204, 174, 130 in FT-Raman spectra have been designated to ring in-plane and outof-plane bending modes, respectively NO 2 group vibrations The most characteristic bands in the spectra of nitro compounds are due to NO 2 -stretching vibrations, which are the two most useful group wave numbers, not only because of their spectral position but also for their strong intensity. In nitro compounds the antisymmetric NO 2 stretching vibrations are located in the region 1580±80 cm 1. The symmetric NO 2 stretching vibrations 34 are expected in the region 1380±20 cm 1. In nitrobenzenes, ν s NO 2 appears strongly at 1345± 30 cm 1, in 3-nitropyridine 37 at 1350±20 cm 1 and in conjugated nitroalkenes 38 at 1345±15 cm 1. In the present case, the band observed at 1638 cm 1 in the IR spectrum is assigned as asymmetric NO 2 mode. The ν s NO 2 mode is observed at 1375 cm 1 in the IR spectrum as a strong band. The B3LYP calculations give 1640 and 1372 cm 1 as asymmetric and symmetric stretching modes, respectively. According to some researchers 39-41, the NO 2 scissoring 36 occur in the region 850±60 cm 1 when conjugated to C=C or aromatic molecules, with a contribution of the νcn, which is expected to be near

9 JANAKI et al.: MOLECULAR HYPERPOLARIZABILITIES AND INTRAMOLECULAR CHARGE TRANSFER cm 1. For nitrobenzene (or) nitroaniline, δno 2 is reported 41 at 852 cm 1. For the title compound, the B3LYP calculations give the band at 773 cm 1 as the deformation band of NO 2. The band observed at 778 cm 1 in the IR spectrum is assigned as δno 2 vibration. In aromatic compounds, the wagging mode, ωno 2 is assigned at 740±50 cm 1 with a moderate to strong intensity a region in which γ CN is also active 36. ωno 2 is reported at 701 and 728 cm 1 for 1,2-dinitrobenzene and at 710 and 772 cm 1 for 1,4-dinitrobenzene 36. For the title compound, the band at 622 cm 1 in the IR spectrum, and 623 cm 1 (B3LYP) are assigned as ξno 2 vibration. In aromatic compounds, the rocking mode, σno 2 is active with region 545±45 cm 1. Nitrobenzene 36 shows this rocking mode at 531 cm 1. In the present case, the B3LYP calculations give rocking mode of NO 2 at 538 cm 1, Sundaraganesan et al. 42 reported the deformation bands at 839, 744 and 398 cm 1 (experimentally), and 812, 716,703 and 327 cm 1 theoretically. The other NO 2 vibrations are also falls within their characteristic regions. 4.3 HOMO-LUMO The analysis of the wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state and is mainly described by one electron excitation from the highest molecular orbital (HOMO) to the lowest unoccupied orbital (LUMO). The LUMO of nature, (i.e., benzene ring) is delocalized over the whole C C bond. By contrast, the HOMO is located over NH 2 atoms; consequently the HOMO LUMO transition implies an electron density transfer to aromatic part of conjugated system from NH 2 group. Moreover, these are significantly overlap in their position of the ring. The frontier molecular orbital pictures are shown in Fig. 4. The electronic transition absorption corresponds to the transition from the ground to the first excited state and is mainly described by an electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO-LUMO energy gap of DCNA was calculated at the B3LYP/ G** level which shows that the energy gap reflects the chemical activity of the molecule. LUMO as an electron acceptor represents the ability to obtain an electron, HOMO represents the ability to donate an electron. Fig. 4 Frontier orbitals of 2,6-dichloro-4-nitroaniline HOMO energy = au LUMO energy = au HOMO-LUMO energy gap au 4.4 Prediction of first hyperpolarizability-nlo property There is an intense current research activity in the area of molecular linear and non-linear optics, devoted to the search for efficient, stable, simple organic molecules exhibiting large hyperpolarizabilities Even though several promising structural motifs have been identified, aromatic backbone molecules are still common and show large non-linear optical properties. Organic non-linear materials have attracted a keen interest in recent years owing to their potential applications in various photonic technologies. Significant effects have focused on studying the

10 610 INDIAN J PURE & APPL PHYS, VOL 51, SEPTEMBER 2013 electronic and structural properties of donor-acceptor substituted -conjugated organic molecules with large molecular non-linear optical (NLO) response (, firstorder hyperpolarizability). Two factors are attributed to NLO properties of such molecules in an electric field: the altered ground state charge distribution by the donor and acceptor moieties and the enhanced -electronic charge redistribution through the -conjugation. The experimental spectroscopic behaviour described above is well accounted for by ab-initio calculations and conjugated systems predict exceptionally large Raman and infrared intensities for the same normal modes. It is also observed in our title compound that the bands at 1276, 1063, 922, 822, and 511cm 1 in FT-IR spectrum have their counterparts in Raman at 1274, 1061, 924, 818, and 515 cm 1, shows that the relative intensities in IR and Raman spectra are comparable. The first hyperpolarizability is associated with the intramolecular charge transfer (ICT), resulting from the electron cloud movement through the -conjugated framework from electron donor to acceptor groups. The electron cloud is capable of interesting with an external electric field and thereby altering the dipole moment and the first hyperpolarizability. A reliable prediction of molecular hyperpolarizability requires adequate basis sets and therefore, must involve both diffuse and polarization functions. As the basis becomes larger, one expects a better description of the molecule and accordingly, more accurate results. In the view of these points, B3LYP/ G** method has been used for present study in order to see the effects of the level of theory and basis sets. The title molecule fully optimized at B3LYP/ G** method in the Gaussian 09 program. The tensor components of the static first hyperpolarizabilities, vec are calculated for the title molecule by taking into account the Kleinman symmetry relations and the squared norm of the Cartesian expression for the tensor. The relevant expressions used for the calculation are as given below. The total static dipole moment is: µ = ( µ + µ + µ ) /2 x y z The isotropic polarizability is: α xx + α yy + α zz α0 = 3 The polarizability anisotropy invariant is: α = 2 [( α α ) + ( α α ) 1/2 2 2 xx yy yy zz + α α + α 2 ( zz xx ) 6 xx ] The average hyperpolarizability is: β = ( β + β + β ) /2 x y z vec /2 β = 3 / 5[( β x + β y + β z ) ] and β = β + β + β x xxx xyy xzz β = β + β + β y yyy xxy yzz β = β + β + β z zzz xxz yyz The total static dipole moment, polarizabilities and first hyperpolarizabilities of DCNA were calculated. Table 5 lists the values of the electric dipole moments (Debye) and dipole moment components, polarizabilities and hyperpolarizabilities of the DCNA. Analyzing the and vec, it can be seen that there is the additive contribution off-diagonal -vectors to the total due substitution in benzene. Such kind of behaviour (large off diagonal contributions) has high practical utility in the NLO materials research 46. The calculated first hyperpolarizability of DCNA is esu and presented in Table 5. The total dipole moment (µ) and mean polarizability are Debye and esu, respectively. The µ value shows that there is significant increase in optical nonlinearities of the title molecule. Table 5 B3LYP/6-311+G(d,p) calculated electric dipole moments (Debye), Dipole moments components, polarizability (a.u), components and (10 30 esu) value of 2,6-dichloro-4- nitroaniline Parameters Values Parameters Values µ x β xxx µ y β xxy µ z β xyy µ(debye) β yyy α xx β xxz α xy β xyz α yy β yyz α xz β xzz α yz β yzz α zz β zzz α 0 (esu) β(esu) α(esu) vec (esu)

11 JANAKI et al.: MOLECULAR HYPERPOLARIZABILITIES AND INTRAMOLECULAR CHARGE TRANSFER NBO analysis The first two columns in the Table 6 give the type of orbital and occupancy between 0 and electrons. The type can be bonding, lone pair and antibonding. A normal Lewis structure would not have any anti-bonding orbital, so the presence of antibonding orbitals shows deviation from normal Lewis structure. Anti-bonding localized orbital s are called non-lewis NBO s. If the occupancy is not 2.000, then there is deviation from an ideal Lewis structure. The DCNA molecule shows some deviations, otherwise is well approximated using Lewis structure. In Table 6, BD(1)C1 N7 orbital with electrons has 42.71% C1 character in a sp 2.76 hybrid and has 57.29% N7 character in a sp 2.25 hybrid. The sp 2.76 hybrid on C has 73.37% p-character and the sp 2.25 hybrid on H has 69.17% p-character. An idealized sp 2 hybrid has 75% p-character. The two coefficients, and are called polarization coefficients. The sizes of these coefficients show the importance of the two hybrids in the formation of the bond. The nitrogen has larger percentage of this NBO, at 57.29% and gives the larger polarization coefficient of because it has the higher electronegativity. Similar results are found in all the BD(1)C N, and BD(1)N O orbitals. At the end of the Table 6, lone pair nitrogen molecules are expected to the Lewis structure. 4.6 Perturbation theory energy analysis Delocalization of the electron density between occupied Lewis type (bond (or) lone pair) NBO orbital s and formally unoccupied (anti-bond (or) Rydberg) non Lewis NBO orbital s corresponding to a stabilizing donor-acceptor interaction. The energy of this interaction can be estimated by the second-order perturbation theory 47. Table 7 lists the calculated second-order interaction energies (E (2) ) between the donor-acceptor orbital s in DCNA. Table 6 Natural bond orbital analysis of 2,6-dichloro-4-nitroaniline BOND(A B) ED/ ENERGY (a.u) ED A (%) ED B (%) NBO S (%) P (%) BD(1)C1 C (sp 1.72 ) C (sp 1.63 ) C BD(1)C1 C (sp 1.74 ) C (sp 1.62 ) C BD(1)C1 N (sp 2.76 ) C (sp 2.25 ) N BD(1)C2 C (sp 1.63 ) C (sp 1.81 ) C BD(1)C2 Cl (sp3.18)c (sp5.06)cl BD(1)C3 H (sp 1.87 ) C (sp 1.67 ) H BD(1)C4 N (sp 3.00 ) C (sp 1.84 ) N BD(1)C5 H (sp 2.39 ) C (sp 0.00 ) H BD(1)C6 C (sp 3.25 ) C (sp 4.95 ) N BD(1)N7 H (sp 2.91 ) N (sp sp 0.00 ) H BD(1)N7 H (sp 2.91 ) N (sp sp 0.00 ) H BD(1)N12 O (sp 2.09 ) N (sp 2.77 ) H BD(1)C1 C (sp 1.72 ) C (sp 1.63 ) C BD(1)C1 C (sp 1.74 ) C (sp 1.62 ) C BD(1)C1 N (sp 2.76 ) C (sp 2.25 ) N BD(1)C2 C (sp 1.63 ) C (sp 1.81 ) C Contd

12 612 INDIAN J PURE & APPL PHYS, VOL 51, SEPTEMBER 2013 Table 6 Natural bond orbital analysis of 2,6-dichloro-4-nitroaniline Contd BOND(A B) ED/ ENERGY (a.u) ED A (%) ED B (%) NBO S (%) P (%) BD(1)C2 Cl (sp 3.18 ) C (sp 5.06 ) Cl BD(1)C3 H (sp 2.38 ) C (sp 0.00 ) H BD(1)C4 N (sp3.00)c (sp1.84) N BD(1)C5 H (sp1.81) C (sp1.84) H BD(1)C6 Cl (sp3.25) C (sp4.95) Cl BD(1)N7 H (sp2.91) N ( sp0.00) H BD(1)N7 H (sp2.91) N (sp sp0.00) H BD(1)N12 O (sp2.09) N (sp2.77) O BD(1)N1 O (sp2.09) N (sp2.77) O LP(1)N sp LP(1)Cl Sp LP(1)O Sp LP(1)O Sp LP(1)Cl Sp BD 2-centred bonding; ED electron density; ED A electron density of A atom; ED B electron density of B atom; LP lone pair Table 7 Second-order perturbation energies E (2) (kcal/mol) corresponding to the most important charge transfer interactions (donor-acceptor) in the compound studied by B3LYP/ G** method Donor NBO (i) Acceptor NBO (j) E (2) (kcal/mol) E(j) E(i) (a.u) F(i,j) (a.u) BD(1)C1 C2 BD*(1)C1 C BD(1)C1 C2 BD*(1)C1 N BD(1)C1 C6 BD*(1)C1 C BD(1)C1 C6 BD*(1)C1 N BD(1)C1 C6 BD*(1)N7 H BD(1)C1 N7 BD*(1)C1 C BD(1)C2 C3 BD*(1)C1 N BD(1)C2 C3 BD*(1)C3 C BD(1)C2 C3 BD*(1)C4 H BD(1)C2 Cl10 BD*(1)C1 C BD(1)C3 C4 BD*(1)N12 O BD(1)C3 H11 BD*(1)C4 N BD(1)C4 C5 BD*(1)N12 O BD(1)C4 N12 BD*(1)C2 C BD(1)C5 H15 BD*(1)C4 N BD(1)N7 H8 BD*(1)C1 C BD(1)N12 O14 BD*(1)C4 N LP(1)O13 BD*(1)C3 C LP(1)O13 BD*(1)C4 N LP(1)O14 BD*(1)C4 N LP(1)O14 BD*(1)N12 O LP(1)Cl16 BD*(1)C1 C LP(1)Cl16 BD*(1)C5 C BD 2-centred bonding; BD* 2-centred antibonding; LP lone pair

13 JANAKI et al.: MOLECULAR HYPERPOLARIZABILITIES AND INTRAMOLECULAR CHARGE TRANSFER 613 The important interaction energies, related to the resonance in the ring are electron donating from the BD(1)C1 C2, BD(1)C1 C6, LP(1)O13 and LP(1)O14 to the anti-bonding acceptor BD*(1)C1 C6, BD*(1)C1 C2, BD*(1)C4 N12 and BD*(1)N12 O13 orbital s and their corresponding energies are 4.52, 4.56, and kcal/mol, respectively. 5 Conclusions The FT-IR and FT-Raman spectra have been recorded and the detailed vibrational assignment is presented for DCNA. The equilibrium geometries, harmonic vibrational frequencies, IR and Raman spectra of DCNA are determined and analyzed by HF/ G** and B3LYP/ G** level of theory. The vibrational frequency analysis by B3LYP method agrees satisfactorily with experimental results than the HF method. We have clearly shown the relevant role played by the molecular frame where electron delocalization takes place. Finally, the calculated HOMO LUMO energies show that charge transfer occur within the molecule which is responsible for bioactive and NLO properties of the molecule. References 1 Dong X, Minhja J & Zhongke T, Acta Chimica Sinica, 41 (1983) Lashmana Perumal C K, Arul Chakkaravarthi A, Rajesh N P, Santhana Raghavan P, Hwang Y C, Ichimura M & Ramasamy P, J Cryst Growth, 240 (2002) Lashmana Perumal C K, Arul Chakkaravarthi A, Balamurugan N, Santhana Raghavan P & Ramasamy P, J Cryst Growth, 265 (2004) Crata V, Ravindrachary V, Lakshmi S, Pramod S R, Shridar M A & Shshidhara Prasad J, J Cryst Growth, 275 (2005) Tommasini M, Castiglioni C, Zoppo M D & Zerbi G, J Mol Struct, (1999) Altun A, Gokuk K & Kumru M, J Mol Struct (Theochem), 625 (2003) Krishnakumar V & Balachandran V, Spectochim Acta A, 61 (2005) Ravikumar C & Hubert Joe I, Phys Chem Chem Phys, 12 (2010) Kurt M, Trudakul M & Yurdakul S, J Mol Struct (Theochem), 711 (2004) Sundaraganesan N, Priya M, Meganathan C, Dominic Joshua B & Cornard J P, Spectrochim Acta A, 70 (2008) Parr R G & Yang W, Density Functional Theory of Atoms and Molecules. Oxford University Press, New York, Frisch M J, Trucks G W, Schlegel H B, et al., Gaussian- 2009, Gaussian, Inc., Pittttsburgh, PA, Frisch A, Nielsen A B & Holder A J, Gaussview Users Manual, Gaussian Inc., Pittsburg, Pulay P, Fogarasi G, Pongor G, Boggs J E & Vargha A, J Am Chem Soc,105 (1983) Rauhut G & Pulay P, J Phys Chem, 99 (1995) Keresztury G, Holly S, Varga J, Besenyei G, Wang AY & Durig J R, Spectrochim Acta, 49A (1993) Keresztury G, Chalmers J M & Griffith P R (Eds.), Raman Spectroscopy: Theory, Handbook of Vibrational Spectroscopy, Vol. 1, John Wiley, New York, Sundius T, J Mol Struct, 218 (1990) (a) Sundius T, Vib Spectrosc, 29 (2009) 89; (b) Molvib,V.7.0: Calculation of Harmonic Force Fields and Vibrational modes of molecules, QCPE Program No.807 (2002). 20 Palafox M A, Nunez J L & Gil M, J Mol Struct (Theochem), 593 (2002) Lister G D, Tyler J K, Hog J H & Larsen N W, J Mol Struct, 23 (1974) Altun A, Gökuk K & Kumru M, J Mol Struct (Theochem), 637 (2003) Subashchandrabose S, Akhil Krishnan R, Saleem H, Parameswari R, Sundaragenesan N, Thanikachalam V & Manikandan G, Spectochim Acta A, 77 (2010) Thompson J, Spectrochim Acta B, (1958) Subramanian M K, Anbarsan P M & Manimegalai S, J Raman Spectrosc, 40 (2009) Pagannnone N, Formari B & Mattel G, Spectrochim Acta A, 43(1986) Anbarasu P, Arivazhagan M & Balachandran V, Indian J Pure & Appl Phys, 50 (2012) Sathyanarayana D N, Vibrational Specttroscopy-Theory and Applications,Second ed., New Age International (P) Ltd.Publishers, New Delhi, Furnell Vogel B S, Textbook of Practical Organic Chemistry, 5 th ed., Longman, New York, Wade L G, Advanced Organic Chemistry, Fourth ed., Wiley, New York, (1992) Krishnakumar V, Prabavathi N & Muthunatesan S, Spectrochim Acta A, 70 (2008) Murali M K & Balachandran V, Indian J Pure & Appl Phys, 50 (2012) Sundaraganesan N, Saleem H, Mohan S, Ramalingam M & Sethuraman V, Spectrochim. Acta A,62 (2005) Jeyavijayan S & Arivazhagan M, Indian J Pure & Appl Phys, 50 (2012) Varsanyi G & Szoke S, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, Roeges N P G, A Guide to the Complete Interpretation of Infrared Spectra of Organic Structures. Wiley: New York, Perjessy A, Rasala D, Tomasik P & Gawinecki R, Collect Czech Chem Commun, 50 (1985) Brown J F Jr., J Am Chem Soc, 77 (1955) Green J H S, Kynanston W & Lindsery A S, Spectochim Acta, 17 (1961) Exnner O, Kovac S & Solcaniova E, Collect Czech Chem Commun,37 (1972) 2156.

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