Molecular Spectroscopic Characterization and Electronic Structure Analysis of N-benzylaniline-A DFT Approach

The FT-IR and FT-Raman spectra of N-benzylaniline[NBZA] have been analysed in the region 4000-450cm -1 and 4000-50cm -1 respectively. The geometrical structure, harmonic vibrational frequency, infrared intensity, Raman activities and bonding features of the title compound were carried out by DFT method with B3LYP/6-311++G(d, p) basis set. The complete vibrational frequency assignments were made by normal co-ordinate analysis following the scaled quantum mechanical force field methodology (SQM). The charge transfer and hyperconjugative interactions have been analysed using natural bond orbital (NBO) and HOMO-LUMO analysis. The reactivity sites are identified by mapping the electron density into electrostatic potential surface (MESP). The thermodynamic properties of the title compound at the different temperatures have been calculated. The mulliken charges, the values of electric dipole moment (μ) of the molecule were computed using DFT calculations and the results are interpreted. The UV absorption spectrum of the title compound has been described at the experimental level.


INTRODUCTION
N-benzylaniline is an N-alkylated derivative of aniline.Aniline, a primary aromatic amine, is a weak base which forms salts with mineral acids.Aniline derivatives are widely used in pharmaceutical manufacturing, electro-optical, dye stuff and other commercial and industrial applications [1].Some of the para substituted derivatives of aniline are commonly used as local anesthetics.Aniline and its derivatives are also important in the manufacture of rubberprocessing chemicals and varnishes.Aniline derivatives such as phenylenediamines and diphenylamine are antioxidants.Fluoroanilines are important molecules in chemical industries for the synthesis of pigments, plastics and conducting polymers [2] and in the manufacture of agrochemicals [3].Krishnakumaret al. [4] have studied the spectral analysis of 2, 6 -dibromo -4nitroaniline and 2-(methylthio) aniline based on density functional theory.Sundaraganesanet al. [5] investigated the experimental vibrational spectra of 3, 4-dimethoxy aniline by using ab initio and DFT methods.
In this aniline series, N-benzylaniline acts as a protecting agent against cerebral ischemia injury and it has an analgestic action for chronic pathologic pains.It is used as a drug for cerebral apoplexy, neuropathic and inflammatory pain [6].Owing to the industrial and biological importance, an extensive spectroscopic study on N-benzylaniline has been carried out in this present work.The molecular geometry and vibrational spectra of NBZA were calculated by applying the density functional theory (DFT) method with B3LYP functional at 6-311++G (d,p) basis set.A complete vibrational analysis of NBZA was performed by combining the experimental and theoretical information using Pulay's DFT based on the scaled quantum mechanical (SQM) approach.The redistribution of electron density (ED) in various bonding, antibonding orbitals and E(2) energies have been calculated by natural bond orbital (NBO) analysis to give clear evidence of stabilization originating from the hyper conjugation of various intra-molecular interactions.HOMO-LUMO analysis has been used to elucidate information regarding charge transfer within the molecule.The Molecular electrostatic potential (MEP), thermodynamic properties and Mulliken charge analysis have also been studied.

EXPERIMENTAL SECTION
The compound N-benzylaniline (NBZA) in the solid form was purchased from the Sigma Aldrich chemical company (USA) with a stated purity of 98% and used as such without further purification.FT-Raman spectra were recorded in the range of 4000-50cm -1 using BRUKER, Model RFS 100/s FT-Raman spectrophotometer.The FT-IR spectrum of the sample was recorded using Perkin Elmer RXI spectrometer in the region 4000-450cm -1 usingtheKBr pellet technique.UV-Vis absorption spectra were recorded in the polar solvents water and ethanol in the region 200-400 nm using Perkin Elmer LAMDA UV-vis NIR spectrometer.The polar solvents were used for the better stabilization of the exited state.

COMPUTATIONAL DETAILS
The DFT computation of NBZA had been performed using Gaussian 03 program package [7] at the Becke 3 -Lee -Yang -Parr (B3LYP) functional with the standard 6 -3ll++G(d, p) basis set.The optimized structural parameters were evaluated for the calculations of vibrational frequencies at different methods.The harmonic vibrational frequencies had been analytically calculated by taking the second order derivative of energy using the same level of theory.The scaling of the force field was performed according to the scaled quantum mechanical procedure (SQM) [8,9] using selective scaling in the natural internal co-ordinate representation [10,11] to obtain a better agreement between the theory and the experiment.Normal co-ordinate analysis had been performed in order to obtain the detailed interpretation of the fundamental modes using the MOLVIB program version 7.0 written by Sundius [12,13].The Raman activities (Si) calculated by the Gaussian 03W program and converted into relative Raman intensities (Ii) during the scaling program with MOLVIB using the following relationship derived from the basic theory of Raman scattering [14,15].
The NBO calculations [16] were performed using NBO 3.1 program as carried out in the Gaussian 03 W package at the DFT/B3LYP level in order to understand the various second order interactions between the filled orbitals of one sub system and vacant orbitals of another sub system, which is a measure of delocalization or hyper conjugation.The all calculations presented in the work were also been carried out at the same level of theory of NBO calculations using 6-311++G (d, p) basis set.

Molecular Geometry
The optimized geometry performed at B3LYP/6-311++G (d,p) basis set of the title molecule is shown in Fig. 1.with atom numbering scheme.The comparative optimized structural parameters such as bond lengths, bond angles along with its experimental data obtained by the X-ray analysis of NBZA [17] are presented in Table 1.All the optimized C-C bond lengths are larger than the experimental values, due to the fact that theoretical calculations belong to isolated molecules in the gaseous phase and the experimental results belong to the molecule in the solid state.
The C-C bond lengths in aniline and benzyl ring of the NBZA lie in the range 1.387-1.409A 0 and 1.392-1.400A 0 respectively by B3LYP/6-311++G(d, p) method ,but due to the substituent of NH2 the bond lengths C2-C3(1.409A0 ) and C2-C7(1.405A0 ) in the aniline ring show a considerable increase .All the C-C bond lengths in the aniline ring and benzyl ring except C2-C3, C2-C7 are in agreement with literature values [18].The bond length C8-C9 (1.513A 0 ),which forms a bridge between the two ring systems, is found to be longer than that of the standard C-C bond length of about 1.47 A 0 [19].In the title compound, the shortening of the C-N bond lengths C2-N1 (1.392A 0 ) and C8-N1 (1.459A 0 ) from the normal single bond length (1.48A 0 ) reveals the effect of resonance [20] on that part of the molecule.But due to the coulomb repulsive interaction of methylene, an asymmetry is observed in C2-N1 and C8-N1 bond lengths.

Potential energy surface scan
The potential energy surface (PES) with the B3LYP/6-311++G(d,p) level of theoretical approximation is performed for the title molecule by varying the dihedral angle C8-N1-C2-C3 in steps of 0 0 ,10 0 ,20 0 , . . . ..,360 0 and the resultant minimum energy curve as a function of dihedral angle is shown in Fig.

Vibrational spectral analysis
The vibrational spectral assignments of NBZA have been carried out with the help of normal co-ordinate analysis.A non-redundant set of local symmetry co-ordinates are constructed by suitable linear combination of internal co-ordinates following the recommendations of Pulay et al. [8] are presented in Table 2.The computed wave numbers are selectively scaled according to the SQM procedure suggested by Rauhut and Pulay [8].The observed and simulated FTIR and FT -Raman spectra of the compound are shown in Figs 3&4.The observed and calculated frequencies using B3LYP/6-311++G(d,p) basis set along with their relative intensities, probable assignments and potential energy distribution (PED) of NBZA are summarized in Table 3.

N-H Vibrations
The methylene and amino groups are generally referred to as electron donating substituents.In aromatic compounds, the associated N-H stretching vibration appears as moderate to strong band in the region 3410±70cm -1 [23].For NBZA molecule, the N-H stretching mode is observed at 3415cm -1 in FT-IR as a strong band which agrees well with the computed wavenumber at 3415cm -1 by B3LYP/6-311++G(d, p) method with a PED contribution of 100%.The N-H inplane and out of plane bending vibrations are expected in the range 1530±50cm -1 , 635±35cm -1 respectively [23].In the title molecule, the strong broad band at 1489cm -1 in the FT-IR spectrum and a weak band in the FT-Raman spectrum at 1492cm -1 are assigned to N-H in plane bending modes.The N-H out of plane bending vibration is at 457cm -1 in FT-IR spectrum.The N-H in-plane and out of plane bending vibrations computed by B3LYP/6-311++G (d, p) method show good agreement with the recorded spectral data.

C-H Vibrations
In the aromatic compounds, the C-H stretching vibrations normally occur at 3100-3000cm -1 [24].These vibrations are not found to be affected due to the nature and position of the substituent.In NBZA, the FT-IR bands at 3077, 3060, 3050 and 3025cm -1 and the FT-Raman bands at 3050 and 3025cm .The frequencies of the C-H out of plane deformation vibrations are mainly determined by the number of adjacent hydrogen atoms on the ring [25].The C-H out-of-plane bending vibrations calculated at 945 -725cm -1 , show good agreement with the observed values of 794 and 727cm -1 in the FT-IR and 802cm -1 in the FT-Raman. - by B3LYP/6-311++G(d, p) method.For mono substituted aromatics, the bands due to the in-plane ring deformation vibration and out-of-plane ring deformation vibration appear in the region 830 -435cm -1 and 710 -405cm -1 respectively [23].The in-plane ring deformation vibrations of NBZA are observed at 983,626,609 and 506cm -1 as strong and weak bands in the FT-IR and at 1000,987 and 617cm -1 in Raman spectrum.The out-of -plane ring deformation vibrations appear at 695 and 670 cm -1 in FT-IR spectrum and at 411cm -1 in Raman spectrum.The calculated bending modes found at 998,986,619,604,586,502 cm - 1 and at 687,674,411 and 406cm -1 by B3LYP/6-311++G(d, p) method are assigned to C-C in-plane and out-of-plane ring deformation vibrations respectively.

C -N vibrations
The CN stretch, connected with the amine twist, appears as a moderate to strong band in the region 1075±25 cm -1 [23].For NBZA , the strong CN stretching vibration band observed at 1117cm -1 both in FT-IR and FT-Raman agree well with the theoretically calculated frequency at 1110cm -1 by B3LYP/6-311++G(d,p) method.Primary aromatic amines (e-g anilines) have a weak to medium intensity band at 345-445cm -1 due to the C-N in-plane deformation.For mono substituted amino benzenes with electron donating substituents, this band is observed between 400-500cm -1 [25].Accordingly, in the present study, the FT-Raman band assigned at 500cm -1 and FT-IR band assigned at 486cm -1 to C-N inplane bending vibration of NBZA, which is in good agreement with the calculated value 501cm -1 by B3LYP/6-311++G (d, p) method.

. Methylene group vibrations
The CH2 group frequencies basically six fundamentals can be associated, namely, symmetric, asymmetric, scissoring, rocking, wagging and twisting vibrations.The methylene asymmetric stretching is found at 2930±15 cm -1 and the symmetric stretching at 2870 ±20 cm -1 [23].In NBZA molecule, the computed wavenumbers at 2927 cm -1 and 2846 cm -1 by B3LYP/6-311++G(d, p) method are assigned to CH2 asymmetric and symmetric stretching vibrations.In the present work, the strong band observed at 2923 cm -1 in the FT-IR and a medium band observed at 2927 cm -1 in the FT-Raman spectrum of NBZA are assigned to the CH2 asymmetric modes of vibrations.Similarly, the band at 2846 cm -1 in the FT-IR and FT-Raman spectrum of NBZA is assigned to symmetric stretching mode of vibrations.The methylene deformation of -CH2NH2 compounds absorbs in the region 1450±20cm -1 with moderate intensity [23].In title molecule, the δCH2 vibration is observed at 1447 cm-1 as a strong band in the FT-IR and at 1447cm -1 as weak band in the FT-Raman spectrum.This agrees well with the computed wavenumbers at 1447 cm -1 by B3LYP/6-311++G(d, p) method.The methylene wag appears with a weak to moderate intensity in the region 1360±25 cm -1 [23].For NBZA molecule, the wagging mode of CH2 is observed at 1360cm -1 as a medium band in FT-IR and at 1361 cm -1 as weak band in the FT-Raman spectrum.The theoretically computed CH2 wagging vibration at 1363 cm -1 by B3LYP/6-311++G (d,p) level shows good agreement with experimental results.The weak to moderate band at 1290±45 cm -1 is mostly assigned to the methylene twist [23].The CH2 twisting vibration of NBZA is observed at 1183cm -1 as a weak band in the FT-Raman spectrum.The computed wavenumber for the twisting mode is assigned at 1186 cm -1 by B3LYP/6-311++G (d, p) method.The methylene rocking vibrationsare observed in the region 890±55 cm -1 with band intensities varying from weak tomoderate [23].The CH2 rocking vibration of NBZA is attributed to 960 cm -1 in the FT-IR spectrum.The corresponding calculated wavenumber is at 960 cm -1 using B3LYP/6-311++G (d, p) basis set.

Natural bond orbital (NBO) analysis
Natural Bond Orbital analysis provides an efficient analysis for studying intra and intermolecular bonding and interaction among bonds.It also provides a convenient basis for investigating charge transfer to conjugate interaction in molecular systems.The second -order Fock matrix has been carried out to evaluate the donor-acceptor interactions from NBO analysis [26].The interactions result in the 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 determined as qi is the donor orbital occupancy.Ei, Ej is the diagonal elements and Fij is the off diagonal NBO Fock matrix element.The larger the stabilization energy E(2) value, the more intensive is the interaction between electron donors and electron acceptors.
Strong intramolecular hyper conjugative interactions (ICT) are formed by orbital overlap between LP(N1) and π*(C-C) bond orbital which result in ICT causing stabilization of the system.These interactions are observed as an increase in electron density (ED) in C-C antibonding orbital that weakens the respective bonds.The possibility of ED delocalization between the lone pair donor atom to antibonding acceptor atoms of the title molecule is shown in Table 4.It is evident from the Table, that the strongest electron transition is observed between the lone electron pair of N1 and neighbouring antibonding orbital of π*(C2-C7) which leads to a maximum stabilization energy of 33.18 kcalmol -1 .This is due to the fact that greater the value of E (2), the more intensive is the interaction between electron donors and acceptors.In NBZA, for the donor LP(N1), the occupancy is highly deviated from 2 and for the corresponding acceptor π*(C2-C7) there is an increase in occupancy(0.40788e).These results indicate a hyperconjugative interaction occurring between the donor LP(N1) and acceptor π*(C2-C7).The strong conjugation of electron -donating amino group with the π ring system leads to some changes of the bond lengths and bond angles of the aromatic ring.

Frontier molecular orbital analysis
The most important frontier molecular orbitals (FMOs) such as highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) play an important role in the chemical stability of the molecule [27].The HOMO represents the ability to donate an electron and LUMO as an electron acceptor represents the ability to accept an electron.The energy gap between HOMO and LUMO also determines the chemical reactivity and chemical hardnesssoftness of a molecule [28].The HOMO and LUMO energies are predicted at B3LYP method with 6-311++G(d, p) basis set.According to the results, the NBZA molecule contains 49 occupied molecular orbitals and 275 unoccupied virtual molecular orbitals.

Molecular electrostatic potential
The Molecular electrostatic potential (MESP) surface is plotted over the optimized electronic structure of NBZA using density functional B3LYP method with 6-311++G (d, p) basis set.The MEP generated in space around a molecule by the charge distribution is used to understand the reactive sites for nucleophilic and electrophilic attack in hydrogen bonding -interactions [29].The computationally observed MEP surface directly provides information about the electrophilic (electronegative charge region) and nucleophilic (positive charge region) regions.
The electrostatic potential at any point V(r) is the energy required to bring a single positive charge from infinity to that point.The electrostatic potential V(r) at any point in space around a molecule by charge distribution is given by Where ZA is the charge on the nucleus A, located at RA and ρ(r | ) is the electron density function of the molecule.To predict reactive sites for electrophilic (blue colour) and nucleophilic (red colour) MESP (Fig. 6) was carried out.As seen from the Fig. 6, there are two possible sites on the compound for electrophilic attack.The negative regions are mainly localized on the electronegative atoms.A maximum positive region is localized on the methylene group of the molecule and over the carbon atom C9 in the benzyl ring with a maximum value of 0.448 indicating a possible site for nucleophilic attack.The negative region is mainly over the aniline ring carbon atoms C3, C4, C5 with a maximum -0.318.

Thermodynamic properties
On the basis of vibrational analysis at B3LYP/6-311++G(d,p) level, the standard statistical thermodynamic functions: heat capacity(C) ,entropy(S) and enthalpy(∆H) for the title compound were obtained from the theoretical harmonic frequencies listed in Table 6.From the table, it can be observed that these thermodynamic functions are increasing with temperature ranging from 100 to 1000K due to the fact that molecular vibrational intensities increase with temperature [30].The values of some thermodynamic properties such as thermal energy, vibrational energy, zero-point vibrational energy, rotational constants and dipole moment of NBZA by B3LYP/6-311++G(d,p) method are listed in Table 7.
The correlation equations between heat capacity, entropies, enthalpy changes and temperatures were fitted by quadratic formulas using the corresponding fitting factor (R 2 ).The corresponding fitting equations are as follows and the correlation graphs are shown in Fig.

Mulliken atomic charges
The charge distribution of the molecule is calculated on the basis of the Mulliken method [31] using B3LYP/6-311++G(d,p) level calculation.This calculation depicts the charges of every atom in the molecule.The distribution of positive and negative charges is vital to the variation of bond length between the atoms.Atomic charge affects dipole moment, polarizability, electronic structure and other molecular properties of the system.Atomic charges of the title compound computed by mulliken method are presented in Table8.
In Table 8, it is observed that all the hydrogen atoms have a net positive charge.The positive charge of the H atom in the N-H group is found to possess a high atomic charge.This is due to the attachment of highly electronegative nitrogen atom.The magnitudes of the carbon atomic charges for the compound are found to be both positive and negative.The negative values on C2,C3,C4,C5 and C6 atoms in the aniline ring and C8,C10,C11,C12,C13 and C14 atoms in the benzyl ring lead to redistribution of electron density.

UV-Vis analysis
The electronic absorption spectra of NBZA compound were recorded in the polar solvents ethanol and water as displayed in Fig. 8.The observed λmax in ethanol and water are 246nm and 238nm respectively.The peaks arise mainly because of n→π* transition.Due to When NH2 group is attached to benzene ring, its absorption changes from λmax(255nm) to λmax(280nm) [32].But in the title compound, the λmax is blue shifted both in the solvents water and ethanol due to the attachment of benzyl ring with the aniline ring.From HOMO LUMO analysis, HOMO is existed on the aniline ring and the LUMO is residing on the benzyl ring.Hence, HOMO → LUMO transition is due to the excitation of electrons from LP(N) to π* orbitals of benzyl ring.This could also be verified in NBO analysis by the large delocalization of n→π* transition (Table.4) A p r i l 1 7 , 2 0 1 5

Conclusion
DFT studies of N-benzylaniline molecule were performed to obtain the detailed vibrational properties and other structural properties at B3LYP/6-311++G (d, p) level of theory.The geometrical structural analysis shows that the calculated bond lengths and bond angles are in good agreement with the experimental values.Substitution of NH2 and methylene groups distorts the molecular geometry.Four local global minimum were observed at the dihedral angles 30 0 , 180 0 , 200 0 and 350 s for τ(C8-N1-C2-C3) in potential energy surface scan analysis.The vibrational spectra of the compound were recorded, and on the basis of experimental results and PED calculations, assignments were given for all the fundamental vibrational frequencies.From the NBO analysis, the hyperconjugative interactions were observed from the amino group LP (N) atom to aromatic π*(C2-C7) and it leads to the change in bond lengths and bond angles.From the HOMO-LUMO analysis the effective charge transfer from aniline to the benzyl ring was observed in pictorial format.The Molecular electrostatic potential analysis revealed the two possible electrophilic and nucleophilic attacks aniline ring and methylene group respectively.The increasing of thermodynamic functions with temperature was established in thermodynamic property analysis.Mulliken atomic charges were analysed for the substitution effect in the molecule.The UV-Vis analysis indicates the substituent effect of amino group by the shift of the maximum absorption band.

ACKNOWLEDGEMENT
The authors are thankful to Sophisticated Analytical Instrumentation Faculty (SAIF), IIT Chennai for the spectral measurements.

Fig. 1
Fig.1 Molecular structure of N -benzylaniline along with numbering of atoms.

Fig. 5
shows the distributions of the HOMO-1, HOMO, LUMO and LUMO+1orbitals for the Nbenzylaniline molecule.As seen from Fig. 5, in HOMO, the charge clouds are mainly located in aniline ring and methylene group.But the corresponding LUMO is mainly located in the benzyl ring.Consequently the HOMO-LUMO transition implies an electron density transfer from the aniline ring to benzyl ring.The HOMO-LUMO energy gap of NBZA has been calculated at the B3LYP/6-311++G(d,p) level.The calculated HOMO -LUMO gap energy shows that chemical hardness of the molecule.EHOMO(a.u)=-0.19097 a.u.ELUMO(a.u)=-0.00882a.u.EHOMO-ELUMO(a.u)= 0.18215 a.u.

Table 4 :
Second order perturbation theory analysis of fock matrix in NBO basis for NBZA