In the present study Electronic structures calculations, at the RHF/6-31G* and B3LYP/6-31G* has been carried out on 3, 4-difluoroaniline (DNA). A complete revised assignment of the experimental spectra has proposed in terms of molecular parameters and PEDs obtained from force field calculations. The electronic structure calculations have been performed using Gassian 03W8; the PED calculations with the MOLVIB9, giving optimized structures, energies harmonic vibrational frequencies, depolarization ratios, infrared intensities, Raman activities, force constants and PEDs. The proposed assignments on the basis of experimental IR and Raman spectra have been reviewed. All the computed harmonic frequencies have been scaled10. Excellent agreement between experimental and theoretical anharmonic inversion-torsion frequencies of fundamental, overtone and combination modes in aniline has been obtained in the correlated ab initio studies, which gives a confidence to thecalculated. Potential energy surface and the evidence to a rather high nonplanarity of aniline molecule [8, 9]. Comprehensive studies of the molecular and electronic structures, vibrational frequencies and infrared and Raman intensities of the aniline radical cat-ion have been performed by using the (UB3LYP) and (UMP2) methods with the extended 6-31111G ~

The liquid sample of 3, 4-difluoroaniline (DAN) was supplied by Aldrich Chemical Co. and was distilled in vacuum before use. The infrared (IR) spectrum of the sample in KBr pellet was recorded at room temperature on Perkin-Elmer model 377 double beam automatic Spectrophotometer. The Laser Raman Spectrum of the sample was recorded on Cary-model Laser Raman Spectrometer using Ar+ laser with two Czerny-Littrow monochromators.

i) Electronic structure calculation methods: The development of theoretical methods and in particularly quantum chemistry, allowed not only an

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Computational chemistry is simply the application of quantum mechanical and computing skill to the solution of physical, chemical problems. Here, one uses computer softwares like GAUSSIAN to generate information such as properties of molecules or simulated experimental results. The Schrödinger equation is the basis for the computational methods. The Born-Oppenheimer approximation is the first of several approximations made while trying to solve Schrödinger‘s equation for more complex systems with more than one or two electrons. A few electronic structure computation methods are discussed in brief as follows. ii) In the ab initio Hartree-Fock self-consistent Field (HF- SCF) method: The wave function is an antisymmetrized determinantal product of one-electron orbitals (the "Slater" determinant). Schrödinger‘s equation is transformed into a set of Hartree-Fock equations. The wave function of each electron is optimized under the mean potential averaged over all the other electrons in the system, as well as the electrostatic potential generated by the fixed nuclei. In another words, during the Hartree-Fock calculation, the electron cannot ‗see‘ other electrons, instead of averaged‗electron gas‘. The primary deficiency of Hartree-Fock method is that the correlation of electron motion cannot be accounted adequately. iii) Density Functional Theory (DFT) method: This method is one of the most popular approaches to quantum mechanical many-body electronic structure calculations of molecular and condensed matter systems. It has been proved that for molecules, the ground-state molecular energy is uniquely determined by the ground-state electron probability density. Therefore, all the ground-state molecular properties can be calculated from the electron density, without having to find the molecular wave function. In addition, according to the Hohenberg-Kohn variational theorem, the true ground-state electron density minimizes the energy functional. The DFT method is variational. A number of density functional theoretical methods have been developed, one such method is B3-LYP (Becke‘s three parameter mixing of exchange functional with Lee Yang Parr correlation functional). In order to model structures and vibrational spectral properties, we performed electronic calculations with the ab initio,s RHF/6-31G* and hybrid DFT,s B3LYP/6-31G* using the Gaussian 03W suite of programs [15]. Molecular electronic energies, equilibrium geometries, All the computed harmonic frequencies have been scaled with scaling factors 0.8929 (RHF) and 0.9616 (B3LYP) [17]. iv) MOLVIB: MOLVIB is a FORTRAN program for the calculation of harmonic force-fields and vibrational modes of molecules upto 100 atoms. To aid the vibrational assignments, we carried out normal coordinate calculations using MOLVIB package [16]: force fields and PEDs have been computed for B3LYP/6-311G* results for all the compounds.

The optimized molecular structure of the DAN is shown in Fig.7. DAN belongs to C1 symmetry due to nonlinearity of amino group. The molecular electronic energy is -486.0607 hartrees at B3LYP/6-31G*. The experimental infrared spectrum taken from reference [14] is presented in Fig -8. We present the optimized geometrical parameters in Table - 4. The results are compared with the available experimental data [19, 20]. The Table – 5 includes stretching and bending force constants derived from a normal coordinate calculations using B3LYP/6-31G* results. Table –6 presents experimental IR and Raman (Fig-9) and theoretical fundamental vibrational frequencies with assignments (in % PED). The observed IR and Raman vibrational frequencies were analyzed and assigned to different normal modes of the molecule. The error obtained was in general very low. The observed IR and Raman vibrational frequencies were analyzed, revised and assigned to different normal modes of the molecule. We point out that the proposed assignments are correlated with the benzene modes, though, it is true, and several of the modes of DAN show contributions from different bond oscillators. This correlation would serve the object of gaining an understanding as to changes in the normal modes of benzene to aniline derivatives. Characteristic benzene as well as C-X vibrations are in the correlation range [8-14]. The deformation vibrations of DAN have appeared with single dominant contribution as evident from PED. We also note that the in-plane and out-of–plane C-X bending vibrations show dominance in the region below 1000 cm-1. All the assignments are in agreement with the literature [8-14]. Other general conclusions have also been deduced. There is good agreement between theory and experiment.

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Assignments:

These vibrations show characteristic bands, as in the benzene derivatives, in the region 3100-3000 cm-1 [3-7]. These vibrations are assigned to medium strong IR/Raman bands at 3080, medium weak IR/Raman bands at 3040 and to a medium weak IR band at 3010 cm-1 where corresponding Raman band was not observed. These modes are predicted as weak at RHF and B3LYP levels. The numbers inside bracket are relative intensities in case of frequencies and % contributions in case of the PEDs.ν-stretch, β- in-plane bending, - Out-of-plane bending .The predicted bands agree well with the observed ones within ~1% for the two methods. It may be seen that the force constants, 5.600–5.688 mdyne/Å, obtained from PED calculations, corresponding to C-H stretching vibrations show little changes in magnitudes compared to the known values [6, 7], suggesting the dominance of a single vibration.

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The numbers inside bracket are relative intensities in case of frequencies and % contributions in case of the PEDs.ν-stretch, β- in-plane bending, - Out-of-plane bending .The predicted bands agree well with the observed ones within ~1% for the two methods. It may be seen that the force constants, 5.600–5.688 mdyne/Å, obtained from PED calculations, corresponding to C-H stretching vibrations show little changes in magnitudes compared to the known

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vibration.

The CH in-plane bending vibrations are substitution sensitive, normally showing the bands in the region 1300-1000 cm-1 [6, 7]. Medium Strong IR bands at 1170, 1130, 975 cm-1 with corresponding medium weak Raman bands at 1165, 1125,980 cm-1 are assigned to the CH in-plane bending vibrations. It is seen from the PEDs that the modes show couplings of the bond oscillators of the ring, NH2 and fluorine. The predicted bands are medium weak at RHF and B3LYP. These frequencies agree well with the observed ones within ~1%.

Bands involving the out – of – plane C-H vibrations appear in the range 1000-675 cm-1 [6, 7]. These vibrations are assigned to strong IR bands at 940, 860, 770 cm-1. The corresponding strong Raman bands are at 945,865,770 cm-1. The frequencies with RHF level agree well with the observed ones within ~2%, whereas B3LYP level deviates by~5 %.

The C-C stretching vibrations occur in a wider spectral range covering 1650-650 cm-1 [3-7]. The five medium weak to strong IR bands at 1605, 1575, 1430, 1315, 735 cm-1 with the corresponding medium to strong Raman bands at 1600, 1570, 1420, 1320, 735 cm-1

corresponding Raman band not observed, are assigned to the C-C stretching vibrations. As the bands corresponding to 1575, 1315 cm-1 are nearly pure the force constants values fall in the known interval, 5.114 - 5.412 mdyne/Å [3, 6]. The bands at 1529 (RHF), 1514 (B3LYP) cm-1 are predicted as very strong where experimental IR band is also very strong. It is seen from the PED that the bands observed at 1480, 1430, 735 cm-1 are mixed with contributions from C-X (X= NH2, F) vibrations. The predicted frequencies agree well with the observed ones within ~3% error.

Three strong IR bands at 715, 450, 390 cm-1 with corresponding medium weak to strong Raman band at 710, 445, 395 cm-1 are assigned to CC in-plane-bending vibrations. The above bands are in the expected range, as in similar systems [6-8]. The three predicted weak bands agree well with the observed ones within~3%.

In the substituted benzenes, CC out–of -plane vibrations are expected in the range 730-425 cm-1 [6-9].

Three weak to strong IR bands at 520, 405,330 cm-1

with corresponding weak to strong Raman bands at 515, 410, 335 cm-1 are assigned to the CC out-of-plane vibrations. These frequencies agree well with the observed ones within ~3%.

Aromatic fluorine compounds give stretching bands in the region 1270-1100 cm-1 [2]. Strong IR bands at 1285, 1245cm-1 with corresponding medium Raman bands at 1285, 1250 cm-1 are assigned to C-F stretching vibrations. The νC-F vibrations are mixed with νCC, νC-NH2 and βCH vibrations. The predicted

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expected in the general range 420-375 cm[2].

For the aniline, Pavel Hobza et al observed two absorption bands of approximately equal infrared intensities, positioned at 3508 and 3422 cm-1. They assigned these bands to the NH2 antisymmetric and symmetric stretching vibrations, respectively [10]. In this work, two medium strong IR band at 3440 and 3375 cm-1 with corresponding Raman bands not observed, are assigned to the NH2 antisymmetric and symmetric stretching vibrations, respectively. The NH2 scissoring vibration in the aniline contributes to two modes, these modes have been assigned to the bands at 1608 and 1618 cm-1, in the IR spectrum of aniline in argon matrix [10]. We assigned this vibration to a medium IR/Raman band at 1620 cm-1 IR spectroscopic studies of aniline show two bands, at 1054 and 1115 cm-1, involve NH2 rocking vibration [10]. We assigned this vibration to a strong IR band at 1070 cm-1 with corresponding medium Raman band at 1075 cm-1. The NH2 wagging vibration in aniline was assigned to a strong IR band at 541 cm-1[10].

A complete vibrational assignment for 3,4-difluoroaniline has been proposed for the experimental IR and Raman spectra, aided by the ab initio (RHF), hybrid density functional methods (B3LYP ) using 6-31G* basis set and PED analysis. A B3LYP/6-31G* optimization leads to stable form with C1 symmetry. Characteristic benzene as well as C-X vibrations is in the correlation range. The C-F We also note that the in-plane and out-of-plane C-X bending vibrations show dominance in the region below 700 cm-1. All the assignments are in agreement with the similar systems.

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Dept. of Physics, Hirasugar Institute of Technology, Nidasoshi, India