Harmonic Analysis of Vibrations of 4-Chloro-2-Fluroaniline: A Scaled Quantum Mechanical Approach

Introduction

Quantum chemical computational methods have proved to be an essential tool for interpreting and predicting the vibrational spectra [1-3].  A significant advancement in this area was made by combining semi empirical quantum mechanical method; ab initio quantum mechanical method and density functional theory (DFT), each method having its own advantage.  In scaled quantum mechanical (SQM) approach, the systematic errors of the computed harmonic force field are corrected by a few scale factors which are found to be well transferable between chemically related molecules and were recommended for general use. The aim of this work is to check the performance of the B3LYP density functional force field for simulation of the FT-IR and FT-Raman spectra of 4C2FA (that have not been subjected to vibrational analysis before) with the use of the large B3LYP/6-311+G^** basis sets, and comparing the effect of simpler and more elaborate versions of scaling, while paying attention to ensuring correct band assignments.

Experimental details

The fine samples of 4C2FA were obtained from Lancaster Chemical Company, UK, and used as such for the spectral measurements. The room temperature Fourier trans­form infrared spectra of the title compounds were measured in the region 4000-400 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 splitter and globar source. The FT-Raman spectra were recorded on the same instrument with FRA 106 Raman accessories in the region 3500-100 cm^-1. Nd:YAG laser operating at 200 mw power with 1064 nm excitation was used as source.

Computational details

Quantum chemical calculations for 4C2FA were performed with the       GAUS­SIAN 98W program [4] using the Becke-3–Lee–Yang–Parr (B3LYP) functionals [5,6] supplemented with the 6-311+G^** basis sets (referred large basis sets), for the Cartesian representation of the theoretical force constants have been computed at the fully opti­mized geometry by assuming C_s point group symmetry.

Molecular geometry   

The optimized molecular structure of 4C2FA was shown in Fig.1. The global minimum energy obtained by the DFT structure optimization was presented in Table 1.  The optimized geometrical parameters obtained by the large basis set calculation were presented in Table 2.

Detailed description of vibrational modes can be given by means of normal coordinate analysis (NCA). For this purpose, the full set of 49 standard internal coordinates containing 13 redundancies were defined as given in Table 3.  From these, a non-redundant set of local symmetry coordinates were constructed by suitable linear combinations of internal coordinates following the recommendations of Fogarasi et. al [7,8] are summarized in Table 4.  The theoretically calculated DFT force fields were transformed in this later set of vibrational coordinates and used in all subsequent calculations.

Analysis of vibrational spectra

The 36 normal modes of 4C2FA are distributed among the symmetry species as G_3N-6= 25 A’ (in-plane) + 11 A² (out-of-plane), and in agreement with C_s symmetry. All the vibrations were active both in Raman scattering and infrared absorption. In the Raman spectrum the in-plane vibrations (A’) give rise to polarized bands while the out-of-plane ones (A²) to depolarized band. The detailed vibrational assignments of fundamental modes of 4C2FA along with calculated IR, Raman intensities and normal mode descriptions (characterized by TED) were reported in Table 5.  For visual comparison, the observed and simulated FT-IR and FT-Raman spectra of 4C2FA are produced in a common frequency scales in Fig. 2 &  Fig. 3.

C-F Vibrations

The vibrations belonging to the bond between the ring and the halogen atoms are worth to discuss here, since mix­ing of vibrations are possible due to the presence of heavy atoms on the periphery of the molecule [9]. C–F bond show lower absorption frequencies as compared to C–H bond due to the decreased force constant and increase in reduced mass. F cause redistribution of charges in the ring. In 4C2FA, the C–F stretching and out-of-plane bending vibrations appeared at 1246,1210 and 1247,1212 cm^-1 in FT-IR and FT-Raman, respectively. The C–F in-plane bending vibration was found at 564 and 560 cm^-1 in FT-IR and FT-Raman spectrum and second position of aniline ring significantly changes the normal modes. Finally, the charge on the F and its variation during vibration signifi­cantly contribute to many of the IR lines, due to heavy atoms substituent in 4C2FA. The very similar behaviour in the IR intensities of νC-Cl and νC-F were also observed.

In 4C2FA, the very strong and the medium strong FT-IR bands observed at 1246, 1210, 689, 683 cm^-1  and Raman bands are observed at 1247, 1212, 668 cm^-1  are assigned to νC-F. The very strong IR bands obtained at 810 and 783 cm^-1  and Raman bands at 786 cm^-1 in the observed spectra are assigned to νC-Cl.

Strong characteristic absorptions due to the C-F stretch­ing vibrations are observed in this study. In the organic halogen compounds the band due to C-F stretch­ing vibrations may be found over a wide frequency range, 1360–1000 cm^-1, since the vibration is easily influenced by adjacent atoms or groups. The C-Cl stretching vibrations give generally strong bonds in the region 760–505 cm^-1.

C-Cl Vibrations

In present investigation, The C–Cl stretching frequency is generally observed in the region 800–600 cm^-1 depending on the configuration and conformation of the compound [10]. Based on this, the FT-IR and FT-Raman bands observed at 858,810,786 and 783 cm^-1 has been assigned to C–Cl stretching modes show strong mixing with several planar modes. However, the pla­nar C–Cl bending modes appear to be relatively pure modes. The C–Cl out of plane bending modes were identified at 668 cm^-1 and 580 cm^-1 for IR and Raman, respectively.

NH₂ Vibrations

According to Socrates [11] the stretching, scissoring and rocking deformation of amino group appeared around 3500–3000, 1700–1600, 1150–900 cm^-1, respectively in absorption spectra. In 4C2FA, the antisymmetric, symmetric stretching modes of NH₂ group found at 3466, 3382, 1712, 1630 cm^-1 in FT-IR and 3077, 1631 cm^-1 in FT-Raman respectively.

Conclusion

Based on the SQM force field obtained by DFT calculations at B3LYP/6-311+G** levels, a complete vibrational properties of 4C2FA have been investigated by FT-IR and FT-Raman spectroscopies. The roles of chloro, fluro and nitro groups in the vibrational frequencies of the title compounds were discussed The assignment of the fundamentals is confirmed by the qualitative agreement between the calculated and observed band intensities and polarization properties as well and is believed to be unambiguous.

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