Molecular structure and vibrational spectra of 3-chloro-4-fluoro benzonitrile by ab initio HF and density functional method

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Abstract

In this work, the experimental and theoretical spectra of 3-chloro-4-fluoro benzonitrile (3C4FBN) were studied. The Fourier transform infrared and Fourier transform Raman spectra of 3C4FBN were recorded in the solid phase. The optimized geometry was calculated by HF and B3LYP methods with 6-311++G(d,p) basis set. The harmonic-vibrational frequencies, infrared intensities and Raman scattering activities of the title compound were performed at and HF/B3LYP/6-311++G(d,p) level of theories. The scaled theoretical wave number showed very good agreement with the experimental values. The thermodynamic functions of the title compound was also performed at HF/6-31G(d,p) and B3LYP/6-311++G(d,p) level of theories. A detailed interpretation of the infrared and Raman spectra of 3C4FBN was reported. The theoretical spectrograms for FT-IR and FT-Raman spectra of the title molecule have been constructed.

Introduction

Benzonitrile is a phenyl cynaide compound. It is a colourless liquid with a boiling point of 197 °C and having a smell of bitter almonds. Many derivatives of benzonitrile are widely used in industry and medicinal fields. The main products of benzonitrile–benzoic acid are used in medicine as urinary antiseptic in the form of salt and in vapour form for disinfecting bronchial tubes. Benzonitrile derivatives are used in dye industry for making aniline blue and also used for preserving food products [1].

Vibrational spectra of benzonitrile and mono-substituted benzonitriles have been extensively studied [2], [3], [4], [5], [6], [7], and also studies have been done on vibrational spectra of some di-substituted benzonitriles [8], [9], [10]. Force field calculations have also been made in a few cases [10] using the classical method developed by Wilson to support the vibrational analysis. Mohan et al. [11] reported the laser Raman spectrum of 2-chloro-6-methyl benzonitrile. They carried out assignment of most of the prominent bands in the spectrum based on the assumption that the benzonitrile belongs to the Cs point group.

Recently Rastogi et al. [12] reported vibrational wave numbers and several thermodynamic parameters were calculated using ab initio quantum chemical methods for the 3,5-difluorobenzonitrile molecules. The results were compared with experimental values with the help of three specific scaling procedures, the observed vibrational wave numbers were assigned to different normal modes.

Literature survey reveals that to the best of our knowledge, neither the complete Raman and IR spectra nor the force fields for 3C4FBN have been reported so far. Therefore, the present investigation was undertaken to study the vibrational spectra of this molecule completely and to identify the various normal modes with greater wave number accuracy. Ab initio HF and density functional theory (DFT) calculations have been performed to support our wave number assignment. 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 the basis set deficiencies and for the anharmonicity [13], [14], [15], [16], [17], [18].

Section snippets

Experimental

The compound 3C4FBN in the solid form was purchased from Sigma–Aldrich Chemical Company (USA)) with a stated purity of greater than 99% and it was used as such without further purification. The FT-Raman spectrum of 3C4FBN has been recorded using 1064 nm line of Nd:YAG laser as excitation wavelength in the region 100–4000 cm−1 on a Brucker model IFS 66 V spectrophotometer. The FT-IR spectrum of this compound was recoded in the region 400–4000 cm−1 on IFS 66 V spectrophotometer using KBr pellet

Method of calculations

All calculations were performed at Hartree-Fock (HF) and B3LYP levels on a Pentium IV/3.02 GHz personal computer using Gaussian 03 W [19] program package, invoking gradient geometry optimization [20]. Initial geometry generated from standard geometrical parameters was minimized without any constraint in the potential energy surface at Hartree-Fock level, adopting the standard 6-31G(d,p) basis set. This geometry was then re-optimized again at HF and the gradient corrected density functional theory

Molecular geometry

Our optimized bond lengths and angles in 3C4FBN are given in Table 1, along with atom numbering scheme given in Fig. 3. All the geometries determined belong to a true minimum proven by real wave numbers in the vibrational analysis. Experimental data of the isolated molecules in gas phase has not been reported. Since bond lengths in the crystal data are usually smaller than in the gas phase, the DFT calculations may describe the bond lengths of 3C4FBN in the gas phase correctly. In general the

Other molecular properties

Several calculated thermodynamic parameters are presented in Table 3. Scale factors have been recommended [60] for an accurate prediction in determining the zero-point vibration energies (ZPVEs), and the entropy, Svib(T). The variations in the ZPVEs seem to be insignificant. The total energies are found to decrease with the increase of the basis set dimension. The changes in the total entropy of 3C4FBN at room temperature at different basis set are only marginal. Due to symmetry of the

Conclusions

We have carried out ab initio and density functional theory calculation on the structure and vibrational spectrum of 3C4FBN. Comparison between the calculated and experimental structural parameters indicates that B3LYP are in good agreement with experimental ones. Vibrational frequencies, infrared intensities and Raman activities calculated by B3LYP/6-311++G(d,p) method agree very well with experimental results. On the basis of agreement between the calculated and observed results, assignments

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