Vibrational Spectroscopic Studies of 2-Amino Picoline

Computational Details

Calculations of the title compound were carried out with Gaussian03 program⁵ using the Hartree-Fock and DFT (B3LYP) levels of theory using the standard 6-31G* set to predict the molecular structure and vibrational wavenumbers. Molecular geometry (Figure 1) was fully optimized by Berny’s optimization algorithm using redundant internal coordinates. Harmonic vibrational wavenumbers were calculated using the analytic second derivatives to confirm the convergence to minima of the potential surface. The wavenumber values computed contain known systematic errors and hence we have used scaling factors 0.8929 and 0.9613 for HF and DFT methods.⁶ The absence of imaginary wavenumbers of the calculated vibrational spectrum confirms that the structure deduced corresponds to minimum energy. The optimized geometrical parameters (DFT) are given in Table 1.

Figure 1: Optimized geometry (DFT)

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Table 1: Geometrical parameters (DFT) of the title compound

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Results and Discussion

The calculated scaled wavenumbers, experimental wavenumbers given by Jose and Mohan³ and the assignments are given in Table 2. The NH₂ stretching modes are expected in the region⁷ 3250-3480 cm^-1 and the DFT calculation give 3532 and 3425 cm^-1 as asymmetric and symmetric NH₂ stretching modes. Jose and Mohan³ reported bands at 3435, 3431 and 3407 cm^-1 as NH₂ stretching modes. The NH₂ scissoring vibrations, expected⁷ around 1650 cm^-1 appear at 1642 cm^-1 Raman spectrum and at 1635 cm^-1 in the IR spectrum. The DFT calculations give this mode at 1613 cm^-1. The δNH₂ scissoring vibrations are reported at 1629 cm^-1 for sulfanilamide⁸ and at 1637 cm^-1 in IR , 1634 cm^-1 in Raman and 1642 cm^-1 in HF for orthanilic acid.⁹ According to Roeges⁷ ρ/τNH₂ vibration is expected in the region 1070 ± 50 cm^-1 and in the present case the DFT calculation give this mode at 1031 cm^-1. Kurt et al.¹⁰ observed the ωNH₂ vibration at 667 cm^-1 in the IR spectrum and at 695 cm^-1 theoretically. Tzeng et al.¹¹ calculated the wavenumber of the wagging vibration of amino group at 649 cm^-1 and experimentally at 665 cm^-1. For the title compound the DFT calculation give the wagging mode of amino group at 601 cm^-1. Primary aromatic amines with nitrogen directly attached to the ring absorb in the region 1330-1260 cm^-1 due to the stretching of the ring carbon-nitrogen bond.^11.12 The band observed at 1276 cm^-1 in IR, 1266 cm^-1 in Raman and 1273 cm^-1 (DFT) is assigned as C-N stretching mode.

The asymmetric stretching vibrations of the methyl group^7,12 are expected in the range 2905-3000 cm^-1 and symmetric methyl stretching vibration in the range 2860-2900 cm^-1. In the present case the DFT calculation give the methyl stretching modes at 3007, 2980 and 2929 cm^-1 which are in agreement with the reported values.³ The methyl deformation bands are expected in the range^7,12 1400-1500 cm^-1 and the bands at 1482, 1454, 1419 cm^-1 given by DFT is assigned as these modes. The methyl rocking vibrations⁷ are expected around 1045 cm^-1 and in the range 970 ±70 cm^-1. In the present case ρMe are assigned at 1047 and 965 cm^-1 theoretically. The methyl and amino torsions are often assigned in the low wavenumber region.⁷

The pyridine CH stretching vibrations^13-15 are observed in the range 3000-3100 cm^-1. Jose and Mohan³ reported CH stretching vibrations at 3135, 3059 cm^-1 in the IR spectrum and at 3049 cm^-1 in the Raman spectrum. The DFT calculations give these modes at 3077, 3057 and 3043 cm^-1. The pyridine ring stretching vibrations¹⁶ occur in the general region 1600-1300 cm^-1. These vibrations involve stretching and contraction of all the bonds in the ring and interaction between the stretching modes. In the present case the DFT calculations give υPy modes at 1596, 1558, 1465, 1385, 1316 cm^-1. The pyridine ring breathing mode is assigned at 997 cm^-1 (DFT).¹⁷ The in-plane and out-of-plane CH deformations are expected above 1000 and below 1000 cm^-1 and all these bands (Table 2) are assigned.⁷

Table 2: Calculated (scaled) wavenumbers and assignments

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Analysis of organic molecules having conjugated π-electron systems and large hyperpolarizability using infrared and Raman spectroscopy has been evolved as a subject of research.¹⁸ The potential application of the title compound in the field of non linear optics demands the investigation of its structural and bonding features contributing to the hyperpolarizability enhancement, by analyzing the vibrational modes using the IR and Raman spectrum. The calculated first hyperpolarizability of the title compound is 2.47×10^-30 esu, which is comparable with the reported values of similar derivatives.¹⁹ We conclude that the title compound is an attractive object for future studies of non linear optics.

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