FT-IR, FT-Raman, Homo-Lumo and UV-Visible Spectral Analysis of E-(N′-(1H-INDOL-3YL) Methylene Isonicotinohydrazide)

Indole is an aromatic heterocyclic organic compound with a bicyclic structure. It consist of a six-member benzene ring fused with five-member nitrogen containing pyrrole ring. It is of interest as it can be compared with tryptophane residue ¹. The derivative of indole is present in both-animal and plants. The most important compound of this group is tryptophan, an essential amino acid in the human diet, which a 3-substituted indole ². Another important indole derivative is the indole-3-acetic acid, a phytohormone, coordinating several growths processing of plants ³. The biological activity of the indole derivatives is in connection with the nature of substitution in position 3, on the pyrrole ring ⁴. Indole derivatives have an important role through individual biological functions. It is present in the side chain of amino acid tryptophan. The chemical and spectroscopic properties of indole derivatives have been subject of many experimental and theoretical investigations ^{5, 6, 7}. The indole tryptamine if one of the biogenic monoamines would play anti-tumor effects by either inhibiting cancer cell proliferation or stimulating the anti-cancer immunity ⁸. DFT method has become an efficient tool in the prediction of molecular structure of organic molecules and in evaluating various molecular properties like conjugation, hydrogen bonding, vibrational frequencies and IR & Raman activities of the bioactive molecule ^{9, 10, 11, 12, 13}.

The new Donor-π-Acceptor type dyes D1-3 carrying 3-(1-hexyl-1H-indol-3-yl)-2-(thiophen-2-yl) acrylonitrile as backbone with three different acceptor units were designed and synthesized by Babu, ¹⁴, using sensitizers for solar cell application. The new dyes were characterized by various spectral and elemental analyses. Their optical and electrochemical properties were investigated using spectrophotometry and cyclic voltammetry, respectively. The DFT study was carried out to investigate their Frontier MO energy states. The above study reveals that the dye carrying 4-aminobenzoic acid as an acceptor showed the highest photovoltaic efficiency among the three dyes. This can be attributed to the longer electron lifetime and lower recombination rates. Additionally, a single crystal XRD study confirms the structure of a key intermediate.

The stability of the syn and anti structures of the non-steroidal anti-inflammatory drug indomethacin were investigated by Hassan et al., ¹⁵, using the DFT/B3LYP and ab initio MP2 calculations with the 6-311G(d,p) basis set. The molecule indomethacin was predicted at the DFT and MP2 levels of calculation to have the syn (C1N7C10C18 ~ 40˚) form being about 1.7 and 1.5 kcal/mol, respectively, lower in energy than the anti (C1N7C10C18 ~ 140˚) structure. The calculated CNCC torsional angles for the chlorobenzene and indole rings syn-anti conformational interconversion was in a good qualitative agreement with the reported XRD angles (C1N7C10C18 ~ 29 and 155˚) for the syn and anti conformers, respectively. Indomethacin was estimated from the calculated Gibb’s free energies to have an equilibrium mixture of 95% syn and 5% anti structures at 298.15 K. The vibrational wavenumbers were calculated at the same level of theory. The complete vibrational assignments were provided on the basis of theoretical and NCA combined with experimental IR and Raman data of the molecule. The analysis of the observed spectra supported the presence of indomethacin in only one conformation at room temperature.

The (E)-2-(2-hydroxybenzylidenamino)-3-(1H-indol-3yl) propionic acid was synthesized by Saleem et al., ⁷. To identify the stable structure, the theoretical conformational analysis was performed. The optimized molecular bond parameters were calculated using B3LYP/6-31G(d,p) basis set. The hyperconjugative interaction energy (E(2)) and EDs of donor (i) and acceptor (j) bonds were calculated using NBO analysis. The dipole moment (µ) and first order hyperpolarizability (β0) were calculated. The band gap energy was analyzed by UV–Vis recorded spectra and compared with theoretical band gap (TD-DFT/B3LYP/6-31G(d,p)) value. The intra-molecular hydrogen bonding interaction was studied between nitrogen and hydroxyl hydrogen (N-H---O).

The molecular structure, complete vibrational spectra and the quantum mechanical calculations of the title compound were not yet reported. Therefore, the vibrational spectrum and the quantum mechanical calculations for the title compound in the ground state by DFT method with the standard 6-311++G(d,p) level of basis set were reported. Electronic absorption spectra of the title compound were predicted by using TD-DFT method. The excitation energies, wavelength and oscillator strengths were obtained at the same level of theory. Besides the molecular parameters, dipole moments, NLO, thermodynamic properties, linear polarizability and first hyperpolarizability, were calculated. The results obtained from theoretical calculations and experimental were compared.

In the present study, MEP surface map of ICINH is calculated using B3LYP/6-311++G(d,p) basis set and illustrated in Figure 6 The MEP which is a plot of electrostatic potential map onto the constant ED surface. In the majority of the MEPs, the maximum negative region which is the preferred site for electrophilic attack and the maximum positive region is the preferred site for nucleophilic attack. The importance of MEPs lies in the fact that it simultaneously displays molecular size, shape as well as positive, negative and neutral electrostatic potential regions in terms of colour scheme (Figure 6) and is very useful in research of molecular structure with its physiochemical property relationship ^{26, 27}. The color scheme for the MEP surface is as follows: red for electron rich, partially negative charge: blue correspond to electron deficient, partially positive charge: green for neutral, respectively ^{28, 29}. The electrostatic potential increases in the order of red<orange<yellow<green<blue. The color code of the map range between -9.185 e-2 (deepest red) to 9.185 e-2 (deepest blue). It is seen from the MEP map of the title molecule, the regions having the negative potential over the electronegative atom (oxygen atom), and the regions having the positive potential over all the hydrogen atoms in indole ring. The oxygen atom indicates the strong repulsion with other atom. These two ends of the molecule which are positively and negatively charged are prone to electrophilic and electrophobic reactions with other molecules.

Figure 6.The molecular electrostatic potential map of ICINH

The FT-IR, FT-Raman and UV-Vis spectra of the compound ICINH had been recorded and analyzed. The detailed interpretations of the vibrational spectra had been carried out. The optimized geometrical parameters were calculated and compared with the reported XRD data. The vibrational assignments were further justified with help of the PED analysis. The HOMO-LUMO energy gap indicated the stability and reactivity of the title compound. The good correlation between the UV-Vis, absorption maxima and calculated electronic absorption maxima were found. The donor-acceptor interaction, as obtained from NBO analysis could fairly explains the decrease of occupancies of σ bonding orbital and the increase of occupancy of π* anti-bonding orbitals. In addition, mulliken atomic charges, MEP, Thermodynamic parameters, first order hyperpolarizabilities and dipole moment of the title compound were also calculated.

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