In different literature, a number of established protocols have been reported for the synthesis of quinoline ring, which can be changed to produce a number of differently substituted quinolines [5]. The quinoline ring has been generally synthesized by Skraup synthesis Figure 2.

Using the usual geometric parameters, geometry optimization was performed as the first task in density functional theory calculation without using any constraints. The optimized geometric parameters were used in the vibrational frequency calculations to characterize all the stationary points as minima [7]. The optimized ground state structure is as shown in Figure 3.

Structural parameter like bond distance, length or radius is the common distance between the nuclear of two bonded atoms in a molecule; it has values typically within the range less than 1 to 2 Å. This structural parameter influences the force of attraction binding such a molecule i.e. the smaller the bond length between the bonding atoms, the stronger is the force of attraction between them [2]. The bond distances and angle for quinoline are shown in Table 1 and Table 2 respectively.

The B3LYP method leads to geometry parameters, which are close to experimental data. A statistical treatment of these shows that for the bond lengths B3LYP/6-31+G (d, p) (Figure 4) is slightly better than the bond length (Figure 5). The correlation coefficient for bond lengths was 0.97 for B3LYP/6-31+G (d, p) methods. The slight variation with the experimental value is due to the fact that the optimization performed in an isolated condition. Most of the optimized

bond lengths are slightly longer than the experimental values and the bond angles are slightly different from the experimental ones, because the molecular states are different in the experimental and theoretical processes [8]. It is observed that the influence of the nitrogen substituent on the molecular parameters.

The values of thermodynamic parameters zero point vibrational energy, thermal energy, specific heat capacity, rotational constants, entropy of quinoline at 298.15 K in ground state are listed in Table 3. The variation in zero point vibrational energies (ZPVEs) seems to be significant.

The vibrational frequency deals with the periodic movement of atoms of a molecule relative to each other and usually range from less than 10¹³ Hz to 10¹⁴ Hz, or 300 cm⁻¹ 3000 cm⁻¹, it is important and necessary because it aids in the elucidation of molecular transition and molecular structure [2]. The calculated wave-numbers and observed IR with relative intensities are shown in (Table 4 and Figure 6).

In Table 5, the equilibrium rotational constants for the Quinoline calculated at the G09 level of theory are shown below. The error difference between the calculated and experimental rotational constants data for quinoline reveal that there is consistency and therefore shows high accuracy in our findings. This work gives the rotational constant values of 3.1010, 1.218 and 0.875 are consistent with reported experimental results of -, -, - GHz [1]. Therefore, for the quinoline with no experimental data, it is concluded that the G9 computational method has predicted the parameters with high accuracy.

Dipole moment is an important molecular property widely used to probe the infrared spectroscopy of molecule. A molecule is infrared active only when there is a change in dipole moments. Thus, provided that there is a dipole moment change during a normal molecular rotation, vibration or a combination of the

two molecular rotation–vibrations, chemical compounds can absorb at the infrared regions and are said to be IR active [1] [2]. For charged systems, its value depends on the choice of origin and molecular orientation [9]. As a result of DFT (B3LYP) calculations, the dipole moment of quinoline (2.004 D) was observed in this study agrees well with the experimental reported literature data was for B3LYP/6-31G+ (d, p).

UV-visible spectroscopy offers some valuable information about the nature of electronic transitions between the two prominent frontier molecular orbitals named as; HOMO (characterized with the aid of using maximum occupied molecular orbital) and LUMO (characterized with the aid of using unoccupied molecular orbital) [10]. The nature of electronic transition has been studied with the aid of using the usage of TD-DFT using 6-31+G (d, p) basis set. The theoretical absorption spectra are shown in Figure 7. The calculated electronic transitions with excessive oscillator strength are listed in Table 6.

HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) are referred to as Molecular orbitals. They were very important electronic parameters for chemist and physicist. The LUMO are referred to as the inner-maximum orbital containing loose locations to simply accept electrons [11]. The HOMO represents the capacity to donate an electron; LUMO as an electron acceptor represents the capacity to obtain an electron. The energy gap between HOMO and LUMO determines the kinetic stability, chemical reactivity, and optical polarizability and chemical hardness-softness of a molecule [9]. The HOMO–LUMO energy gap for quinoline has been calculated DFT level. The Eigen values of LUMO–HOMO energy gap reflect the chemical activity of the molecule [8]. The atomic orbital compositions of the molecular orbitals are sketched in Figure 8. The calculated energies and the energy gap is

HOMO energy = −6.646 eV,

LUMO energy = −1.816 eV,

HOMO—LUMO energy gap = −4.83 eV.

The lower with inside in the HOMO and LUMO energy gap explains the eventual charge transfer interaction taking place within the molecule, due to the strong electron-accepting ability of the electron acceptor group. The strong charge transfer interaction is responsible for the bioactivity of the molecule.

Based on the above molecular orbitals composition (HOMO and LUMO) information we can determine the molecular properties, related to the reactivity and selectivity of the compounds, were estimated following the Koopmans’s theorem relating the energy of the HOMO and the LUMO (Table 7). Electronegativity is estimated using the following the equation:

x = − 1 2 ( E HOMO + E LUMO ) (1)

Chemical hardness (η) measures the resistance of an atom to a charge transfer [6]. On the basis of frontier molecular orbitals, chemical hardness corresponds to the gap between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) [12].

Chemical hardness is approximated using the equation.

η = − 1 2 ( E HOMO − E LUMO ) (2)

Electron polarizability, also called chemical softness (σ), describes the capacity of an atom or group of atoms to receive electrons and is estimated by using the equation

σ = 1 η = − 2 E HOMO − E LUMO (3)

Mulliken atomic charge calculation is a critical device with inside the software the application of quantum chemical calculation to molecular system because atomic charges influence dipole moment, molecular polarizability, electronic structure of molecular systems. The atomic charge depends on basis set presumably occur due to polarization [9]. The calculated Mulliken charge values of N atom is −0.259007 for DFT/6-31+G (d, p) (Table 8).

The charge of H1, H2, H6, H8, and H13 is positive in DFT diffuse functions. Hydrogen atom exhibits a positive charge, which is an acceptor atom. Considering DFT methods and basis set used in the atomic charge calculation, the carbon atoms (C1, C2, C3, C7, C11 and C12) exhibit a substantial negative charge, which are donor atoms. But the charge of C4 is exhibit a positive charge, which is an acceptor atom.