In the last couple of decades, Schiff base molecules attracted a considerable attention among researchers [1,2] due to the unique properties that they exhibit which favor them for a wide range of applications in the field of medicinal and catalysis chemistry . Some of the features that make Schiff bases very special species are being anti-toxic, anti-inflammatory, anti-cancer, and anti-bacterial agents . This class of organic molecules are containing a distinctive functional group which formed by carbon and nitrogen atoms linked via double bond. Schiff bases are prepared from reacting an aldehyde or a ketone with a primary amine molecule. Many studies have concluded that using aromatic compounds that contain carbonyl group will contribute to the high stability of produced Schiff bases compared to aliphatic moieties that increase the polymerization ability. Moreover, aromatic Schiff bases are found to be relatively safer when used as therapeutic agents . In terms of geometrical aspect of the aromatic Schiff bases, some researchers suggest that most of the thermochromic Schiff base molecules are planar, whereas the others that are non-planar are exhibiting photochromism [6,7]. On the other hand, determining the stability of Schiff base structures using theoretical calculations and drug design modeling is considered as a high priority to expand our understanding of this kind of organic species . These computational methods can predict various molecular properties, including electrostatic potentials, vibrational frequencies, non-linear optical merits [9,10]. One of the famous methods that suit these kinds of calculations is density functional theory DFT which represent a post-HF approach that enables the researchers to predict the molecular energies , the characteristics of structures , and spectroscopic properties with high accuracy . Koopmans’ theorem and total energy calculations are performed in this study for the titled molecules, including these that are connected with frontier molecular orbitals. Some other quantities are evaluated in the framework molecular orbitals providing a valuable insight when it comes to the molecular properties of the studied compounds.
Materials and methods
The computational calculations performed in this study are according to the density functional theory method using hybrid functional of three parameters (the Becke) and correlation functional of (LYP) which represents one the hybrid family with the most robust functional. The Basis set that is used for herein calculation was 6-31+g(d,p) which is very suitable for this kind of molecules as reported by many researchers. The studied molecules listed in (Table 1) were prepared and characterized previously [14,15]. Koopmans’ theorem was applied to carry out the electronic properties of the Schiff base and obtain the frontier molecular orbitals . The electron affinity (EA) and ionization potential (IP) were calculated depending on the HOMO-LUMO values. The other parameters were obtained according to these values, including softness (r), hardness (χ), electrochemical potential (µ), electronegativity (η), and energy gap (ΔE) as stated in Equations 1-4 .
Results and discussion
The structural geometries of the studied molecules were determined after optimization using B3LYP/6-31+g(d,p) basis set of DFT method in terms of bond lengths and angles as well as the dihedral angles as listed in Tables 4 to 8). The optimization parameters were obtained for the most stable geometries (Figure 1) after many optimizing steps which indicate that (Pan-N) compound has the least total energy with the following trend for stability:
Pcl-N < Ani-N < Pto-N < Xyl-N < Pan-N
The other parameters of optimization were included in Table 2. All the geometries of investigated compounds have a symmetry point group of C1 with stationary points that are related to the local minima structures on the PE surfaces. Figure 1 illustrates the optimized structures of the studies molecules with atoms numbering that is consistent with the tables of geometry parameters.
To obtain the details of the electrostatic effect at a certain point in the space that surround the molecule, the electrostatic potential maps are used to illustrate the correlation of the net charge distribution, including the nuclei and electron charges, with chemical reactivities, partial charges, electronegativities, and dipole moments. The ESP method provides an efficient data to visualize the relative polarities of the molecules under study . The negative electrostatic potential illustrates the highly attractive properties of the condensed electron density to attract the proton, whereas the electrostatic potential that is positive illustrate the highly repulsive properties of low electron density regions, which represent the atomic nuclei charges that are shielded incompletely, toward the protons . The electrostatic potentials were calculated for all studied compounds using the same method used for optimization and their surfaces were illustrated in Figure 2 where the reactive sites as well as the densities, shapes, and sizes of the molecular charges are depicted . Different colors are used to evaluate the values of electrostatic potentials, including blue color reflecting the electrostatic potential that is highly positive, red color reflects the highly negative electrostatic potential, and green color reflects the regions that are possessing zero potential . All the surfaces of studied compounds showed a highly negative charge distribution around the nitro groups indicating the ability of reacting with electrophilic site, whereas the hydrogen atoms of the rings and the methyl groups showed poor charge densities indicating the high ability of reacting with nucleophilic site.
To determine the nature of interaction between the studied molecules and the other species, frontier molecular orbital study is the suitable for such calculations. The electrons that are located at the outermost orbitals tend to be donated which can be obtained by determining the highest occupied orbitals (HOMO). On the other hand, the free spaces of the innermost orbitals tend to accept electrons obtained by determining the lowest unoccupied orbitals (LUMO). The energy of HOMO and LUMO can be connected directly to the ionization potential and electron affinity, respectively. One of the important properties concluded from these energies is the energy gap which represents the difference in energy between HOMO and LUMO orbitals, and this property determines the kinetic stability and chemical reactivity of the molecules . Small values of energy gap indicate that the molecule is highly polarized, and therefore, being a soft molecule. Moreover, energy gap is commonly used as electron conductivity indicator to determine the molecules’ bioactivity when inferred from the intra-molecule charge transfer (ICT). The studied molecules have shown relatively low energy gaps in the following trend:
Pan-N < Xyl-N < Pto-N < Pcl-N < Ani-N
The FMOs of the molecules under study are depicted in (Figure 3) and the energy values of HOMO, LUMO, and the other electronic parameters are listed in (Table 3).
According to the computational calculations carried out based on B3LYP/6-31+g(d,p) basis set of the DFT method, the optimized structures of Schiff bases compounds are determined in terms of bond lengths, angles, and torsion angles. The FTOs study showed that the studied molecules exhibit small values of energy gaps which indicate the high bioactivity. The MEP maps illustrate that high electronic density in terms of negative potential located on the nitro groups of all studied compounds, whereas the regions of hydrogen atoms of the ring moieties exhibited positive potential. It can be concluded from the values of total energies of the most stable structures that the Pan-N compound possesses the highest stability with the least energy value.
The author would like to acknowledge the AJF for the assistance with sample acquisition and technical support.
Conflict of Interest
There is no conflict of interest.
Ali J.A. Al-Sarray: https://orcid.org/0000-0001-6484-1763
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