The visualization of the optimized geometrical structure and atomic labeling of [Zn(2-Aminothiazole)₂Cl₂] complex are presented in Figure 1.

The results obtained from the geometry optimization of [Zn(2-Aminothiazole)₂Cl₂] in gas-phase and in water- phase cannot be verified experimentally. The agreement between the optimized geometry (in gas and in water phase) and the experimental crystal structure [14] (insolid phase and in the presence of intermolecular interac- tions) is excellent showing that the geometry optimization almost exactly reproduces the experimental con- formation as one can expect. The optimized structural parameters (bond lengths, bondangles, dihedral angles) of [Zn(2-Aminothiazole)₂Cl₂] were compared with the experimental one and are listed in Table 1.

Quantum chemical calculations provide the ability to calculate the net atomic charges (q), which are localized at the corresponding atoms as a result of the redistribution of the electrons in the molecule. Though they are not connected with physical properties and cannot be observed experimentally, they allow the understanding of the distribution of the electronic density in a system of connected atoms and predict some chemical properties of the

molecules. The nitrogen atoms exhibit their electro negative nature as expected. The net atomic charges, calculated by Mulliken and the order of the bonds in the investigated compound are presented in Table 2. Obviously, the thiazole bond orders are in the range 1.218 - 1.774. These bond orders values suggest a relatively strong aromatic character for the five-membered ring of thiazole.

The presence of amino groups leads to the electronic coupling between ring electrons and nitrogen lone pair electrons which provides stabilization to the molecular structure. Hence it is important to study the electrostatic

potential distribution in the molecule. The molecular electrostatic potential is a property that the electrons and nuclei of a molecule create at each point an electrical potential in the surrounding space [15] . Electrostatic potential provides very useful information to explain hydrogen bonding, reactivity and structure-activity relationship of molecules and correlates with dipole moment, electro negativity, partial charges and site of chemical reactivity of the molecule. It gives a visualization to understand the relative polarity of a molecule. The regions with negative electrostatic potential, correspond to the areas of high electron density representing a strong attraction between the proton and the points, on the molecular surface have the brightest red color. The positive valued regions, areas of lowest electron density, have deep blue color indicating the regions of maximum repulsion. The electron density isosurface onto which the electrostatic potential surface was mapped is shown in Figure 2 for [Zn(2-Aminothiazole)₂Cl₂]. It is visible that the region of the most negative electrostatic potential is spread over the Cl²². Regions of most positive electrostatic potential is over the amino groups.

The molecular electrostatic potential is widely used as a reactivitymap displaying most probable regions for the electrophilicattack of charged point-like reagents on organic molecules.The negative (red color) regions of electrostatic potential arerelated to electrophilic reactivity and the positive (bluecolor) onesto nucleophilic reactivity.

The frontier orbitals (highest occupied molecular orbital―HOMO) and (lowest unoccupied molecular orbital― LUMO) are very important indefining its reactivity. The HOMO exhibits the ability to donate an electron and LUMO as an electron acceptor serves the ability to obtain an electron. The HOMO and LUMO energy calculated by B3LYP/6-31G(d, p) level of theory show the energy gap which reflects the chemical activity of the molecule (Figure 3).

High values of E_HOMO have a tendency of themolecule to donate electrons to appropriate acceptor molecules with low energy, empty molecular orbitals. The energy of the lowest unoccupied molecular orbital indicates the ability of themolecule to accept electrons. The lower value of E_LUMO, the more probable it is that the molecule wouldaccept electrons. Consequently, concerning the value of theenergy of the gap ΔE_LUMO-HOMO, larger values ofthe energy difference will provide low reactivity to achemical species. Lower values of the energy difference will render good inhibition efficiency, because the energy toremove an electron from the last occupied orbital will below. The results for the calculations of the ionization potential (I) and the electron affinity (A) by appli- cation of the Koopmans’ theorem [16] are shown. According to the Hartree-Fock theorem, the frontier orbital energies are given by: I = −E_HOMO = 617.491 kJ/mol; A = −E_LUMO = 86.852 kJ/mol. This theorem establishes a relation between the energies of the HOMO and the LUMO and the ionization potential and the electron affinity, respectively. Although no formal proof of this theorem exists within DFT, its validity is generally accepted. Electro negativity (χ), chemical potential (µ) and global hardness (η), their operational and approximate definitions for [Zn(2-Aminothiazole)₂Cl₂] are: χ = −µ= (I + A)/2 = 352.172 kJ/mol; η = (I − A)/2 = 265.320 kJ/mol.

The vibrational spectra of 2-Aminothiazole and [Zn(2-Aminothiazole)₂Cl₂] were calculated by DFT with B3LYP functional having extended basis sets 6-31G(d, p). The experimental assignments of IR for several vibrations, IR theoretical and relative intensities are reported in Table 3.

Frequencies recorded experimentally for 2-Aminothiazole and for [Zn(2-Aminothiazole)₂Cl₂] in far IR were compared with the calculated frequencies obtained by DFT/B3LYP methods with 6-31G(d, p) basis sets. In order to improve the calculated values in agreement with the experimental values, it is necessary to scale down the calculated harmonic frequencies. After scaling, the theoretical frequencies will match well with the experimental ones. These assignments are important to understand the molecular structureand biological activity of the title molecule. Any discrepancies noted between the observed and the calculated wavenumbers due to the fact that the calculations were actually performed on single (isolated) molecules in the gaseous state. Thus some reason- able deviations from the experimental values seem to be justified.

The calculated several thermodynamic parameters are presented in Table 4. The zeropoint vibrational energies, rotational constants, rotational temperature, thermal energy, molar capacity at constant volume and entropy were calculated by B3LYP/6-31G(d, p) in gas and in water phase.