Theoretical Studies on Electrophilic Aromatic Substitution Reaction for 8-Hydroxyquinoline

Theoretical investigations of organic molecules for the objective of their structural stability are the most important techniques in this regards. Recently calculations and simulation reactions utilizing theoretical studies become attractive conventional method for the researchers. Density function theory (DFT) method was used to study the reaction of 8-hydroxyquinoline with 4-ethoxycarbonyl-benzene diazonium chloride as electrophilic aromatic substitution reaction. To study any reaction there are two explanations: first explanation depends on the reactant molecules and second explanation depends on the stability of the product molecules. Determine the stability of the molecule by comparing the energies (total energy, energy level of (HOMO), and energy gap), we have three stable molecules, are: HQ-7-YBAEE (II) for the total energy, HQ-6-YBAEE (II) for the energy level of (HOMO) and HQ-2-YBAEE (II) for the energy gap. The molecule HQ-4-YBAEE (II) is always at least stability in all data.

Theoretical studies had been quite used to investigate the reaction mechanism, explain the reaction products and clarify chemical reactions mystery.Theoretical studies are valuable approaches to explore the mechanism of reactions in the molecules and their electronic structures levels in addition to electronic parameters that acquired by means of theoretical calculations employ the computational methods of quantum chemistry 1,2 .The improvement in theoretical studies and applications have accomplish a point where predicted features of logical accuracy can be obtained from DFT (density functional theory) studies 3,4 .Geometry of the molecules in its ground state, as well as the nature of their molecular orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are involved in the properties of activity of molecules reaction.Density Function Theory (DFT), a DFT calculation adds an additional step to each major phase of a Hartree-Fock calculation.This step is a numerical integration of the functional (or various derivatives of the functional) 5 .Thus in addition to the sources of numerical error in Hartree-Fock calculations (integral accuracy, SCF convergence, and CPHF convergence), the accuracy of DFT calculations also depends on number of points used in the numerical integration.The "fine" integration grid is the default in Gaussian 09.This grid greatly enhances calculation accuracy at minimal additional cost 6 .We do not recommend using any smaller grid in production DFT calculations.Note also that it is important to use the same grid for all calculations where you intend to compare energies (e.g., computing energy differences, heats of formation, and so on).Larger grids are available when needed (e.g.tight optimization of certain kinds of systems).
An alternate grid may be selected by including in the route section 7 .The B3LYP density functional theory calculations exhibit good performance on the molecular geometry and vibrational properties of organic compounds 8,9 .To extend our studies on the design and synthesis of new compounds [10][11][12][13][14][15][16][17][18][19][20][21][22] , we describe here, investigate the dependence of stability of the molecules on theoretical chemical parameters such as total energy, energy level of (HOMO), and energy gap.We have three stable molecules, HQ-7-YBAEE (II) for the total energy, HQ-6-YBAEE (II) for the energy level of (HOMO) and HQ-2-YBAEE (II) for the energy gap.The molecule HQ-4-YBAEE (II) is always at least stability in all data.

The Calculation Method
To calculate ground-state geometries, Gaussian 09, revision A.02 23 was optimized to a local minimum without symmetry restrictions using basis set 6-31G 24,25 .The combination of the Becke three-parameter hybrid (B3) 26 exchange functional and the Lee-Yang-Parr (LYP) (Lee, Yang, Parr, 1988correlation functional (B3LYP) 27,28 , was used for all geometry optimizations, Highest Occupied Molecular Orbital Energies (E HOMO ), Lowest Unoccupied Molecular Orbital Energies (E LUMO ), and physical properties for the molecules in this study.

RESULTS ANd dISCUSSION
The reaction 8-hydroxyquinoline with 4-ethoxycarbonyl-benzene diazonium chloride described below as in Scheme 1, was selected for study the electrophilic aromatic substitution reaction theoretically by using density function theory (DFT).
T h e m o l e c u l e 8 -hy d r ox y q u i n o l i n e electrophilic aromatic substitution reaction theoretically has six possibilities due to the presence of six hydrogen on aromatic carbon atoms which is associated with the C-2 to C-7, for the purpose of discussing the possibilities of this reaction there are two explanations: first explanation depends on the reactant molecules or call the name (explanation of the pre-reaction), which now has a lot of research, the second explanation depends on the stability of the product molecules, which we will focus on in this work [29][30][31][32][33] .

Explanation of the pre-reaction
Figure 1.shows the electronic distribution of the molecule 8-hydroxyquinoline when the energy level of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), if we look to the electronic distribution of HOMO note electron density high for each of the carbon atoms C-2 to C-6, which means the probability equal to a substitution reaction, making it difficult to determine directing substitution on site given alone.

Explanation of the post-reaction
This explanation depended on the stability of the product molecules; first, you must know the number of the product molecules and second to study of these molecules through find the total energy and the energy level of (HOMO), the number of the product molecules are twelve are shown in Figure 2. and Figure 3. use symbols by name because of the length of the molecule name, 4-ethoxycarbonylbenzene diazonium is attached to an aromatic system (usually hydrogen) is replaced by an electrophile, six hydrogens will be replaced; so we will have six  All twelve molecules were studied using the method of calculation (RB3LYP/6-31G) and the results found in the table 1.We note that the second isomer (II) when the value of dihedral angle to be about 180 degrees is more stable than first isomer (I) when the value of dihedral angle to be about zero degrees due to decrease repulsion energy because of the steric affect, thus been reduced possible molecules to six molecules are [HQ-2-YBAEE (II), HQ-3-YBAEE (II), HQ-4-YBAEE (II), HQ-5-YBAEE (II), HQ-6-YBAEE (II), and HQ-7-YBAEE (II)].Of the total energy and the energy level of (HOMO) can find the most stability of the molecule and thus can be expected to substitution on any site is a high percentage, molecule HQ-7-YBAEE (II) is more stable than the rest of the other molecules based on the total energy have equal value (-1084.55187au), can rearrangement of the molecules in descending order for the decrease stability, also at the bottom.While for energy level of (HOMO), molecule HQ-6-YBAEE (II) is more stable than the rest of the other molecules based on energy level of (HOMO) have equal value (-6.0730 eV), can rearrangement of the molecules in descending order for the decrease stability, also at the bottom.Energy gap; the energy gap is also called band gap, the gap energy generally refers to the energy difference (in electron volts) between the Low Unoccupied Molecular Orbital (LUMO) and the High Occupied Molecular Orbital (HOMO) in insulators and semiconductors.This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, therefore, the molecule is stable, which has a large energy gap, molecule HQ-2-YBAEE (II) is more stable than the rest of the other molecules based on energy gap have equal value (3.2684 eV), can rearrangement of the molecules in descending order for the decrease stability, also at the bottom.So we have three stable molecules, are: HQ-7-YBAEE (II) for the total energy, HQ-6-YBAEE (II) for the energy level of (HOMO), HQ-2-YBAEE (II) for the energy gap.The molecule HQ-4-YBAEE (II) is always at least stability in all data.

CONCLUSION
A quantum chemistry calculation is carried out using the density function theory (DFT) method to study optimized for twelve molecules, to study the electrophilic aromatic substitution reaction using two explanations, first explanation depends on the reactant molecules and second explanation depends on the product molecules.We have three stable molecules, are: HQ-7-YBAEE (II) for the total energy, HQ-6-YBAEE (II) for the energy level of (HOMO), HQ-2-YBAEE (II) for the energy gap.The molecule HQ-4-YBAEE (II) is always at least stability in all data.