Theoretical studies on some chlorinated pesticides compounds namely: Heptachlor, Heptachlorepoxide and Chlordane as a first group, Aldrin and Endrin as a second group, and DDT, Methoxychlor as a third group, were performed using semi-empirical quantum chemical calculation. Modified Neglect of Diatomic Overlap MNDO method was used to investigate the effect of positional variation of chlorine and oxygen atoms on the electronic and structure properties of the studied chlorinated pesticides. The structure optimization of some electronic parameters such as heats of formation, total energies, electronic energy, binding energy, nuclear energy, ionization energies, electron affinities and dipole moment were calculated and discussed. The energy of the Highest Occupied Molecular Orbital (HOMO) and the energy of the Lowest Unoccupied Molecular Orbital (LUMO) are also calculated. The LUMO - HOMO energy gap have been calculated and discussed. The stability and activity of the studied compounds based on the obtained values have been investigated.

A theoretical study of 1, 3 cycloaddition has been carried out using density functional theory (DFT) methods at the B3LYP/6-31G* level. The regioselectivity of the reaction have been clarified through different theoretical approaches: Case of a Two-Center Process (domingo approach), HSAB principle (Gazquez and Mendez approach), and the activation energy calculations. The analysis of results shows that the reaction takes place along concerted asynchronous mechanism and the isomer meta is favored, in agreement with the experiment results.

The stereodynamics of the reaction H($^2$S) + NH $(υ= 0, 1, 2, 3; j = 0)$→N($^4$S) + H$_2$ are studied using the quasi-classical trajectory method on a double many-body expansion potential energy surface to understand the alignment and orientation of the product molecules in the collision energy range of 2–20 kcal·mol$^{-1}$ The vibrational–rotational quantum number of the NH molecules is specifically investigated for $v = 0, 1, 2,$ and $3$ and $j = 0.$ The $p(\theta_r),$ $p(\phi_r),$ $p(\theta_r, \phi_r)$, differential cross section $[{\rm DCS}; (2\pi/\sigma)(d\sigma_{00}/d\omega_t)],$ and average rotational alignment factor $\langle p_2(cos\theta_r) \rangle.$ are calculated. The stereodynamics results indicate that the reagent vibrational quantum number and initial collision energy significantly affect the distributions of the $k-j'$, $k-k'-j'$ and $k-k'$ vector correlations along with $\langle p_2(cos\theta_r) \rangle.$ In addition, while DCS is extremely sensitive to the collision energy, it is not significantly affected by the vibrational excitation of the reagents.