Structural and SQMFF study of potent insecticide 4′,4′-DDT combining the FT-IR and FT-Raman spectra with DFT calculations

Abstract

The experimental FT-IR and FT-Raman spectra in the solid state have been employed to vibrationally characterize the potent insecticide dichlorodiphenyltrichloroethane (4′,4′-DDT). The complete vibrational assignments of 4′,4′-DDT in gas phase and in acetone and diethyl-ether were performed combining hybrid B3LYP/6-31G* and B3LYP/6–311++G** calculations with the internal coordinates, the scaled quantum mechanical force field (SQMFF) methodology and the experimental spectra. Here, the molecular structures of 4′,4′-DDT were theoretically determined in gas phase and in acetone and diethyl-ether solutions by using those two methods. The self-consistent reaction field (SCRF) method and the integral equation formalism variant polarised continuum model (IEFPCM) model were employed to study the properties in solution while the solvation energies were predicted by using the solvation model. The B3LYP/6–311++G** calculations show higher solvation energy in the solvent with lower permittivity, diethyl-ether. A clear and direct correlation was found with both basis sets between solvation energy and dipole moment for those two solvents. NBO studies clearly show that dichlorodiphenyl groups confer to 4,4′-DDT higher stability than the trichloroethyl group due to the fact that the lone pairs of Cl21 and Cl22 atoms present the n→π* transitions while the AIM study shows the participation of trichloroethyl group in halogen bonds clearly justifying the stability of 4′,4′-DDT in the three media and with both basis sets. The MK and NPA charges evidence different behaviors in both media and with both basis sets. The frontier orbitals show that the reactivity of 4,4′-DDT strongly depends on the basis set while it is more reactive than dieldrin, hexachlorobenzene and saxitoxin but 3,3′,4,4′-tetrachloroazobenzene, 3,3′,4,4′ tetrachloroazoxybenzene, CN− and CO species present higher reactivity than 4,4′-DDT. The number of Cl atoms could justify the differences observed in the nucleophilicity indexes of 4,4′-DDT, dieldrin, hexachlorobenzene, 3,3′,4,4′-tetrachloroazobenzene and 3,3′,4,4′ tetrachloroazoxybenzene.