Structural study and vibrational assignments of FT-IR and FT-Raman spectra of powerful pesticide 2,4’-DDT. Its comparison with 4,4’-DDT

Abstract

In this work, the S(+) and R(-) forms of powerful pesticide 2,4’-DDT have been experimentally characterized by using the experimental FT-IR and FT-Raman spectra in the solid phase accomplished with calculations derived from the density functional theory (DFT) together with the 6-31G∗ and 6-311++G∗∗ basis sets. The optimizations in gas phase with both levels of calculations reveal that the R(-) form is the most stable of 2,4’-DDT, as was experimentally observed in the solid state. Here, the geometrical parameters, atomic charges, bond orders, main delocalization energies, the topological parameters, force constants and the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps for both forms of 2,4’-DDT were compared with reported for R-4,4’-DDT. Complete assignments of 78 vibration normal modes for R-2,4’-DDT were performed by using the scaled quantum mechanical force field (SQMFF). Comparisons between both DDT derivatives show that the structural change of one Cl atom in para position in 4,4’-DDT to ortho position in R-2,4’-DDT produce changes in the assignments, intensities and positions of IR bands between 1400 and 1200 cm−1 region while a set of bands in the region close to 1000 cm−1 are only observed in the Raman spectra of R-2,4’-DDT. The vibration CCl3 and C-Cl modes of groups or bonds common to the two DDT species are predicted in practically the same positions. Natural bond orbital (NBO) and atoms in molecules (AIM) calculations evidence higher stability of form R-2,4’-DDT as compared with the S one while the gap values support lower reactivities of both forms of 2,4’-DDT as compared with 4,4’-DDT. Finally, the scaled force constants related to diphenyl rings and trichloroethane groups of both derivatives present practically the same values.