Theoretical study of the internal rotational barriers in nitrotoluenes, nitrophenols, and nitroanilines

Abstract

The molecular geometries and internal rotational barriers of the nitro group of 3-nitrotoluene (3-NT), 4-nitrotoluene (4-NT), 3-nitrophenol (3-NP), 4-nitrophenol (4-NP), 3-nitroaniline (3-NA), and 4-nitroaniline (4-NA) were calculated by five different types of density functional theory (DFT) methods with three different levels of basis sets. Analyses of the torsional angles of the nitro, methyl, amino, and hydroxyl groups indicate that 3-NP, and 4-NP are planar molecules, but 3-NT, 4-NT, 3-NA, and 4-NA are not planar molecules. Internal rotational barriers of the nitro group were calculated as the V₂ barrier, and the NO₂ torsional potentials for each molecule were given. The heights of the V₂ barrier vary with the DFT methods, the basis sets, and the kinds and positions of substituents. The average values of the V₂ barriers for 3-NT, 4-NT, 3-NP, 4-NP, 3-NA, and 4-NA are 6.44, 6.92, 6.64, 7.93, 6.38, and 9.13 kcal/mol, respectively. Torsional potentials of the OH and NH₂ groups of nitrophenol and nitroaniline derivatives were also studied by a B3LYP/6-31G* approach. Except for the OH group in 2-NP, these derivatives have the V₂ barrier.