Conformation and Internal Rotation of Nitroaromatic Amines in Solution as Detected by Proton Magnetic Resonance

The 60-Mc/sec proton magnetic resonance spectra of N-methyl-2,4,6-trinitroaniline (I) and of N-methyl-2,6-dinitroaniline (II), as well as of the analogous N-diphenylamino compounds, show that the 3- and 5-phenyl protons are inequivalent at low temperatures in solution, but act as equivalent at sufficiently high temperatures. The N-unsubstituted anilines and N,N-dimethyl anilines corresponding to I and II show no such inequivalence. The coalescence with temperature of the AB quartet of the picryl protons in the picrylaniline (I) and diphenylpicrylhydrazine in dichloromethane solution could be described quantitatively by the line-shape equations for two coupled protons exchanging with each other. The analysis yielded the lifetimes of the ground state configurations of these molecules, with Arrhenius activation energies of 14.5±0.3 and 12.5 ± 0.2 kcal/mole and frequency factors of 1.4×1015 and 6×1010 sec—1, respectively, for the intramolecular exchange process. A model for these observations, including Hückel MO calculations, is presented in terms of the barrier to rotation about the amino nitrogen-ring carbon bond, the orientation of the nitro groups ortho to the amino group, and hydrogen bonding between the amino group and these nitro groups. The chemical shift and spin—spin coupling parameters for these compounds and for the 2,4-dinitro analogs are tabulated and compared. The chemical shift values exhibit regularities in excellent agreement with the model introduced.