Knowledge Bank

Vapor phase far-infrared spectra from 35 to $675cm-1$ of the nine ortho-, meta- and para-substituted fluoro-, chloro- and bromoanilines obtained with an interferometer are reported. The $v1=1,2,$ and 3 levels of the amino group inversion vibration are established from the observed transitions, and the barriers to inversion are derived from these, using a simple double minimum harmonic well potential with a Gaussian barrier. Calculated barrier heights range between $385.9cm-1$ (2-bromoaniline) and $599.9cm-1$ (4-fluoroaniline), and these are discussed in terms of the electronic effects of the halogen substituents. Intramolecular hydrogen bonds and/or steric effects lead to remarkably low inversion barriers in 2-chloro- and 2-bromoaniline- The following out-of-plane vibrations are also assigned: amino group wag, substituent wag, and ring deformation. The $\nu \tau \pm \nu i$ combinations lead to deduced frequencies for the torsional vibration, $v\tau$, in a number of cases.