The microwave spectrum and large-amplitude vibrations of N-methylaniline

Microwave spectra of the normal and amine-deuterated isotopic species of N-methyl-aniline in the ground and some low-lying excited vibrational states have been observed. The inertial defect indicates that the dihedral angle of the N-CH₃, bond with respect to the ring plane is somewhat less than that for the N—H bonds in aniline. The position of the amino-hydrogen atom is very poorly determined by the isotopic substitution method because of large zero-point effects. The excited vibrational states are consistent with a double-minimum potential for the inversion of the HNCH₃ group and there is some evidence for a lower barrier than in aniline. The excited states of the HNCH₃ torsion indicate a barrier in the range 8 < V₂ < 25 kJ mol^?1 to the internal rotation of this group. No splitting of the ground-state lines attributable to the torsion of the methyl group has been observed, which implies a barrier of V₃ > 8 kJ mol^?1.