A molecular orbital study of nitrogen inversion in aniline with extensive geometry optimization

The geometry and energy of aniline have been calculated using the 6-31G and 6-31G^*(5D) basis sets for the planar structure and various pyramidal structures, assuming that the ring and the N-atom bonded to it lie in the same plane, but otherwise with full geometry optimization. With the 6-31G basis set the planar structure is predicated to be the most stable, whereas the inclusion of polarization functions in the 6-31G^*(5D) basis set finds a pyramidal structure with the out-of-plane angle f=42.3° to be most stable and the energy barrier to inversion via the planar transition state to be 1.59±0.02 kcal mol^-1, in close agreement with experiment. Completing the optimization, allowing the N-atom and the C- and H-atoms of the ring to take up equilibrium out-of-plane positions increases the calculated energy carrier to inversion by less than 0.1 kcal mol^-1 to 1.66 kcal mol^-1. The ring adopts a very shallow inverted boat-type conformation with ?N₇-C₁?C₄ = 2.0°.