Molecular structure of aniline in the gaseous phase: A concerted study by electron diffraction and ab initio molecular orbital calculations

Abstract

The molecular structure of free aniline has been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G*(6D) basis set. Least-squares refinement of a model withC s symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C-C-C angle at theipso position of the benzene ring, α=119.0±0.2‡ (where the uncertainty represents total error). This parameter provides information on the extent of the interaction between the nitrogen lone pair and the π system of the benzene ring, and could not be determined accurately by microwave spectroscopy. The angles at theortho, meta, andpara positions of the ring are 120.3±0.1‡, 120.7±0.1‡, and 119.0±0.3‡, respectively. Important bond distances are 〈r g(C-C)〉=1.398±0.003 a andr g(C-N) =1.407±0.003 a. The effective dihedral angle between the H-N-H plane and the ring plane, averaged over the large-amplitude inversion motion of the amino group, is 〈¦Τ¦〉=44±4‡. The equilibrium dihedral angle is calculated to be 41.8‡ at the HF level and 43.6‡ at the MP2 level, in agreement with far-infrared spectroscopic information. The MO calculations predict that the differencer(Cortho-Cmeta) -r(Cipso-Cortho) is 0.008–0.009 a. They also indicate that the nitrogen atom is displaced from the ring plane, on the side opposite to the amino hydrogens. The displacement is 0.049 a at the HF level and 0.072 a at the MP2 level. The two calculations, however, yield very different patterns for the minute deviations from planarity of the ring carbons.