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 with Cs symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C-C-C angle at the ipso 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 the ortho, meta, and para positions of the ring are 120.3 ± 0.1°, 120.7 ± 0.1°, and 119.0 ± 0.3°, respectively. Important bond distances are 〈rg(C-C)〉 = 1.398 ± 0.003 Å and rg(C-N) = 1.407 ± 0.003 Å. 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 difference r(Cortho-Cmeta) - r(Cipso-Cortho) is 0.008-0.009 Å. 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 Å 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.
ASJC Scopus subject areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry