MOLECULAR-STRUCTURE OF ANILINE IN THE GASEOUS-PHASE - A CONCERTED STUDY BY ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

The molecular structure of free aniline has been investigated by gas-p hase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G(6D) basis set. Least-squares re finement of a model with C-s symmetry, with constraints from MP2 calcu lations, has led to an accurate determination of the C-C-C angle at th e ipso position of the benzene ring, alpha = 119.0 +/- 0.2 degrees (wh ere the uncertainty represents total error). This parameter provides i nformation on the extent of the interaction between the nitrogen lone pair and the II. system of the benzene ring, and could not be determin ed accurately by microwave spectroscopy. The angles at the ortho, meta , and para positions of the ring are 120.3 +/- 0.1 degrees, 120.7 +/- 0.1 degrees, and 119.0 +/- 0.3 degrees, respectively. Important bond d istances are <r(g)(C-C)> = 1.398 +/- 0.003 Angstrom and r(g)(C-N) = 1. 407 +/- 0.003 Angstrom. 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 (\tau\) = 44 +/- 4 degrees. The equilib rium dihedral angle is calculated to be 41.8 degrees at the HF level a nd 43.6 degrees at the MP2 level, in agreement with far-infrared spect roscopic information. The MO calculations predict that the difference r(C-ortho-C-meta) - r(C-ipso-C-ortho) is 0.008-0.009 Angstrom. They al so indicate that the nitrogen atom is displaced from the ring plane, o n the side opposite to the amino hydrogens. The displacement is 0.049 Angstrom at the HF level and 0.072 Angstrom at the MP2 level. The two calculations, however, yield very different patterns for the minute de viations from planarity of the ring carbons.