The position of the lone pair of 17 amines has been located with the help of Bader's AIM methodology. This position has been compared with those estimated with simple geometrical models. It appears that these models are convenient only for some symmetrical or undistorted cases.

A b initio calculations on the structure of pyrazole have been carried out at different levels of accuracy. At the Hartree-Fock (HF) level, the performance of several basis sets, namely 3-21G, 6-31G, 6-31G**, and 6-311G* * was investigated.The influence of electron correlation effects also was studied by carrying out geometry optimizations at the MP2, MP4, and QCISD levels. The performance of a density functional method also was evaluated. We have also investigated the possible influence of the frozen core approximation on the final optimized geometry. Three different statistical analyses were considered in determining which geometry is closest to the experimental microwave geometry-namely Paul Curtin's diagrams, cluster analysis, and multidimensional scaling. From these analyses, we conclude that there is no asymptotic approach to the experimental geometry by increasing the quality of the theoretical model, although, as expected, the more reliable structures are those obtained at the MP2, MP4, and QCISD levels, as well as those obtained by the B3LYP density functional method. We have also found that the values of the rotational constants are a tight criterion to define the quality of a molecular geometry. 0 1995 by

Ninety‐two unsaturated hexagonal compounds (from benzene to hexaphosphinine, including azines and azaphosphinines) have been theoretically studied at DFT level, B3LYP/6‐311++G(d,p). Lone pair/lone pair repulsion between adjacent N atoms has been deduced using an empirical partition of the energy. Aromaticity based on magnetic criteria and on Bader analysis was studied finding that both criteria were almost orthogonal. The structure that are nonplanar or broken have been analyzed in function of their geometry or the fragments, respectively.

No abstract