The pseudorotational process of pyrrolidine (PYR) and the conformational preference at the N-H position have been thoroughly reinvestigated by means of ab initio methods. To examine electron correlation effects and basis set dependencies, Hartree-Fock (HF), post-Hartree-Fock (MP2, CC, QCI, and CI) and several density functional (DFT) methods with a large variety of basis sets have been employed. It has been found that both post-Hartree-Fock and DFT methods predict opposed energy differences between the N-H axial and N-H equatorial conformers depending on the size of the basis set. However, according to HF and B3LYP computations with the aug-cc-pVQZ basis set, it could be concluded that the N-H equatorial structure is the most stable conformer of PYR. This prediction is in agreement with the last microwave free jet experiment of Caminati et al. [

The generalized anomeric effects in nitrogen, phosphorus, and arsenic compounds were examined in detail by means of ab initio calculations. The conformational preferences can be considered adequately described at the HF/6-311G**//MP2/6-311G** level, since these results agree with those obtained using larger basis sets and including electron correlation up to the MP4 level. The favored conformers show two or one anti orientations between the X lone pair (Lp-X) and the X−C polar bond. According to the NBO analysis of the Hartree−Fock wave functions, the preferences for the anti Lp-X−C−X orientations and the barriers to internal rotation are due mainly to charge delocalization, which is always stronger than the electrostatic and steric contributions included in the Lewis term. These features are much larger for second-row substituents. From the comparison with the previously reported data for the corresponding oxygen, sulfur, selenium, and tellurium compounds, an increase of the stability of the conformers favored by anomeric orientations and also of the rotational barriers can be observed from group 15 to group 16 of the periodic table. The reason for this fact, more noticeable for second-row compounds, is the predominant role of the Lewis energy, that is, the nonhyperconjugative contributions. The calculated energies for the group separation reactions also increase when moving to the right through the periodic table, but they are not a reasonable measurement of the generalized anomeric effect, since they do not have a direct relationship with the conformational preferences.

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A theoretical study of the reaction of S with C2H has been carried out. This reaction is a possible step in the generation of sulfur-containing cumulenes in interstellar clouds and circumstellar envelopes. The potential energy surfaces were computed by means of the G2, G2(QCI), and CBS-Q methods in the case of local minima and saddle points. The energy profiles for the interaction of S and C2H in all states associated with the lowest energy electron configurations have received special attention. The MR-AQCC/aug-cc-pVTZ method was used as the basic level of computation; spin−orbit interactions and basis set superposition corrections were also taken into account. We found only two neatly attractive potential energy surfaces, corresponding to the 2Π3/2 and 2Π1/2 electronic states. We employed an approximate classical trajectory method to compute the capture rate. According to our computations the reaction is relatively fast, its rate for T = 300 K being not too far from the typical values for ion−molecule reactions. The main product should be SC2(3Σ-)+H(2S). However, we have encountered a slight decrease in the rate coefficient with decreasing temperature.