Ab initio study and NBO interpretation of the anomeric effect in CH2(XH2)2 (X = N, P, As) compounds

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.