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The rotational energy barriers in CH2—X—CH2species (X = NH, O, CH2) are calculated by means of abinitioSCF + CI calculations. With the most sophisticated CI calculations, they are found to be equal to 36.1, 20.6, and 1.1 kcal/mol for X = NH, O, and CH2, respectively. These values are briefly discussed with respect to the valence bond analysis of the wave function.

Ab initio calculations have been performed on unsubstituted (H2N)2C&dbd;C(NH2)2 and methylated (Me2N)2C&dbd;C(NMe2)2 tetrakis(amino)ethylene. In the unsubstituted system, idealized geometries have been optimized first. Geometrical relaxations by pyramidalization or rotation of the amino groups have been carefully studied. Full geometry optimization leads to a Y-shaped lowest energy structure. Optimized geometry resulting from a synchronous rotation of the amino groups is located 8.1 (HF level) and 6.7 (MP2 level) kcal/mol above in energy. In methylated species where strong steric effects are at work, only two minima are found. Their energy difference is small (1.0 (HF level) and 0.6 kcal/mol (MP2/HF level)). The most stable isomer is Y-shaped, while the secondary minimum exhibits an almost conrotatory motion of the amino groups. This latter minimum is geometrically very close to the experimentally observed molecule. It is concluded that (i) from an electronic point of view, Y-shaped geometry is preferred and (ii) steric effects disfavor this geometry, which becomes energetically competitive with that obtained from a conrotatory motion of the amino groups. It is finally suggested that the existence of two minima close in energy should be at the origin of the phase transitions that are experimentally observed in this material.

(studied by means of Extended Hückel calculations.