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The results from a thorough natural bond orbital analysis of several transition states (TSs) for nucleophilic additions to carbonyl function are discussed. It is demonstrated that the absence or presence of the Cieplax type [sigma]->[sigma.sup.*#] interaction depends on whether the TS in question is of early or late character.

Natural bond orbital (NBO) analysis of several early TSs does not support the sigma --> sigma*# hypothesis. The sigma --> pi*(C=O) interaction controls the carbonyl pyramidalization that, in turn, controls the pi-selectivity of a nucleophilic addition. In contrast, late TSs are devoid of sigma --> pi*(C=O) interactions, and they benefit from sigma --> sigma*# interactions that control pi-selectivity. The evidence in favor of Anh-Felkin's sigma# --> sigma* hypothesis is weak. The electron-withdrawing sigma(C-F) in the 2-fluoropropanal-LiCN TS did not align anti to the incipient bond even though there was complete conformational freedom. The initial guess for the TS in which sigma(C-F) was held anti to sigma# optimized to what had lost the said geometrical relationship. Furthermore, in the TS for axial addition of LiCN to 2-ax-F-cyclohexanone, the net sigma --> sigma*# interaction was considerably larger than the net sigma# --> sigma* interaction. The relative TS energies require that the equatorial addition of LiCN to 2-ax-F-cyclohexanone be favored over the axial addition in good compliance with the available experimental results.