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MNDO and STO-3G calculations were performed to determine relative stabilities of rotamers for α-substituted acetones, CH2XCOCH3, X = F, Cl, OH, SH, and NH2. It was found that rotamers corresponding to gauche forms are preferred for all the α-substituents except for X = F and NH2, for which the cis forms were the preferred ones. The stability of gauche form was dictated by the stabilizing two-orbital-two-electron interaction σcx-π*co, operating uniquely in the gauche form due to the substantial vicinal overlap and energy gap narrowing between σcx and π*co orbitals. The energy gap narrowing was caused by the lowering of π*co level due to the hyperconjugative σ*cx-π*co interactions; the red shift in the n-π* transition was another effect of the relatively large σ*cx-π*co splitting. Various σ-π interactions in the gauche form were found to be stronger in the third-row hetero atom system, X = Cl and SH. Interactions between nonbonding orbital on N, nN and vicinal C-C σ bond were shown to be stronger in the trans than in the cis orientation.
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