Abstract
MNDO and STO-3G calculations were performed to determine relative stabilities of rotamers for ${\alpha}$-substituted acetones, $CH_2XCOCH_3$, X = F, Cl, OH, SH, and $NH_2$. It was found that rotamers corresponding to gauche forms are preferred for all the ${\alpha}$-substituents except for X = F and NH$_2$, for which the cis forms were the preferred ones. The stability of gauche form was dictated by the stabilizing two-orbital-two-electron interaction ${\sigma}_{cx}$-${\pi}_{co}^*$, operating uniquely in the gauche form due to the substantial vicinal overlap and energy gap narrowing between ${\sigma}_{cx}$ and ${\pi}_{co}^*$ orbitals. The energy gap narrowing was caused by the lowering of ${\pi}_{co}^*$ level due to the hyperconjugative ${\sigma}_{cx}^*$-${\pi}_{co}^*$ interactions; the red shift in the n-${\pi}^*$ transition was another effect of the relatively large ${\sigma}_{cx}^*$-${\pi}_{co}^*$ splitting. Various ${\sigma}-{\pi}$ 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, $n_N$ and vicinal C-C ${\sigma}$ bond were shown to be stronger in the trans than in the cis orientation.
${\alpha}$-치환아세톤($CH_2XCOCH_3$, X = F, Cl, OH, SH 및 $NH_2$)의 상대적 형태안정성을 결정하기 위하여 MNDO 및 STO-3G계산을 수행하였다. 계산결과 cis형이 더 안정한 F 및 $NH_2$ 치환 아세톤의 경우를 제외하고 모두 gauche형에 해당하는 형태이성체가 안정함을 밝혔다. gauche형에 대한 안정성은 ${\sigma}_{cx}$와 ${\pi}_{co}^*$궤도간의 two-orbital-two-electron상호작용에 의한 것으로 생각되며 이것은 gauche형에서의 vicinal overlap이 상당히 크다는 점과 두 궤도간의 에너지간격이 작다는 점에서 비롯된다. 아울러 이러한 에너지간격의 좁힘현상은 ${\sigma}_{cx}^*$-${\pi}_{co}^*$궤도간의 hyperconjugation상호작용 때문에 ${\pi}_{co}^*$궤도가 낮아졌기 때문이다. 또한 gauche형에서의 여러가지 ${\sigma}-{\pi}$상호작용들은 X가 Cl 및 SH인 3주기 원자들에게 더 강함을 보였다. 한편 질소원자의 비결합궤도함수 $n_N$과 인접 C-C ${\sigma}$결합의 상호작용은 cis일 경우 보다 trans배향일 경우에 더 강하였다.
