Theoretical Studies on Orbital Interactions and Conformation of ${\alpha}$-Substituted Acetones

${\alpha}$-치환 아세톤의 궤도간 상호작용과 형태에 관한 이론적 연구

  • Ikchoon Lee (Department of Chemistry, Inha University) ;
  • Kiyull Yang (Department of Chemistry, Inha University) ;
  • Wang Ki Kim (Department of Chemical Education, Chonnam National University) ;
  • Byung Hoo Kong (Department of Science Education, Chungbuk National University) ;
  • Byung Choon Lee (Department of Science Education, Chungbuk National University)
  • 이익춘 (인하대학교 이과대학 화학과) ;
  • 양기열 (인하대학교 이과대학 화학과) ;
  • 김왕기 (전남대학교 사대 화학교육과) ;
  • 공병후 (충북대학교 사대 과학교육과) ;
  • 이병춘 (충북대학교 사대 과학교육과)
  • Published : 1986.02.20

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.

Keywords

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