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Molecular Orbital Theory of the Electronic Structure of Molecules. 35. ,&Substituent Effects on the
 

Summary: 590I
Molecular Orbital Theory of the Electronic Structure
of Molecules. 35. ,&Substituent Effects on the
Stabilities of Ethyl and Vinyl Cations. Comparison
with Isoelectronic Methyl Boranes. The Relative
Importance of Hyperconjugative and Inductive Effects
Yitzhak Apeloig,'" Paul v. R. Schleyer,*Iband John A. PopleIc
Contribution from the Departments of Chemistry, Princeton University,
Princeton, New Jersey 08540 and Carnegie-Mellon University, Pittsburgh,
Pennsylvania 15213, and the Institut fur Organische Chemie der Universitat
Erlangen-Nurnberg, 8520 Erlangen, West Germany. Received October 13, 1976
Abstract:The effect of @-substituents,X, on the stabilities of ethyl and vinyl cations is studied by standard ab initio procedures.
The ethyl cations are examined in perpendicular (1) and eclipsed (2) conformations. X is varied systematically along the whole
series of first short period substituents, Li, BeH, BH2, CH3, NH2, OH, and F. Electropositivesubstituents are extremely effec-
tive in stabilizing ethyl cations, e.g., 88.9 and 27.1 kcal/mol (RHF/4-31G) for @-Liand @-BeH,respectively. Hyperconjuga-
tive contributions are larger than inductive effects for most substituents. Hyperconjugation between the C-X bond and the
empty 2p cationic orbital is stabilizing for electropositive (relative to hydrogen) substituents which thus prefer conformation
1, but destabilizing for electronegative substituents which prefer conformation 2. Very high barriers for rotation around the
C-C+ bond were found for P-lithio- and 0-beryllioethyl cations, 49.8 and 22.8 kcal/mol (RHF/4-3 IC),respectively. A linear
correlation exists between the rotation barriers in the ethyl cations and the isoelectronic boranes, the cations being 2.3 times

  

Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology

 

Collections: Chemistry