SUBSTITUENT EFFECTS ON STRAIN ENERGIES

Ab initio calculations employing the STO-3G basis set have been performed on numerous substituted derivatives of the strained (or unsaturated) molecules ethylene, cyclopropane, cyclobutane, acetylene, spiropentane, bicyclobutane, cyclopropene, tetrahedrane, and cubane. Substituents investigated include Li, BeH, BH2, BH3-, CH3, CH2+, CH2-, CN, CF3, NH2, NH3+, OCH3, OH, O-, and F. Stabilization of strained molecules is achieved with σ-donating, π-withdrawing, and most π-donating substituents. σ withdrawal destabilizes unsaturated linkages. Effects decrease in the order of increasing saturation, e.g., ethynyl > vinyl > cyclopropyl. The stabilizing effects of Li and O- substituents are particularly striking. The strain in tetralithiotetrahedrane is calculated to be less than one-fourth of that in tetrahedrane. © 1979, American Chemical Society. All rights reserved.