Shape and angle strain in organic intermediates. A model study of alkyl radicals, anions, and cations

Abstract

The Walsh-Mulliken diagrams for methyl are constructed from ab initio wave functions and are used to explain the shape and energetics of alkyl cations, radicals, and anions. Two angles, HCH and the out-of-plane CH bend, define the energy surfaces. The minimum energy conformation of the cation is always planar and that of the radical is also planar except for an HCH angle of 90°, in which case CH is bent out-of-plane by 32.4°. For the anion, HCH angles of 90, 109.47, 120, and 135° lead to minimum energy out-of-plane CH angles of 72, 63.4, 56.6 and 32.8°, respectively. It is shown that the entire energy surface can be explained solely in terms of the occupancy and hybridization of the highest occupied molecular orbital. It is expected that the shape of all alkyl intermediates will be governed by the simple rules established in this model study and it is shown that existing experimental and computational data fit this pattern.