A MO Study of β‐Substituted Ethyl Radicals

The preferred ground state conformation of β‐substituted ethyl radicals, with the C_β heteroatom substituent group being PH₂, SH and CI₁ have been determined by ab initio spin‐unrestricted self‐consistent field gradient optimization techniques. All the ground state geometries are found to have the C_β‐heteroatom bond coplanar with the C_α‐centered radical orbital. No tendency towards heteroatom group bridging between C_α and C_β is found in the systems studied here. The radical centers are calculated to be non‐planar, with the C_α hydrogen atoms tilting towards the C_β substituent group. Calculated rotation energy profiles about the C_α‐C_β and C_β‐heteroatoms bonds, as well as the atomic orbital spin population distributions, are interpreted in terms of possible interactions between the C_α‐centered radical orbital and the C_β‐heteroatom σ bond orbitals (hyperconjugation, type I) or the heteroatom 3d (type II) or lone pair (type III) atomic orbitals. Type I is the only significant stabilizing interaction found in these systems, whereas the type III (two orbital, three electron) interaction is actually found to be destabilizing.