1,2-Rearrangement in β-substituted ethyl radicals. A molecular orbital study

Ab initio self-consistent field (SCF) calculations in a Gaussian orbital basis set have been carried out on both the equilibrium (open) and bridging (ring) structures of various β-substituted ethyl radicals (C₂H₄X, X = Cl, SH, and PH₂) in order to study substituent 1,2-migration. For X = Cl the C_2v symmetrically bridged (SB) structure is found at all geometric points to be above the C₂H₄ + Cl dissociation energy limit in a multiconfiguration (MC) SCF framework, although single-configuration unrestricted SCF calculations show very shallow minima (for ²A₁ and ²B₁ states) at long C₂H₄-Cl distances. In the MC calculations on the steeply repulsive ²A₁ state charge transfer (C₂H₄ → Cl) is found to be an important stabilizing feature, suggesting a method for stabilizing the ring structure. Bridging structures (both symmetric and slightly unsymmetric) involving relative twisting of the methylene groups are found to be more stable than the SB structures. For both X = SH and PH₂ the unrestricted SCF results for the ring structures are similar to that for C₂H₄Cl.