1,2-Rearrangement of protonated-heterosubstituents in 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" (3) and bridging (4) structures of various protonated β-substituted ethyl radicals (C₂H₄ A +′H; A′H = ClH, SH₂, PH₃). In contrast to the findings for the unprotonated species (1 and 2), heteroatom protonation, which stabilizes both the "open" (3) and bridged (4) forms, results in paths for 1,2-A′H⁺ migration whose energies are substantially below the energy of dissociation to [CH₂CH₂ + A′H]· ⁺· moieties. In the case of CH₂CH₂C l +H, intramolecular ClH⁺ migration can proceed without significant barrier. This raises the possibility that apparent chlorine atom migrations to vicinal radical termini may in fact be ClH⁺ migrations. It is found that protonation of the "open" structures 1b and 1c (A′ = SH, Cl) to give 3b and 3c (A′H = SH₂, ClH) respectively, considerably enhances hyperconjugative interaction between the C_α single electron orbital and the C_βA bond orbitals. The contrasting result that protonation of 1a (A′= PH₂) to 3a (A′H = PH₃) does not enhance hyperconjugation has been rationalized on the basis of rehybridization of the heteroatom bonds.