Tova Hoz

1,2-Rearrangement of protonated-heterosubstituents in ethyl radicals: a molecular orbital study

Abstract 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 (C 2 H 4 A + ′H; A′H = ClH, SH 2 , PH 3 ). 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 [CH 2 CH 2 + A′H]· + · moieties. In the case of CH 2 CH 2 C 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 = SH 2 , ClH) respectively, considerably enhances hyperconjugative interaction between the C α single electron orbital and the C β A bond orbitals. The contrasting result that protonation of 1a (A′= PH 2 ) to 3a (A′H = PH 3 ) does not enhance hyperconjugation has been rationalized on the basis of rehybridization of the heteroatom bonds.