Hyperconjugation at Play

Boris Karpichev, Hanna Reisler, Anna I. Krylov, and Kadir Diri

J. Phys. Chem. A, 112, 9965 (2008)

Structures of hydroxymethyl and hydroxyethyl radicals.

Based on electronic structure calculations we offer a physical explanation for the observed large decrease (0.9 eV) in ionization energies (IE) in going from hydroxymethyl (CH₂OH) to hydroxyethyl radical (CH₃CHOH). This effect is attributed to hyperconjugative interactions between the σ_CH orbitals of the methyl group in CH₃CHOH, the singly occupied p orbital of C, and the lone pair p orbital of O.

In hydroxymethyl, the lone pair of oxygen is close in energy to the carbon p_z orbital, so they form bonding and antibonding π-type orbitals.

Analyses of the vertical and adiabatic IEs and hyperconjugation energies computed by NBO suggest that the decrease is due to the destabilization of the singly occupied molecular orbital in CH₃CHOH radical, as well as due to geometrical relaxation of the cation, which maximizes hyperconjugation. The stabilization is achieved through the shortening of the CO and CC bonds, whereas changes in torsional angles have a small effect on the total hyperconjugation energies and, therefore, on the IEs.

In hydroxyethyl, the π*_CO fragment can interact with the σ_CH orbitals of the methyl group.