Effect of hyperconjugation on ionization energies of hydroxyalkyl radicals
Based on electronic structure calculations and molecular orbital analysis, we offer a physical explanation of the observed large decrease (0.9 eV) in ionization energies (IE) in going from hydroxymethyl to hydroxyethyl radical. The effect is attributed to hypercongutaive interactions between the sigma(CH)$ orbitals of the methyl group in hydroxyl, the singly occupied p orbital of carbon, and lone pair p orbital of oxygen. Analyses of vertical and adiabatic IEs and hyperconjugation energies computed by Natural Bond Orbital procedure reveals that the decrease is due to the destabilization of the singly occupied molecular orbital in hydroxyethyl radical, as well as structural relaxation of the cation maximizing the hyperconjugative interactions. The stabilization is achieved due to contracted CO and CC bonds, whereas large changes in torsional angles bear little effect on total hyperconjugation energies and, consequently, IEs.