Studies on the structure and β-bond scission reactions of primary alkyl radicals, CH₃(CH₂)_nCH_2•, for n = 1-6

Abstract

HF/6-31G* optimized geometries are reported for n-alkyl radicals from n-propyl to n-octyl in two extended chain conformations, one where the radical carbon 2p orbital housing the unpaired electron is eclipsed to a β-CH bond and another where the orbital is eclipsed to a β-CC bond. In all cases, the β-CH eclipsed conformer is ≈100-200 cal/mol lower in energy than the β-CC eclipsed conformer. The geometry at the radical center is nonplanar by approximately 14°, indicative of some "s" character in the carbon 2p orbital containing the unpaired electron. In the radicals studied here, the bond lengths of the eclipsed β-CH and β-CC bonds are longer than the corresponding noneclipsed β-bonds, attributed to a hyperconjugative interaction. Potential functions for internal rotation about the α, β, and γ bonds reveal the following: rotation about α-CC bonds is free but rotation about β- and γ-bonds have barriers of ≈3 kcal/mol. Bond scissioning and isomerization reactions for n-alkyl radicals are calculated and compared with experimental data. The ΔE values, the changes in total energies between products and reactants, including zero point energies, for C-H rupture are of the order of 33 kcal/mol, while for C-C scissioning, ΔE ≈ 20 kcal/mol. 1,3-, 1,4- and 1,5-isomerization reactions have much lower ΔE values and, on a relative basis, appear to be the preferred reaction pathway for the radicals. © 1993 American Chemical Society.