Alkoxy radicals in the gaseous phase: β-scission reactions and formation by radical addition to carbonyl compounds

The structures and reactivities of the alkoxy radicals methoxy (CH ₃O·), ethoxy (CH₃CH₂O·), 1-propoxy (CH₃CH₂CH₂O·), 2-propoxy ((CH₃)₂CHO·), 2-butoxy (CH₃CH ₂CH(CH₃)O·), tert-butoxy ((CH₃) ₃CO·), prop-2-enoxy (CH₂=CHCH₂O· ), and but-3-en-2-oxy (CH₂=CHCH(CH₃)O·) have been investigated at the B3-LYP/6-31G(d) and CBS-RAD levels of theory. Enthalpies of formation (Δ_fH₂₉₈^o) were calculated with CBS-RAD for all the alkoxy radicals, the carbonyl and radical products of β-scission reactions, and the transition structures leading to them. The mean absolute deviation between the predicted and available experimental Δ_fH₂₉₈^o values is 5.4 kJ mol ^-1. Eyring (ΔH₀‡, ΔH ₂₉₈‡, ΔG₂₉₈‡) and Arrhenius (log A, E_a) activation parameters for both the forward (β-scission) and reverse (radical addition to carbonyl) pathways were calculated. Agreement with available experimental data is very good, generally within 1-5 kJ mol ^-1 for E_a, and 0.5 for log A. The transition structures are found to be substantially polarized, with the departing radical slightly positive, the O atom negative, and the rest of the molecule positive. The barriers for the β-scission reactions decrease with decreasing endothermicity and with decreasing ionization energy of the departing radical.