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

Abstract

The structures and reactivities of the alkoxy radicals methoxy (CH3O·), ethoxy (CH3CH2O·), 1-propoxy (CH3CH2CH2O·), 2-propoxy ((CH3)2CHO·), 2-butoxy (CH3CH2CH(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 (ΔfH°298) 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 ΔfH°298 values is 5.4 kJ mol–1. Eyring (ΔH‡0, ΔH‡298, ΔG‡298) and Arrhenius (log A, Ea) 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 Ea, 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.Key words: alkoxy, alkoxyl, radical, addition, carbonyl, β-scission, calculaton, electronic structure, B3LYP, CBS-RAD, thermochemistry.