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 (CH3O.), ethoxy (CH3CH2O.), 1-propoxy (CH3CH2CH2O.), 2-propoxy ((CH3)(2)CHO.), 2-butoxy (CH3CH2CH(CH3)O.), tert-butoxy ((CH3)(3)CO.), 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 (Delta(f)H(298)degrees) were calculated with CBS-RAD for all the alkoxy radicals, the carbonyl and radical products of beta-scission reactions, and the transition structures leading to them. The mean absolute deviation between the predicted and available experimental Delta(f)H(298)degrees values is 5.4 kJ mol(-1). Eyring (DeltaH(0)(double dagger), DeltaH(298)(double dagger), DeltaG(298)(double dagger) and Arrhenius (log A, E-a) activation parameters for both the forward (beta-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 0 atom negative, and the rest of the molecule positive. The barriers for the beta-scission reactions decrease with decreasing endothermicity and with decreasing ionization energy of the departing radical.