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

摘要

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 (ΔfH298o) 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 ΔfH298o values is 5.4 kJ mol -1. Eyring (ΔH0‡, ΔH 298‡, ΔG298‡) 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.