Theoretical study of the reactivity of ketene with free radicals

The structures and energies for the addition of free radicals R . (R = H, C H3, OH, F, SiH3, Cl) to CH2=C=O to give the radicals RCH2C over dot =O, C o ver dot H-2(C=O)R, CH2=C over dot OR have been calculated by ab initio and B3LYP-DFT methods, and the latter method gives good agreement with availabl e experimental energies. Product radicals C over dot H2C(=O)R for groups R which possess electron lone pairs are stabilized and have predominant spin density on carbon, and this is attributed to conjugation of the carbonyl gr oup in the product with substituents OH, F, and Cl at the alpha-position. A dditions of H and SiH3 have lower barriers to form the more stable product RCH2C over dot =O, which for the latter is favored due to hyperconjugative stabilization by the beta-SiH3. For CH3 attack at both carbons is competiti ve, while for OH, F, and Cl, the barriers are low for attack at either carb on, although attack at C-alpha gives much more stable products. Initial com plexes between ketene and the CH3, OH, SiH3, and Cl radicals are detected, and for Cl using B3LYP this species has the structure of a pi-complex with the C=C double bond that is stabilized by 16.2 kcal/mol relative to the rea ctants and forms C over dot H2C(=O)Cl with a barrier of 2.8 kcal/mol. For F no barriers for addition to either carbon were found, but for B3LYP there is a barrier of 27.6 kcal/mol for conversion of FCH2C over dot =O to C over dot H2C(=O)F, which is more stable by 19.1 kcal/mol. The corresponding rea rrangement of ClCH2C over dot =O has a barrier of 4.6 kcal/mol, and the pre dicted preference for initial attack at C-beta to give the less stable prod uct agrees with experiment.