DFT study of the formation of CH2=CF2 through both methylene insertion and β-fluoride elimination on the Ag(111) surface

Total energy calculations based on (1) density functional theory (DFT) in connection with ultrasoft pseudopotential and generalized gradient spin-polarized approximation (GGSA) and (2) the partial structural constraint path minimization (PSCPM) method have been used to investigate the energetically more favorable pathway for methylene (CH2) insertion into the Ag-CF3 bond followed by beta-fluoride elimination to generate an isolated CH2=CF2(g) above the Ag(111) surface. The diffusion of the fcc-hollow site of CF3(ads) toward the bridge site of CH2(ads) is proposed as an energe*tically more favorable path for CH2 insertion into the AgCF3 bond to form the bridge site of CH2CF3(ads) on the Ag(111) surface. Then we proceed with beta-fluoride elimination to form an isolated CH2=CF2(g) and the bridge site of F-(ads) on the Ag(111) surface. Our calculated energy barrier for beta-fluoride elimination is 0.715 eV higher than that for CH2 insertion on the Ag(111) surface. These calculated results imply that beta-fluoride elimination rather than CH2 insertion on the Ag(111) surface controls the CH2=CF2(g) formation rate as observed from temperature-programmed reaction (TPR) experimental data. Finally, we attribute these different energy barriers to the different transition state structures - largely distorted seven-centered versus less distorted four-centered - involved in these two different processes.