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 β-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 Ag–CF3 bond to form the bridge site of CH2CF3(ads) on the Ag(111) surface. Then we proceed with β-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 β-fluoride elimination is 0.715 eV higher than that for CH2 insertion on the Ag(111) surface. These calculated results imply that β-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.
