DFT STUDY OF THE FORMATION OFCH2=CF2THROUGH 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 theAg–CF3bond followed by β-fluoride elimination to generate an isolatedCH2=CF2(g)above the Ag(111) surface. The diffusion of the fcc-hollow site ofCF3(ads)toward the bridge site ofCH2(ads)is proposed as an energe*tically more favorable path forCH2insertion into theAg–CF3bond to form the bridge site ofCH2CF3(ads)on the Ag(111) surface. Then we proceed with β-fluoride elimination to form an isolatedCH2=CF2(g)and the bridge site ofF(ads)on the Ag(111) surface. Our calculated energy barrier for β-fluoride elimination is 0.715 eV higher than that forCH2insertion on the Ag(111) surface. These calculated results imply that β-fluoride elimination rather thanCH2insertion on the Ag(111) surface controls theCH2=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.