Recently, Chiang's research group successfully carried out the temperature programmed reaction (TPR) spectroscopy under ultrahigh-vacuum conditions to probe the reaction pathways of adsorbed methyl (CH3(ads)) and trifluoromethyl (CF3(ads)) with coadsorbed methylene (CH2(ads)) via the CH2 insertion reaction, leading to the formation of adsorbed 1,1,1-trifluoro-ethyl (CF3CH2(ads)) and ethyl (CH3CH2(ads)) on the Ag(111) surface. Additionally, the authors found it feasible for CF3CH2(ads) to proceed the subsequent β-fluoride elimination on the Ag(111) surface to produce 1,1 difluoroethylene (CF2=CH2(g)) but difficult for CH3CH2(ads) to proceed β-hydride elimination to form ethylene (CH2=CH2(g)) on the Ag(111) surface. To elaborate on this noticeably different reactivity between β-hydride and β-fluoride eliminations on the Ag(111) surface we performed total energy calculations based on density functional theory in connection with ultrasoft pseudopotential and generalized gradient spin-polarized approximation, and partial structural constraint path minimization to establish the energetically more favorable pathways for the CH2 insertion into Ag-CX3 (X=H and F) bonds followed by β-X elimination to generate an isolated CH2=CX2(g) on the Ag(111) surface. Following our proposed reaction pathways, namely, the diffusion of the fcc-hollow site of CX3(ads) toward the bridge site of CH2(ads) through the CH2 insertion and the subsequent β-X elimination to form both isolated CH2=CX2(g) and hcp-hollow site of X(ads) on the Ag(111) surface, our calculated energy barrier for β-hydride elimination is significantly larger than (∼0.661 eV) that for β-fluorite elimination. This unusual high-energy barrier prohibits β-hydride elimination of CH2CH3(ads) to form an isolated CH2=CH2(g) on the Ag(111) surface and explains what we observed from the TPR experimental data. We also attribute this much higher energy barrier to forming a largely distorted seven-center ring transition state structure with a larger distortion of the Ag-C(α)H2C(β)H3-Ag and a less stable Ag-H bond on the Ag(111) surface. Finally, our calculated energy barrier for /3-fluoride elimination to form CH2=CF2(g) is 0.687 eV, in very good agreement with the experimental data.
International journal of quantum chemistry 102(5), pp.858-865