Mode-softening of C–H stretch vibration in alkyl groups on Ag(1 1 1) and the fluorination effect

The surface vibrations of CH2(a), CH3CH2(a) and CF3CH2(a) groups on Ag(1 1 1) have been studied by reflection–adsorption infrared spectroscopy. Compared with the corresponding iodides, obvious C–H stretch mode-softening was observed for adsorbed CH2(a) and CH3CH2(a), but not for CF3CH2(a), which is attributed to the weakening of the charge transfer from silver to the C–H anti-bond of the alkyl fragments by the fluorine atoms. These results provide the indirect evidence that the partial charge-donation into the C–H anti-bond, instead of the formation of an agostic C–H ⋯ M bond, is responsible for the C–H stretch mode-softening of adsorbed hydrocarbons/alkyl fragments on transition metal surfaces.