CH vibrational mode-softening in alkyl groups bound to Cu(111)

The surface vibrations of C₁C₃ alkyl groups bound to a Cu(111) surface have been studied by high resolution electron energy loss spectroscopy. The alkyl groups were generated by the dissociative adsorption of alkyl iodides and bromides. All of the adsorbed alkyls show CH stretching mode-softening, and isotope labelling studies indicate that the softened CH bonds are those at the α-carbon (the one bonded to the surface). Our results suggest that charge donation from the metal to the antibonding orbital of the alkyl groups is the cause of the CH stretching mode-softening.