The origin of soft vibrational modes of alkanes adsorbed on Cu: An experimental and theoretical investigation

A detailed experimental and theoretical analysis of the metal-induced softening of C-H vibrational modes commonly seen for alkanes adsorbed on metal surfaces was described. The results show that the significant mode softening was associated with only some of the bound states of these molecules. A theoretical analysis confirms the surprising experimental observation that cyclopropane on Cu(111) exhibits no soft modes and indicates that the occurrence of soft modes in the vibrational spectra of cyclohexane and propane correlates with a small, but still significant degree of charge transfer from the metal into the molecule.