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 has been carried out for highly symmetric cyclic saturated hydrocarbons and short chain linear saturated hydrocarbons. The softening of the C-H stretching modes of cyclopropane, cyclohexane, as well as propane and its deuterated analogs adsorbed on Cu(111) was examined experimentally using reflection absorption infrared spectroscopy and theoretically using ab initio Hartree-Fock and correlated calculations. The investigation showed that molecules are present in two bound orientations for both cyclopropane and propane. The data further reveal that significant mode softening is associated with only some (not all) of the bound states of these molecules. Soft modes with significant intensity were found to be present for cyclohexane and propane bound on Cu(111) in a planar orientation. A theoretical analysis of the various adsorbed forms of these three types of molecules confirms the surprising experimental observation that cyclopropane has no soft mode that attends its binding on Cu(111). The calculations show that the occurrence of soft modes in the vibrational spectra of cyclohexane (where a broad, intense soft C-H mode exists) and propane (where a weaker, more narrow linewidth soft mode is found) correlates with a small, but still significant degree of charge transfer that governs the alkane-metal interaction. No evidence is found to support a previously proposed theory of a weak hydrogen-bonding interaction that mediates the metal/H-C interaction. (C) 2003 American Institute of Physics.