C-H...M INTERACTIONS AND ORIENTATIONAL CHANGES OF CYCLOHEXANE ON CU(111) - A RAIRS, EELS AND LEED STUDY

The adsorption of cyclohexane on Cu(111) has been studied by reflection absorption infrared spectroscopy (RAIRS), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). The vibrational data indicate that, although cyclohexane retains its molecular identity upon adsorption, some of its C-H groups are significantly perturbed by C-H ... M interactions. These interactions are analogous to hydrogen bonding, and occur between the metal and those C-H groups in closest proximity to the surface. The stretching frequency of 2770 cm-1 exhibited by the perturbed C-H groups and the low desorption temperature of cyclohexane from this surface indicate that this is one of the weakest C-H ... M interactions reported for cyclohexane adsorbed on a metal surface. Detailed analysis of the high resolution IR spectra reveals that, at low coverages, the molecule is adsorbed in C3v symmetry with its carbon skeleton parallel to the surface. This symmetry is subsequently reduced to C(s), induced by increasing coverage in the first layer. The reduction in site-symmetry is attributed to an orientational change in the adsorbed layer, in which the carbon skeleton of the molecule tilts towards the surface normal. This change in adsorbate orientation significantly diminishes the C-H ... M interaction, implying that the extent of C-H mode softening is sensitively dependent on the geometry of the C-H ... M unit.