HYDROCARBONS WITH ORTHOGONAL PI-SYSTEMS - MOLECULAR ADSORPTION VERSUSSTRUCTURAL ISOMERIZATION AT SURFACES

The adsorption of propadiene on Ni(111) and Cu(110) has been studied b y reflection absorption infrared spectroscopy and on evaporated silver films using transmission infrared spectroscopy. Adsorption on Cu(110) and silver films at low temperatures yields a molecularly adsorbed sp ecies in which the orthogonal pi-systems of the molecule are retained and oriented so that one of the CH2 units lies with its plane parallel to the surface, and the other with its plane perpendicular to the sur face. The molecules adopt a preferential orientation in both the monol ayer and the multilayers whereby the C=C=C skeleton is held parallel t o the surface. Adsorption of propadiene on Ni(111) follows a different course, leading to hydrogen-transfer induced isomerisation in which t he two original C=C double bonds of the molecule are replaced by one s ingle and one triple bond, resulting in the formation of a di-delta/di -pi propyne species.