NITROMETHANE AND METHYL NITRITE ADSORPTION ON AU(111) SURFACES

The adsorption of nitromethane (CH3NO2 and CD3NO2) and d(3)-methyl nit rite (CD3ONO) on Au(111) was studied by temperature-programmed desorpt ion (TPD) and high-resolution electron energy loss spectroscopy (HREEL S). These molecules are only weakly adsorbed on Au(111), and adsorptio n is completely reversible in both cases. Adsorbed CH3NO2 and CD3ONO i n the monolayer each give rise to one thermal desorption peak in TPD w ith desorption activation energies of 10.5 and 8 kcal/mol, respectivel y. These desorption energies are close to the values for the heats of adsorption of these molecules, since there is no appreciable activatio n energy for molecular adsorption: HREELS confirms weak, molecular ads orption in the monolayer for both molecules. Furthermore, nitromethane is bonded on Au(111) in an upright, strongly tilted geometry, suggest ing a monodentate coordination to the surface. Methyl nitrite adsorbs on Au(111) with the O-N=O group in a fiat-lying geometry, with evidenc e for both cis and trans forms. The Au(111) surface does not sufficien tly activate nitromethane and methyl nitrite for dissociation or isome rization (CH3NO2 <-> CH3ONO) to occur under UHV conditions. Thus the a ctivation energies for dissociation and isomerization of nitromethane on Au(111) exceed 10.5 kcal/mol.