Nitromethane and methyl nitrite adsorption on Au(111) surfaces

The adsorption of nitromethane (CH₃NO₂ and CD₃NO₂) and d₃-methyl nitrite (CD₃ONO) on Au(111) was studied by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). These molecules are only weakly adsorbed on Au(111), and adsorption is completely reversible in both cases. Adsorbed CH₃NO₂ and CD₃ONO in the monolayer each give rise to one thermal desorption peak in TPD with desorption activation energies of 10.5 and 8 kcal/mol, respectively. These desorption energies are close to the values for the heats of adsorption of these molecules, since there is no appreciable activation energy for molecular adsorption. HREELS confirms weak, molecular adsorption in the monolayer for both molecules. Furthermore, nitromethane is bonded on Au(111) in an upright, strongly tilted geometry, suggesting a monodentate coordination to the surface. Methyl nitrite adsorbs on Au(111) with the O-N=O group in a flat-King geometry, with evidence for both cis and trans forms. The Au(111) surface does not sufficiently activate nitromethane and methyl nitrite for dissociation or isomerization (CH₃NO₂ ↔ CH₃ONO) to occur under UHV conditions. Thus the activation energies for dissociation and isomerization of nitromethane on Au(111) exceed 10.5 kcal/mol.