Coordination and bonding geometry of nitromethane (CH3NO2) on Au(111) surfaces

The adsorption and bonding geometry of nitromethane (CH3NO2) on a Au(111) s ingle crystal surface was studied by temperature programmed desorption and infrared reflection-absorption spectroscopy (IRAS). Nitromethane weakly ads orbs on Au(111) with an adsorption energy of only 10.5 kcal/mol. Adsorption is completely reversible; no thermal decomposition occurs under UHV condit ions. Consistent with weak binding, IRAS does not reveal any significant ch emical shifts for the vibrational modes of CH3NO2 at monolayer and submonol ayer coverages compared to solid multilayer films. Based on the intensities in IRAS and the surface selection rule, we propose that CH3NO2 adsorbs on the Au(111) surface with C-s symmetry in a monodentate geometry using one o xygen atom to bind to the surface with a large tilt of the C-N bond away fr om the surface normal. This bonding interaction is 25% less than that of ad sorbed NO2 on Au(111) which chemisorbs in an O,O'-chelating geometry with C -2 nu symmetry. While isomerization of nitromethane to from methyl nitrite (CH3NO2 <----> CH3ONO) occurs during thermal decomposition in the gas phase , the Au(111) surface does not sufficiently activate CH3NO2 for dissociatio n or isomerization to occur under UHV conditions, and so the activation ene rgies for these processes exceed 10.5 kcal/mol on Au(111). (C) 2001 Publish ed by Elsevier Science B.V.