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

The adsorption and bonding geometry of nitromethane (CH₃NO₂) on a Au(111) single crystal surface was studied by temperature programmed desorption and infrared reflection-absorption spectroscopy (IRAS). Nitromethane weakly adsorbs on Au(111) with an adsorption energy of only 10.5 kcal/mol. Adsorption is completely reversible; no thermal decomposition occurs under UHV conditions. Consistent with weak binding, IRAS does not reveal any significant chemical shifts for the vibrational modes of CH₃NO₂ at monolayer and submonolayer 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 oxygen atom to bind to the surface with a large tilt of the C-N bond away from the surface normal. This bonding interaction is 25% less than that of adsorbed NO₂ on Au(111) which chemisorbs in an O,O′-chelating geometry with C_2v 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 dissociation or isomerization to occur under UHV conditions, and so the activation energies for these processes exceed 10.5 kcal/mol on Au(111).