First attempts at an elucidation of the interface structure resulting from the interaction between methacrylonitrile and a platinum anode: An experimental and theoretical (ab initio) study

The aim of the present paper is to contribute to the elucidation of the molecular structures obtained on a platinum surface as this surface is submitted to an anodic potential (with respect to a silver reference electrode) when dipped into pure 2-methyl-2-propenenitrile (methacrylonitrile). Modified surfaces are examined using X- and UV-photoelectron spectroscopies (UPS and XPS). The results evidence the formation of an ultra-thin (20-40 Angstrom) grafted oligomer film, which is not classical polymethacrylonitrile (PMAN), as obtained through a radical or anionic mechanism: spectral characteristics argue in the sense of a cationic polymerization of methacrylonitrile through its nitrile groups, as evidenced by a lowering of the gap as well as by the UPS and XPS (N 1s region) spectra. Molecular models of the reactants and reaction intermediates are proposed for the cationic polymerization of methacrylonitrile, and show that this polymerization is about as feasible as that of acetonitrile, at least on kinetic control grounds. Two different mechanisms are nonetheless possible, leading either to a quasi conjugated poly-imine type -(N=C)(n)-, or to a poly-cumulene type -(N=C=C)(n)- network. Theoretical considerations on reactants properties lead us to select the poly-imine way as the most plausible. Along with literature data concerning chemisorbed nitriles on platinum surfaces, a molecular model of the final state of the poly-imine reaction is then designed, comprising a three atom cluster to render the grafting site, and a dimer to render the grafted structure. A full geometry optimization is performed on the organic moiety at the Hartree-Fock (ab initio) level of theory, and a rough evaluation of the spectral footprint of the interface bond in the N 1s region is performed on the basis of Koopmans theorem with calibration on the bulk polymer peak. A preliminary 2.7 eV downward shift is predicted for N 1s interface nitrogens with respect to the polymer peak, which can be compared with the low-energy contribution, found about 2.0 eV below the polymer peak, in the experimental spectrum. The directions in which the molecular model of the interface need be improved are discussed. On the basis of the present results, as well as those obtained previously in the methacrylonitrile/nickel cathode interaction, the conclusion examines the proposition that the structure of the very interface in the final stare of electropolymerization reactions is the frozen footprint of the initial stages of the interaction, and alludes to electrochemistry as a tool to monitor molecule/surface interactions.