FIRST ATTEMPTS AT AN ELUCIDATION OF THE INTERFACE STRUCTURE RESULTINGFROM 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 th e molecular structures obtained on a platinum surface as this surface is submitted to an anodic potential (with respect to a silver referenc e electrode) when dipped into pure 2-methyl-2-propenenitrile (methacry lonitrile). Modified surfaces are examined using X- and UV-photoelectr on spectroscopies (UPS and XPS). The results evidence the formation of an ultra-thin (20-40 Angstrom) grafted oligomer film, which is not cl assical polymethacrylonitrile (PMAN), as obtained through a radical or anionic mechanism: spectral characteristics argue in the sense of a c ationic 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 methacry lonitrile, and show that this polymerization is about as feasible as t hat of acetonitrile, at least on kinetic control grounds. Two differen t mechanisms are nonetheless possible, leading either to a quasi conju gated poly-imine type -(N=C)(n)-, or to a poly-cumulene type -(N=C=C)( n)- network. Theoretical considerations on reactants properties lead u s to select the poly-imine way as the most plausible. Along with liter ature data concerning chemisorbed nitriles on platinum surfaces, a mol ecular model of the final state of the poly-imine reaction is then des igned, comprising a three atom cluster to render the grafting site, an d a dimer to render the grafted structure. A full geometry optimizatio n is performed on the organic moiety at the Hartree-Fock (ab initio) l evel of theory, and a rough evaluation of the spectral footprint of th e interface bond in the N 1s region is performed on the basis of Koopm ans theorem with calibration on the bulk polymer peak. A preliminary 2 .7 eV downward shift is predicted for N 1s interface nitrogens with re spect to the polymer peak, which can be compared with the low-energy c ontribution, found about 2.0 eV below the polymer peak, in the experim ental spectrum. The directions in which the molecular model of the int erface need be improved are discussed. On the basis of the present res ults, as well as those obtained previously in the methacrylonitrile/ni ckel cathode interaction, the conclusion examines the proposition that the structure of the very interface in the final stare of electropoly merization reactions is the frozen footprint of the initial stages of the interaction, and alludes to electrochemistry as a tool to monitor molecule/surface interactions.