Cationic polymerization within clusters composed of unsaturated molecules

Electron impact ionization of large neutral clusters composed of simple olefinic molecules (ethene, 1,1-difluoroethene and propene) display highly anomalous ion intensity distributions. These cluster ion distributions typically exhibit a magic number at n = 4, for cluster ions of the type {M}_n⁺, where M is an olefinic molecule. The explanation which best explains this observation is that these small ‘cluster’ ions are in actuality covalently bound molecular ions formed by sequential ion- molecule association (polymerization) reactions which have occurred within the cluster ion. This size dependent cluster chemistry is accounted for by kinetic bottlenecks which occur in the ionic chain growth reactions and which terminate with the formation of cyclic molecular ions (i.e. cyclopentanes and cyclohexanes ions). The condensed phase counterparts of these intracluster reactions are the free cationic polymerization reactions initiated by radiolysis of bulk samples of the olefins. In addition to reviewing our own efforts in this field, we will discuss previous work on cationic polymerization which is relevant to the new chemistry we have observed within clusters of olefinic molecules. We also will present work concerning acetylene and methylacetylene cluster ions where we also observe similar intracluster ion-molecule reactions giving rise to what are most likely benzene and trimethylbenzene ions. Lastly, we will show how mixed cluster of acetylene-acetone also are capable of producing covalently bonded cyclic molecular ions again via intracluster ion-molecule polymerization reactions. We therefore feel that these new cluster condensation reactions constitutes a more general family of chemical reactions for all cluster ions containing unsaturated molecules.