INTRACLUSTER POLYMERIZATION REACTIONS WITHIN ACETYLENE AND METHYLACETYLENE CLUSTER IONS

We report the observation of large (n up to approximately 25) acetylene and methylacetylene cluster ions-the largest yet reported from ionization of neutral {C2H2}n or {CH3CCH}n clusters. The cluster ion intensity distributions of both systems display prominent magic numbers at n = 3. This finding is indicative of intracluster ion-molecule reactions giving rise to what are most likely benzene and trimethylbenzene ions from {C2H2}n+ and {CH3CCH}n+ cluster ions, respectively. The acetylene cluster ion intensity distributions observed under efficient clustering conditions are further characterized by unexpected features, most notably a sharp break at n = 7 and a strong magic number at n = 14. The mass spectra of methylacetylene display a less dramatic break at n = 7 and a weak magic number around n = 10. These surprising structures may arise as a result of the formation of particularly stable covalently bonded molecular ions formed via intracluster polymerization reactions.